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Ask an Astrophysicist: Cosmology

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Cosmology

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Library of Past Questions and Answers

    Common Questions about Cosmology

Where in the Universe did the Big Bang originate? (Submitted March 27, 1998)

The Question

Is there an area in the universe where cosmologist believe the Big Bang originated?

Depending on where our galaxy (the Milky Way) is located in relationship to the origin of the Big Bang, might it not appear that the Universe were expanding or not?

The Answer

The question you have asked is a good one, and it involves concepts which are foreign to most of our everyday experiences. You may have heard something of Einstein's contributions to physics, in particular his theory of relativity. He introduced many unusual and counter- intuitive concepts that derived ultimately from simple, almost child- like thought experiments. One of his contributions was showing that, in a Universe with no matter and no energy, time itself ceases to exist. Now, one corollary of this that comes into play as far is the Big Bang is its location. Some think of the Big Bang as a localized and very powerful explosion. This is not quite accurate. All things that exist now or ever existed in the past were also present, although perhaps in different form, when the Universe was created. Therefore the Big Bang occurred everywhere all at once. You could not assign a location to it. The Big Bang was not so much an explosion really as the start of a great expansion, which continues even now. The rate of expansion would appear to be roughly the same, aside from local anisotropies, no matter where in the Universe you are.

J.K. Cannizzo
for Ask an Astrophysicist

Question ID: 980327a

What was before the Big Bang? (Submitted October 01, 2003)

The Question

Scientists mention the Big Bang theory often, and i have clearly read about the subject many times, but no one seems to say what the beginning REALLY was. They always say that when the Big Bang occurred, is when time started. But surly there must have been a 'time before the big bang'...What I mean is, where did the matter and energy come from before the explosion occurred? How DID something (matter and energy) come from nothing? I don't see how something can just be without something to create it. What is the TRUE beginning? I understand if you don't have all the answers. but please try and give me your point of view.

The Answer

One thing to remember is that the way it appears our universe works, there is no way for information from any past universe to get to us. Also, if our universe were to collapse into a big crunch, which seems pretty unlikely with what we know presently, any information in this universe would be destroyed, and no information would be transmitted into the next universe.

So, as far as anything we can know, the universe started with a big bang (or rapid expansion from a very hot dense state) about 13.7 billion years ago.

Hope this helps,
Michael Arida for Ask an Astrophysicist

Question ID: 031001a

Was there time before the Big Bang? [Follow-up to above] (Submitted June 29, 2006)

The Question

I have trouble seeing how time came to existence from the Big Bang. If this is the case, it will mean no time will before the Big Bang they would be no time for something to cause the Singularity to explode it will be frozen in time without causing any Big Bang.

The Answer

Thanks for your excellent question. You are among some of our great scientific thinkers when it comes to wondering about the beginning of time. The truth is that, we don't yet have a definitive answer. The Big Bang certainly suggests that time began at the first instant of the Big Bang, since before then, the universe was collapsed into a singularity. The notion of time within the Big Bang scenario is discussed at length by Stephen Hawking. Everything is squeezed down to zero and such physical quantities as spacetime become infinite. The singularity is the point at which time has no meaning.

However, there are other theories about our universe that suggest that there was indeed a time before the initial "bang." Two of the most popular theories are pre-Big Bang theory (using string theory) and the Ekpyrotic scenario (using the collision of "D-branes"). In these theories, the pre-big bang universe was very sparse and weakly interacting, but did eventually give rise to a sudden transition that is similar to our "big bang." We recommend an excellent article on the subject at :

http://www.sciam.com/article.cfm?articleID=00042F0D-1A0E-1085-94F483414B7F0000

Hope this helps,
Georgia & Mike
For "Ask an Astrophysicist"

Question ID: 060629a

If all the distant galaxies are flying away from us, does that mean that we're in the center of the Universe? (Submitted June 11, 1997)

The Question

If all the distant galaxies are flying away from us, does that mean that we're in the center of the Universe?

The Answer

Thanks for your question. Astronomers and physicists interpret the result that all distant galaxies are flying away from us as evidence for the uniform expansion of the Universe. In this case, any observer, at any location in the Universe, observes the same general motion: that the further a galaxy is from us, the faster its relative velocity with respect to the observer is. The famous (and very illustrative) example of this is to imagine a loaf of raisin bread as it is baking. The raisins in the bread spread away from one another as the loaf rises and expands during the baking. Pick any raisin and pretend you are standing on it (you're very small now!) and measuring the rate at which the other raisins are moving away from you. You will find that, no matter which raisin you choose, all other raisins appear to be moving away from you, with the furthest raisins receding the fastest.

The current cosmological model of the Universe supposes that our position within the Universe is typical, not special. We are not located at the center of the Universe, but are rather taking part in its global expansion. I hope this answers your question.

Regards,

Padi Boyd
for the Ask an Astrophysicist

Question ID: 970611e

What is the Universe expanding into? (Submitted October 21, 2001)

The Question

As a Philosophy student, I have linguistic tools to deal with space and time. These are bound intimately with consciousness. Having minimal experience with the astronomer's conception of space, and having only an amazed onlooker's idea of the "space" of the space of the universe, and having a child's wonder of the idea that the universe may be expanding unstoppably onward, then I ask a child's question: what is the universe expanding into? What "space" exists that allows us to say: this is big, and growing bigger into WHAT? Into what "space" is the universe expanding? And I am familiar with the idea that "expansion" and "growth" only happen within space/time parameters- philosophically speaking, reality is only apparent under the conditions that we "appear"... so I have read that the "edge" of the "edge of the universe" idea really does not not hold true when one "does the math." I would like to know more about this math, that conditions a movement that expands, but expands not "into" not "out" or "toward" -anything else...

The Answer

Thank you for your question. Perhaps the simplest way to look at these questions is the following: if the universe includes, by definition, everything -- all of space, time, matter, energy -- than there can be nothing outside of it (and hence no edge), nothing for it to expand into. Its true that this is contrary to our everyday experience, as is much else in physics and astronomy; but of course our everyday experience does not extend to the entire universe. In some ways this line of argument parallels those in refutations of the "argument by design" for the existence of God.

Another way to look at it: if there were a higher-dimensional space in which the universe were embedded and into which it expands (like a two-dimensional balloon expanding into three-dimensional space), we could have no way of ever measuring the existence or characteristics of such a space. Whether such an unobservable space can truly be said to exist at all is a question best addressed by philosophers such as yourself!

Here is an additional reference you may find helpful:

http://www.astro.ucla.edu/~wright/cosmology_faq.html.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"

Question ID: 011021a

What is at the edge of the Universe? (Submitted December 02, 1996)

The Question

I'm a 10 year old boy who has a very hard question to ask. My teacher Mr. Sperling just had us learn about the Solar System. We just made a Internet site. Now my question. What is at the edge of the Universe ?

The Answer

That's an interesting question. The short answer is that there isn't any "edge" to the Universe, as in the edge of your school grounds where there is more property beyond. Science fiction and other dimensions aside, the best way of looking at the Universe is to think of the surface of a balloon. Right now the "balloon" is expanding (being blown up) so the distance between any two points on the balloon is increasing. However, there is no edge to the surface of the balloon. This is where cosmologists (people who study the physical nature and evolution of the Universe) and relativists (people who study Einstein's general theory of relativity) talk about a curved space-time continuum.

One observable effect of this geometry for the Universe is if we look far enough in any direction, we see the same thing. Because light does not travel infinitely fast, the farther into the distance that we look, the farther back into time that we look. In astronomy there is something called the cosmic microwave background. This radiation is left over from the "big bang", the event at the start (in time) of our Universe.

Check out your library and its reference section for books on astronomy for more information.

Steve Snowden
for Imagine the Universe!

Question ID: 961202c

If the universe is expanding like a balloon, is it hollow? [Follow-up to above] (Submitted July 7, 1998)

The Question

If the universe is expanding like a balloon and we are on that balloon, what is inside the balloon, or outside?

Is this balloon hallow? If it is not, are galaxies traveling outward (in radial direction) at the same speed with respect the "ground zero" (Big Bang)?

If it is hollow, should we see nothing if we look inward or outward?

Thank you for your time.

The Answer

The balloon analogy is very confusing. In fact, there is no direction in the universe which corresponds to inward or outward on the balloon. Thus the universe is not like a hollow balloon because there is no "inside" to be hollow.

Other books use the analogy of rising raisin bread dough (where the raisins represent galaxies). This has different problems (e.g., the Universe doesn't have anything analogous to the crust), but it doesn't raise questions of inside and outside.

All analogies break down eventually. The balloon analogy breaks down sooner than most.

David Palmer
for "Ask an Astrophysicist"

Question ID: 980707a

How can we see light that should have already passed us? (Submitted November 03, 2006)

The Question

The Hubble Ultra Deep Field is a snapshot of (that small section) of the galaxy 13 billion years ago. This means that the light that composed the image travelled 13 billion light years, and hence the source is currently ~13 billion light years away. If we say that the universe is 13.7 billion years old, then the observable universe is roughly 13.7 billion light years in radius.

Therefore when the light was emitted, we can assume the observable universe was 700 million light years in radius. Which means that the source should also have been within 1.4 billion light years of Earth. So unless the source and Earth were expanding away from each other at nearly 9/10ths the speed of light, which is obviously untrue...how come the light for this photo didn't pass us billions of years ago?

The Answer

That is a good question and it shows the difficulties and non-intuitive nature to some aspects of our universe. We really would like to have an absolute reference frame to measure all things against, but it just doesn't exist.

It is also easy to get confused about distances and time in cosmology, because there are different definitions used in different situations, or by different authors. As Prof. Ned Wright says, "The time and distance used in the Hubble law are not the same as the x and t used in special relativity, and this often leads to confusion." See:
http://www.astro.ucla.edu/~wright/cosmo_02.htm#DH
http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN
http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2

Some parts of the universe are outside of our horizon, and since the expansion appears to be speeding up, these will never be observable to us.

Some detailed references are:

- Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe
http://arxiv.org/abs/astro-ph/0310808

- No Superluminal Expansion of the Universe
http://arxiv.org/abs/gr-qc/930300

Hope this helps,
Tom, Koji, Andy, Tess and Mike for "Ask an Astrophysicist"

Question ID: 061103a

Could the Big Bang have been a Black/White Hole? (Submitted October 30, 2001)

The Question

Given that the universe may have started from a singularity in a Big Bang, and that it seems that an awful lot of the universe is going to end up at the singularity inside a Black Hole, is it possible there is a connection between the two?

I've seen the depiction of Space-Time as a rubber sheet with each mass causing a distortion to it -(recall Homer Simpson looking into the deep distortion caused by a Black Hole?) - Is it possible for the singularity of a sufficiently massive Black Hole to distort Space-Time to the extent where it ruptures, the singularity exploding into the void beyond giving rise to another Big Bang and the start of another universe?

The Answer

Although the Big Bang also represents a spacetime singularity, it is not really a black hole. Actually, the Big Bang has more of a resemblance to the time-inverse of black holes: white holes (that may not actually exist in nature). But the Big Bang singularity is not really a white hole either -- there are technical differences in the natures of their event horizons and their connection to the rest of the universe and its constituents.

For a more detailed, and somewhat technical, explanation please see
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html.

It is also not necessarily the case that "an awful lot of the universe is going to end up at the singularity inside a Black Hole," since the universe could very well be Open rather than Closed and simply expand forever (in fact the latest evidence seems to point in that direction).

The Big Bang included all of spacetime and so does not represent a rupturing of some space in which it is embedded (that is, there is no "void beyond" the universe), and there is no danger of supermassive black holes causing any sort of miniature Big Bangs although they are responsible for the super-energetic phenomena know as quasars.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"

Question ID: 011030a

Is it possible that there are many Universes? (Submitted March 01, 1998)

The Question

If it is possible to have hundreds of thousands of solar systems, is it possible to have hundreds of thousands of universes?

The Answer

People can, and are, going about extensive observing programs looking for planets around other stars, with a good deal of success recently. So not only can they exist, but we now know a few that actually do.

On the other hand, there is no way to observe any universe other than our own. This is not a practical issue (like there not being good enough telescopes), but rather a fundamental theoretical issue. By definition, our universe is self-contained. No other universe can affect anything in our universe, so we cannot gather evidence about its existence.

Applying Einstein's laws of general relativity, it is possible to come up with solutions that connect one universe to another via a spinning black hole. However even if we were to observe such a thing, it would still not tell us if there was another universe on the other side or if the other side was part of our own universe.

So, from a scientific standpoint the question is moot, but from a philosophical standpoint, it is fascinating.

Jonathan Keohane
for Ask an Astrophysicist

Question ID: 980301d

    The Big Bang and the Expansion of the Universe

What does the evidence tell us about the origin of the Universe and what is the evidence that concludes this point? (Submitted September 22, 1997)

The Question

What does the evidence tell us about the origin of the Universe and what is the evidence that concludes this point?

The Answer

The cosmological model of the origin of the Universe is a big subject, and I will only be able to give you a short sketch in this email, but there are many excellent books written by researchers for the general public that you can refer to.

In 1912 American astronomer Vesto Slipher noticed that virtually every spiral galaxy he observed had a redshifted spectrum. The instrument he used split the light from the galaxies into a spectrum, in the same way a prism splits the light from the Sun into a rainbow. Looking at light in this way, you can measure the intensity as a function of wavelength. Elements found in the galaxies each have "fingerprints": the spectral lines they emit. Since it is straightforward to measure the wavelength at which these spectral lines are emitted in different elements in the laboratory, looking at the spectrum of galaxies can give us a tremendous amount of information.

In addition to seeing that the lines appear, astronomers can measure how far the wavelength we see them at differs from the "rest" wavelength. If the galaxy is moving, the lines will be Doppler shifted: they will appear at shorter wavelengths (bluer) if moving toward us, and at longer wavelengths (redder) if moving away. Edwin Hubble realized in the 1920s that when we look at the motions of all of the galaxies, measured in this way, there is a definite trend. The galaxies are speeding away from each other, consistent with a general expansion of the Universe. This was the first observational evidence to indicate an initial starting point of the Universe as a sort of explosion, from which everything is now still expanding. This is called the Big Bang. Hubble noticed that the measured recession velocity of a galaxy was proportional to its distance from us. This is called Hubble's Law, and the constant of proportionality is called the Hubble constant---the value of which is currently still a very active area of modern observational astronomy.

Hubble's Law has a very interesting implication for the history of the Universe. If we know how fast something is traveling away from us, it is a simple matter to calculate how long it has taken that thing to reach its present distance. If we assume the velocities of the galaxies we measure have been constant in time, then we can conclude that at a time= 1./(Hubble constant) all the galaxies were virtually at the same location, starting their expansion. This time turns out to be about 13 billion years (using a value of the Hubble constant of 75 kilometers per second per megaparsec). It is this beginning that is known as the Big Bang.

The Big Bang theory has many predictions. In the 1940s, physicist George Gamow realized that the very early Universe must have been very dense and very hot. As the Universe expanded and cooled down, this hot radiation should cool down, eventually being observable in the radio region of the spectrum. In the 1960s Penzias and Wilson discovered the cosmic microwave background radiation: a uniform radio hiss that implied a temperature of about 3 degrees Kelvin. Later, the COsmic Background Explorer (COBE) took very detailed measurements of the spectrum and spatial distribution of this radiation, confirmed that it is extremely uniform, is of the spectral shape predicted by theory, and corresponds to a temperature of 2.7 degrees Kelvin. This observation provides strong support for the Big Bang theory.

There are many fascinating branches of the story that I haven't even mentioned. Here are some references you should look at: S. Hawking "A Brief History of Time", C. Sagan, "Cosmos", S. Weinberg "The First Three Minutes". All are extremely readable, and written by great science communicators.

Regards,

Padi Boyd
for the Ask an Astrophysicist

Question ID: 970922g

What was Friedman's theory on the expansion of the universe? (Submitted September 20, 2000)

The Question

What was Freidman's theory on the expansion of the universe? What evidence did he have to support this?

The Answer

Aleksandr Aleksandrovich Friedmann (1888-1925) was a Russian mathematician:

http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Friedmann.html

who correctly identified that Einstein's field theories allowed for expanding and contracting universes as well as static ones. He didn't have any physical evidence for this at the time, but correctly deduced, and Einstein later confirmed, that it was possible.

You can find more information on Friedman's theory at:

http://ned.ipac.caltech.edu/level5/Narlikar/Narlikar_contents.html

Hope this helps,
Michael Arida for Ask an Astrophysicist

Question ID: 000920a

What does the concept of relative time frames mean for the Universe's expansion? (Submitted August 18, 1997)

The Question

Hawking wrote about a singularity (physical as well as mathematical term), and the possibility that space begun its life from one of these. If space is expanding at speed comparable to the speed of light and if every galaxy is (approximately) considered to be homogeneous (speed of all material bodies within one galaxy is the same), would it mean that every galaxy has its own 'TIME' regarding to some reference time in spot of singularity? If so, possibility of parallel worlds would be reality.

The Answer

Your question touches on one of the fundamental concepts of relativity. Observers moving relative to each other have their own 'time' in the sense that they may not agree whether two events happen at the same time. So there is no way to set up a single time system for the whole universe. Since the universe is expanding the relative speed of galaxies increases with distance. This means that there may be galaxies far enough away from us that the distance to them is increasing faster than the speed of light. This might seem to conflict with Special Relativity but it happens because space itself is expanding. We are completely cut off from these regions of spacetime. There is no way to communicate with them because that would require us to send information faster than the speed of light. Maybe this is what you mean by parallel worlds, but they are not really parallel worlds. They are just regions of our universe that we can never reach or communicate with.

Damian Audley and Tess Jaffe
for the Ask an Astrophysicist team.

Question ID: 970818a2

How much energy was released in the Big Bang? (Submitted February 11, 1998)

The Question

What is the amount of energy released in the Big Bang. Expressed in tons of dynamite or H-bombs, etc.

The Answer

Energy wasn't "released" per se - it's still contained within the event horizon, presumably.


Notation:
** is an exponent - ie x**2 means x squared.
* is a multiplication symbol
/ is a division symbol

The total mass-energy content of the universe today is of the order of the critical density,

  3 x H0**2/(8*pi*G) = 5 x 10**(-30) g/cm**3, 

times the volume contained within the present event horizon,

  (4/3)*pi*R**3,
 

where R = the event horizon = c * T (speed of light * age of Universe ) = 3 x 10**10 cm/s x (2/3)*(c/H0). Here H0 is the Hubble constant, assumed to be around 50 km/s/Mpc and Omega = 1 (critical deceleration). For this value of H0, 1/H0 = (app) 20 billion years, making the current age of the Universe about 2/(3*H0) = 13 billion years, so that

  R = (app.) 1.3 x 10**28 cm,
which should be equivalent to 13 billion light-years (1.3 x 10**10 y x 10**13 km/y x 10**5 cm/km).

This gives a total mass-energy mass of about 4.4 x 10**55 grams, equivalent to about 2.6*10**79 protons. The energy equivalent (E = m*c**2) of these protons is about 2.5x10**79 GeV or 2.5x10**88 eV * 1.6x10**-19 J/eV = 4x10**69 Joules.

One ton of TNT releases 4.2 x 10**9 Joules. Thus the energy equivalent of the mass=energy of the universe is about 9.5 x 10**53 Megatons of TNT. This is greater than the mass-energy of the universe, but only because the chemical process involved in exploding TNT is vastly less efficient that E = m*c**2.

Jim Lochner
for Ask an Astrophysicist (with help from Mark Kowitt, Mike Corcoran, and Leonard Garcia)

Question ID: 980211b

Where is the newest information on inflation theory? (Submitted February 02, 1997)

The Question

I have studied a lot about the inflation theory (for example Allan Guth and others). Could you tell me where I can find the newest info on that subject?

The Answer

For an overview of inflation without equations, a good site is:

http://www.lifesci.sussex.ac.uk/home/John_Gribbin/cosmo.htm

This page is by John Gribbin, who is an excellent science writer in my opinion.

NASA's Microwave Anisotropy Probe (MAP) page may also be helpful. For more in depth information with diagrams and basic formulas, try UCLA Prof. Ned Devine's inflation/cosmology tutorial.

You could also try the book "The Inflationary Universe: The Quest for a New Theory of Cosmic Origins" written by Alan Guth and published by Addison-Wesley.

Best wishes,

Koji Mukai
for Imagine the Universe!

Question ID: 970202

Doesn't faster than light inflation violate Relativity? (Submitted October 30, 2003)

The Question

In a article I saw that the size of the Universe was 18 Million LYs across after only 780,000 years. Could you explain how the Universe got so big after just a short amount of time? Is it relativistic in nature? I also saw an article that stated the Universe was a few kilometers across just microseconds after the "big Bang" occured. Could you explain this anomaly? Thank you.

The Answer

What you are referring to is called "Inflation." In an early time after the Big Bang, there is evidence that the universe expanded at speeds greater than light. But special relativity was not violated, because this was an expansion of SPACE and no matter or information was carried between two points at faster than light speed. General relativity allows inflation to be incorporated into Big Bang cosmology.

For more on this, check out these other answers on our site:

http://imagine.gsfc.nasa.gov/ask_astro/cosmology.html#970202
http://imagine.gsfc.nasa.gov/ask_astro/cosmology.html#070904a

Amy C. Fredericks and Michael Loewenstein
for "Ask an Astrophysicist"

Question ID: 031030b

Will the universe acceleration exceed the speed of light? (Submitted September 04, 2007)

The Question

I have read articles that cosmologists believe that the universe is expanding at an accelerated rate. Another theory states that the speed of light is constant and nothing can move faster than the speed of light. My question is if the universe is accelerating, eventually should it not reach a speed faster than the speed of light? Has anyone investigated the question as to what will happen if the expansion of the universe breaks the light speed barrier?

The Answer

Thanks for your question. It is true that nothing can go faster than the speed of light. And it is also true that our universe is expanding faster than the speed of light today. This sounds like a contradiction, but actually it is space itself that is expanding faster than the speed of light, driving objects further apart at an increasing rate. The concept of space expanding faster than the speed of light is not in contradiction with the limit for zero mass particles, ultimate speed. A nice discussion of this can also be found at: http://curious.astro.cornell.edu/question.php?number=575

Hope this helps,
Georgia & Mike
For "Ask an Astrophysicist"

Question ID: 070904a

Does time slow down in distant galaxies? (Submitted April 16, 2007)

The Question

Galaxies at the fartherst reaches of the universe are travelling at near the speed of light correct? Does this mean that time within those galaxies is at a slower rate?

The Answer

Yes, from our point of view. From their point of view, time is moving slower in our galaxy. The situation is completely symmetric. We can see this by imagining that there is a clock in each galaxy sending out regular tick signals. We specify that the clocks are constructed exactly the same. We can count the ticks, compare it with our clock, and see how time is changing in the other galaxy.

Well, there are such clocks: atoms of various elements that have definite frequencies at which they emit light. A frequency is just a number of ticks per second, and all atoms of a given kind, say sodium, have the same characteristic frequencies. Looking at a distant galaxy, we can see that their sodium atoms run slow, the light is redder than it should be. They see the same thing, our sodium atoms look redder to them than theirs.

In addition to the time dilation effect, distant galaxies show the Doppler effect of reddening because they are travelling away from us. The time dilation effect exists for any direction of relative motion.

The fact that each observer sees the other's clock running slow appears to be a paradox because of our limited experience with high speed. But that is just the way things are. For more information on the apparent paradox, see

http://imagine.gsfc.nasa.gov/ask_astro/relativity.html#971109a



Jay Cummings and Jeff Livas
for Ask an Astrophysicist

Question ID: 070416a

Is the Universe rotating? With reference to what? (Submitted December 17, 1996)

The Question

I'm not a rocket scientist, but I play one on TV. Here are a couple of questions I've been pondering.

  1. Is the Universe rotating? With reference to what?
  2. Is dark matter necessarily within the bounds of the Universe? In other words, could something spatially disconnected from the Universe be having a gravitational effect on it?
  3. If you drew a circle of radius 1 billion light years, what would be the value of its circumference divided by its diameter (taking into consideration the curvature of space)? Same question but the largest circle that could be drawn in the Universe?
  4. If the speed of light had been slowing since the big bang, would there be any way of knowing it? (Assuming that everything else would have been slowing with it.)
I think these fall into the category, "there's no such thing as a stupid question, just stupid people who don't ask questions." Thanks in advance for any light you can shed.

The Answer

Thank you again for your interesting questions. In addition to our Ask an Astrophysicist staff at Imagine the Universe!, Demos Kazanas in the Astrophysics Science Division also contributed to these answers to your questions.

  1. As far as we know, the Universe is not rotating. The presence of rotation would induce a type of change in the Cosmic Microwave Background temperature which has not been observed. In addition, the presence of rotation would imply that locations along the axis of the rotation were somehow "special", which violates our understanding of relativity that the Universe appears the same regardless of the location of the observer.
  2. Yes, dark matter as we normally discuss it lies within the bounds of the Universe. The effect of gravity is propagated by space-time, which is the fabric of the Universe itself. "Outside" the Universe there is no space-time, hence gravity has no effect. Remember that the best analogy for the "shape" of the Universe is that of a balloon being blown up. There is no "edge" to it. What lies "outside" it is undefined.
  3. It depends on the value of the spatial curvature, which in turn depends on the density of the Universe compared to the critical density to just halt the expansion of the Universe. If the Universe has any curvature (which results from the density being anything other than the critical density), then the circumference is larger than what it would be on a flat plane. The size of the effect does depend on the radius of the circle.

    To get a sense of scale, the farthest object that we can see to is ~ 20 billion light years away. (Sorry we're not giving you a specific numerical answer on this one !)

  4. This one caused us the most thought and reflection. I found a book entitled "Gravitation and Spacetime" by Hans C. Ohanian (1976, Norton & Co.) to be helpful in settling some issues.

    We'll first tackle the question you asked - whether a change in the speed of light could be detected. The answer is, in principle, yes. One way is that the values for atomic transitions depend on the speed of light. We observe these transitions as lines in a spectrum. Hence, if the speed of light has changed, then the values for these transitions from sources far away (and hence which emitted their light long ago) would be different from present day values. The difficulty is that there are many other factors which cause the observed values of these transitions to change. These factors include the Doppler shift due to the expansion of the Universe, local motions of the object, gravitational redshifts, etc. In practice, it would be difficult to disentangle the effect of a changing speed of light from these.

    Another way is to rely on a technique which utilizes the decay rates of various radioactive isotopes. These decay rates are very sensitive to the physical constants - the speed of light among them. If the values of these constants change with time, then nuclear "clocks" based on different isotopes would disagree. (A practical problem is figuring out which of the "constants" changed.) Nonetheless, evidence shows that the ages of rocks measured by these different clocks agree quite well and hence puts tight constraints on the time variations of the physical constants.

    So in principle it can be done, and there is evidence that the physical constants are quite "constant". However, we should take this further and ask whether we should expect the speed of light to change with time.

    The speed of light is not just something we measure about light, but rather (as I've already implied) is a fundamental physical constant itself. Indeed, the constancy of the speed of light in space and time is an essential feature of special relativity. Time enters in because relativity treats time on an equal footing with space (hence the term, space-time). Consistent equations cannot be written if it is taken to be variable. In addition, general relativity and our understanding of the evolution of the Universe is built upon the same premise.

    So my conclusion is that the speed of light is truly constant.

    You've given us a good workout. We hope our answers are helpful.

    Jim Lochner
    for Imagine the Universe!

    Question ID: 961217a

What holds the Universe together? What holds us together? (Submitted January 08, 1997)

The Question

What holds the Universe together? What holds us together?

The Answer

Your very good questions are related to two different forces. The first, what holds the Universe together, is one that astronomers think about often. On large scales like the Universe, the most important force is gravity. Between any two objects the gravitational attraction is proportional to the product of the masses divided by the square of the distance between them. Gravity is the force responsible for keeping the Earth and other planets in our solar system in orbit around the Sun. Gravity also governs the motions of the Sun and nearly all the stars you can see in the sky, which are orbiting about the center of the Milky Way Galaxy. The Milky Way is part of a gravitationally bound collection of galaxies which includes Andromeda, and is called the Local Group. Apart from observing that objects large and small are gravitationally attracted to each other, astronomers also observe that the Universe is expanding: an after-effect of the birth of the Universe in the Big Bang.

Your second question, what holds us together, is closer to biochemistry than astrophysics. Human beings are composed of different types of large molecules: proteins, nucleic acids, lipids, carbohydrates, etc. These molecules are held together by intermolecular forces. An example is the peptide bond that links amino acids together. This bond is formed when atoms of Hydrogen, Oxygen, Carbon and Nitrogen share electrons. Molecular bonding is governed by the electrostatic force, which on small scales is much stronger than the gravitational force for charged particles. We human beings still feel the effects of gravity though: it keeps us from floating off the Earth.

Regards,
Padi Boyd
for Imagine the Universe!

Question ID: 970108b

What is the longevity of light? (Submitted February 16, 1997)

The Question

My questions relate to two matters that have been troubling me and for which I have not seen any comments by any astrophysicist or astronomer. The first question concerns the possibility for light emitted by a body that is 5 billion light years away from the earth, to survive for 5 billion years without being reduced to nothing during such long period of time. In other words, once light leaves its source it is no longer being fed with energy and thus it only dissipates energy through space and time. That being the case, how is it possible for such light to survive not only the distance but also the time. The only explanation that makes any sense to me would be that which would hold that space is curved and that the distances we think we observe are nor real in a physical sense. Rather, they are relativistic.

My second question relates to the theory of the big bang. If the theory is correct, then all of the observable Universe and beyond must be surrounded in a sphere of light that was created at the moment of the big bang. But because such sphere is expanding at the speed of light, we will never be able to observe it unless some of that light is reflected inwards towards the center of the sphere. Hence, if the Universe will eventually contract, the sphere of light will collapse back towards the point of origin and on its way there at the speed of light, it will illuminate (or burn up) everything in its path including our earth. I do not really understand any of this but I would welcome the comments and views of an appropriate qualified individual.

The Answer

As for your first question, No, light does not dissipate its energy as it travels through space. It can only dissipate its energy if it interacts with matter. Light is a form of energy, and does not need to be "replenished" once it is it emitted. This is because light is actually made up of an electric field and a magnetic field which produce and support each other as the light beam travels through space. If you've ever seen an electric generator/motor you know that the coils of wires being spun inside the magnets can produce electricity (i.e. an electric field). Also near power lines or motors compasses will become deflected because of the magnetic field produced by the electricity flowing through the power line or by the motor. It was in the 19th century that James Clerk Maxwell discovered that a changing electric field produces a magnetic field, and that likewise a changing magnetic field produces an electric field. He also discovered that light was comprised of these changing electric and magnetic fields. Hence, a light beam is "self-sustaining".

The only thing that stops this from going on forever is when a light wave interacts with some form of matter (ie. a planet, dust, gas etc...). Its energy is then absorbed by the matter. Sometimes the material may re-emit the light, but usually at a lower energy. Since space is mostly empty the chances of these waves encountering some matter is relatively small. Hence, light can propagate outward for long lengths of time.

Of course, as the waves spread out the intensity (or brightness) does gets weaker. This is because there are fixed number of waves spreading out into a larger area. This is known as "the inverse square law", because at any given point in space the intensity of the light decreases as the inverse square of the distance from the light emitting source. Nonetheless, its energy remains unchanged.

As for your second question, there is a residual effect from the Big Bang, and we can and have observed it. It's the Cosmic Background Radiation, which is observable in infrared wavelengths. Right after the first instant of the Big Bang, the energy was so great and dense that matter was constantly being created and destroyed (as predicted by Einstein's E = mc^2). The Universe was an expanding and cooling "soup" of energetic particles and photons. Around a year after the Big Bang, the "soup" had expanded and cooled enough that the photons in the soup no longer interacted with matter. This left a "gas" of photons that has since expanded and cooled to 3 degrees Kelvin. This radiation permeates all of the Universe. The Cosmic Background Explorer (COBE) measured this radiation to an unparalleled precision. For example, it has found that all but one part in 3000 of this "photon gas" contains energy from the Big Bang (in other words, the photons have essentially not interacted at all with the rest of the Universe since the Big Bang). You can learn more about COBE from http://www.gsfc.nasa.gov/astro/cobe/cobe_home.html. COBE has shown that much of the Big Bang is a good representation of how our Universe began and has ruled out some competing theories.

We hope this helps you understand better these things that have been puzzling you.

Jim Lochner, Andy Ptak, and Mike Arida
for Imagine the Universe!

Question ID: 970216

How can galaxies merge in a expanding universe? (Submitted April 21, 2011)

The Question

I have seen various reports that in approx. 5 billion years time the Andromeda galaxy will collide with the Milky Way.

I would like to know how this is possible in an expanding universe? I thought all galaxies were getting steadily further apart?

The Answer

Thanks for your question. Yes, in about five billion years the Andromeda and Milky Way galaxies will collide, forming an elliptical galaxy, though the collision itself will take a few billion years. In our current time, the expansion of the universe is only detectable on very large scales. Nearby galaxies and galaxy clusters can have peculiar motions far greater than cosmic expansion, given that the distances to them are relatively short in comparison to the total size of the observable universe. In fact, there are about 7000 galaxies with recorded blue-shifts, meaning these galaxies have a peculiar motion toward us. This is still less than 0.01% of catalogued galaxies, the vast majority of which are red-shifted due to the expansion of the universe.

However, the expansion of the universe is accelerating, and if this continues there will be a point in the future when the observable universe will actually begin to shrink. Light emitted from galaxies a certain distance from us will never be able to reach us due to the accelerating expansion of the space in between. So fewer and fewer galaxies will appear to be approaching us, or even observable at all. It's estimated that in about 100 billion years time this "cosmic horizon" will only include our local galaxy group, which by that point will have merged into one giant elliptical galaxy. The local gravitational attraction between galaxies will keep galaxy clusters together in the face of universal expansion, just as gravity keeps galaxies themselves together

Jack Hewitt
for Ask an Astrophysicist

Question ID: 110421a

Why dosen't the space in and between atoms also expand? (Submitted June 19, 2002)

The Question

I have been trying to understand the idea of an expanding Universe. As I understand what I have read, this concept is based on relativity. In this model, the universe should not be seen as simply a sphere with an expanding radius but rather it should be thought of as space itself expanding. If this is so, it would seem that even "local" space would be expanding and therefore the dimensions at the atomic level would be increasing (i.e., if the distance to a distant qalaxy doubles over a few billion years, then atomic dimensions also double, atomic spectra would shift, etc.) I assume that this is not so, but why? (A reference to a good book or paper would be helpful)

The Answer

This is a good question --- a very short answer is that local forces are dominant on the scale of atoms and they, rather than the large-scale structure of space-time, determine the structure of an atom. Even on the scale of galaxies, our own is being drawn toward Virgo cluster of galaxies. The expansion of the universe becomes the dominant factor only on the largest scales.

For a longer explanation, with references, see this page (part of the Usenet physics FAQ):

http://math.ucr.edu/home/baez/physics/Relativity/GR/expanding_universe.html

Hope this helps,

Koji & Georgia
for "Ask an Astrophysicist"

Question ID: 020619a

If everything expands, how can we measure the expansion of the universe? (Submitted August 20, 2009)

The Question

Friedmann identified that Einstein's field theories allow for an expanding universe. I am puzzled by the fact that in a universe expanding under the laws of Einstein's field theories, everything is expanding, not only the distances between the objects of the universe. Thus a yardstick or a metre stick expands along with the rest of the universe.

On these grounds, I must conclude that the metric distances in a metric space are conserved irrespective of any expansion occurring within the frame of Einstein's field theories. How is it possible that the wavelength of the background microwave radiation is construed to having increased on a metric scale when that scale should have expanded along with the expansion of the wavelength, leaving the wavelength the same as it was in the young universe?

The Answer

This is a good question. You have to keep in mind that Einstein' theory is 4 dimensional, not 3. In the Friedmann solution, the space parts (3 dimensions) are expanding (or contracting) uniformly; however, the time component is not. If the time component were also expanding at the same rate, then one could not observe the universe to be expanding.

A good way to see this is through the example of the CMB which you have brought up. As the CMB travels through space, the wavelength is stretched by the expansion; however, the time has not. If the time component also stretched, we would observe the CMB to have smaller intervals between oscillations. In fact, this factor would exactly cancel out the stretching of the space, and we would observe it at its original optical wavelengths.

Dave Chuss
for Ask an Astrophysicist

PS. Note also that physical yardsticks do not actually expand with the expanding universe. See our answer on that question.

Question ID: 090820a

    The Age of the Universe and the Hubble Constant

How do you determine the age of the Universe? (Submitted March 28, 1997)

The Question

Given the latest value of the Hubble constant at 60-65 Km/s/Mpc and assuming a zero cosmological constant how do you actually calculate the age of the Universe ?

The Answer

Here is a short answer to a complicated problem. One can estimate the age of the Universe from the Hubble constant and the "deceleration parameter", which in turn depends on the energy densities in the Universe (usually expressed as Omega and Lambda). The article at

http://www.lpi.usra.edu/newsletters/lpib/lpib77/black77.html

shows the variation in the age of the Universe with H and Omega - see especially the discussion associated with Figure 2.

Jim Lochner & Koji Mukai
for Imagine the Universe!

Question ID: 970328f

What is the Hubble Constant? (Submitted October 19, 1997)

The Question

I've been trying to figure out exactly what the Hubble Constant is for quite a while. I know that it has to do with the expansion rate of the Universe and that it can also directly yield the distance scale and the age of the Universe. Could you possibly explain it to me in fairly simple terms. I'm in 11th grade and I've had about a month's worth of class on astronomy.

The Answer

Early in this century Edwin Hubble discovered that galaxies are moving rapidly away from us (this was the first evidence for the big bang theory for the creation of the Universe). He also discovered that more distant galaxies were moving away at a higher velocity and proposed the following relationship:

v = H*r

where v is the velocity of the galaxy, r is distance and H is Hubble's constant. Finding a more precise value for Hubble's constant is an area of very active research, since many questions of cosmology are tied up with this value. Here are a few URLs that are related to the Hubble Constant:

http://imagine.gsfc.nasa.gov/ask_astro/cosmology.html#970321d

http://imagine.gsfc.nasa.gov/ask_astro/cosmology.html#970326e

I hope this helps,

Jeff Silvis
For Ask an Astrophysicist

Question ID: 971019c

How has the new Hipparcos data affected the Hubble constant and thus the estimated age of the Universe? (Submitted March 21, 1997)

The Question

I read, with interest, the recent reports of the Hipparcos Satellite results. The revised data for stellar distances gives an age of the oldest stars to be about 11 billion years, less than the 15 billion originally given. This partly solves the paradox of the oldest stars being older than the age of the Universe.

However, how does the new stellar distance data affect the value of the Hubble constant (if at all) and how will this alter the estimated age of the Universe. Also will the revised data have any consequence for the value of omega,the critical mass of the Universe.

The Answer

Since I'm not an expert on this subject, I have asked a colleague, Dr. Richard Mushotzky, for input. Here is a brief summary.

There are two Hipparcos results with cosmological implications.

(1) They measured trigonometric parallaxes of main sequence stars with similar chemical compositions and ages to those of Globular Cluster (GC) main sequence stars. With this, the distances to GCs have been revised upwards, therefore GC stars are intrinsically brighter than previously thought, and therefore younger. (The standard tool for measuring the ages of GCs has been the main sequence turnoff --- which is now estimated to be occurring at a brighter absolute magnitude.)

This has no direct implications for H0 (the Hubble constant); see below for the implications on Omega.

(2) They have also measured trigonometric parallaxes of several nearby Cepheids. Most Cepheids are at or beyond the limits of the capabilities of Hipparcos, and there are some complications, but it appears that the Cepheids are brighter than previously thought. This would reduce the estimates of H0 based on Cepheids.

However, this may not be as important as you might think, because there are now a couple of rapidly improving methods of measuring the value of H0 that are completely independent of the Cepheid distance scale (Sunyaev- Zeldovich effect and time delays in Gravitationally lensed systems). They are all converging to about 60-65 km/s/Mpc (the best Cepheid based values tended to be around 70).

One can derive a constraint on Omega by requiring that the Globular clusters be younger than the Universe (you can also play with the "cosmological constant" here, but let's ignore this possibility for the moment). This is because one needs the value of Omega (which is related to the deceleration of the expansion of the Universe) as well as that of the Hubble constant to calculate the age of the Universe. With this method, the Hipparcos discovery on the GC age raises the allowed value of Omega somewhat, to be roughly consistent with the observationally derived values which tend to be around 0.3.

Best wishes

- Koji Mukai
for Imagine the Universe!
with a big help from Dr. Mushotzky

Question ID: 970321d

How does Polaris being a Cepheid variable in a binary help us understand the expansion of the universe? (Submitted April 22, 2006)

The Question

How does the discovery of Polaris being a binary star affect our thoughts on cepheid variable stars, and how will it help us discover more about the size and growth of our universe?

The Answer

Thanks for your question. Polaris is believed to be a special type of Cepheid in which it is pulsating at its first overtone mode with more than one node of pulsation within the star. Since Polaris has companion stars, the minute changes in Polaris' orbit caused by the interaction with the companion stars gives a measure of the mass of Polaris --which can be used independently to better understand the physics of Cepheid behavior of Polaris. You might want to read more at the following website:
http://www.astro.uiuc.edu/~kaler/sow/polaris.html

Understanding Cepheids in better detail will help to better delineate Cepheids as Standard Candles, ie, objects in space that can be used to measure distances and knowing distance scales better will help us to understand how the universe is growing. You can read about the distance ladder at:
http://heasarc.gsfc.nasa.gov/docs/cosmic/

Hope this helps,
Georgia & Mike
For "Ask an Astrophysicist"

Question ID: 060422a

What is the Sunyaev-Zeldovich Effect and can it be used to calculate the Hubble Constant? (Submitted March 26, 1997)

The Question

I've read that the Sunyaev-Zeldovich Effect (SZE) can be used as an independent means to calculate the Hubble Constant. My question is: What is the SZE and what observing programs are either ongoing or planned to measure it and the Hubble Constant? Also, I'd appreciate any references you can point me to for further reading.

The Answer

The Sunyaev-Zeldovich effect refers to the scattering of cosmic microwave background (CMB) photons off of hot gas. It can be used as a cosmological measure because clusters of galaxies have hot gas associated with them, at temperatures of 10^7 K or more. When the CMB photons pass through the hot gas, the photons scatter off of the hot gas and gain energy (this is called inverse-Compton scattering). This distorts the spectrum of the CMB in the directions of clusters. The amount of distortion depends on the size, temperature, and density of the hot cluster gas. We measure the temperature and density of the gas with X-ray observations. Using the X-ray temp., density and the distortion of the CMB we can estimate the size of the gas-emitting region. This size estimate along with the brightness gives an estimate of the distance to the cluster. This is the key ingredient to measure the Hubble constant. This discussion is a little simplified but it gives the basic idea.

Using radio and X-ray observations, the SZE has been used to estimate Hubble's constant. Measurements reported from 1990 to 1995 give values ranging from 32 to 82 km/s/Mpc. Relevant X-ray measurements are continuing to be done using the ASCA and ROSAT satellites.

For a recent review, written at a level for professional astronomers, see "Comptonization of the Cosmic Microwave Background: The Sunyaev- Zeldovich Effect" by Y. Rephaeli in Annual Review of Astronomy and Astrophysics, Vol 33 (1995), p. 541.

We're not aware of any discussion in a popular level magazine, like Scientific American or Sky and Telescope, but you might check their indices.

Jim Lochner & Andy Ptak
for Imagine the Universe!

Question ID: 970326e

Do you have information on the Loh and Spillar test that can be used to determine the Hubble Constant? (Submitted July 11, 1997)

The Question

I recently read a book which gave reference to the Loh and Spillars test which can be used to find a value of the Hubble Constant. Unfortunately, the book did not give many details of this method nor give any findings for this test. Do you happen to know anything about the Loh & Spillar test? Also do you know of any results this test has provided?

The Answer

The method of Loh and Spillar uses the photometric redshifts of nearly 1000 galaxies to determine cosmological parameters. A photometric redshift is measured by observing the flux of a galaxy through different filters, and combining this information to determine the Spectral Energy Distribution (SED) of the galaxy. This is different than the typical method of determining redshifts, which uses the shift of narrow spectral features. The photometric method concentrates on the broad features and overall shape of the spectrum. It is easier to obtain the photometric data than the spectroscopic data for dim galaxies. Using imaging detectors helps even more when multiple galaxies can be measured simultaneously.

The results of their test indicate that the Universe is "flat", one of three possible overall geometries, and the one that results in the Universe decelerating to zero expansion rate as time approaches infinity.

You will find a lot of details about the method at the web page:

https://web.archive.org/web/20080113034654/http://astrowww.phys.uvic.ca/grads/gwyn/pz/index.html

Regards,

Padi Boyd
for the Ask an Astrophysicist Team

Question ID: 970711a

Is the expansion of the Universe slowing down or speeding up? (Submitted February 10, 1999)

The Question

Is the Universe's expansion rate slowing down or speeding up? I have always been taught that it was slowing down, but I read something in the New York Times that it was found about a year ago that it was actually speeding up.

The Answer

Up until a year or two ago, it was thought that the Universe's expansion rate was decreasing, due to gravity pulling back on the material exploding form the Big Bang.

However, more recently, scientists have been able to measure how fast the Universe was expanding, and the data indicate that it is actually speeding up.

This measurement was made by looking at distant supernovae. If distant supernovae are different in brightness than nearby supernovae (e.g. if there is more dust dimming the light than we think) then the measurement could be wrong. However, most astronomers think that the measurements are strong.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

Question ID: 990210c

Is the existence of a Cosmological Constant being considered? (Submitted August 21, 1998)

The Question

I was listening to a few scientists talk on the radio about a force that is causing or assisting the expansion of the universe. I only had a brief moment to catch the radio show before having to go to work. They where saying that an anti gravity is causing the universe to expand. Is any one familiar with these ideas that may be able to point me in the right direction to get more information on this?

The Answer

You ask very good questions, that astronomers, physicists, and cosmologists are debating intently right now. A group of researchers recently published findings that the universe may be expanding faster now than in the distant past. This would imply a non-zero value for the 'Cosmological Constant' (CC).

Einstein's original cosmological model was a static, homogeneous model with spherical geometry. The gravitational effect of matter caused an acceleration in this model which Einstein did not want, since at the time the Universe was not known to be expanding. Thus Einstein introduced a cosmological constant into his equations for General Relativity. This term acts to counteract the gravitational pull of matter, and so it has been described as an anti-gravity effect. see: http://www.astro.ucla.edu/~wright/cosmo_constant.html for a good but technical description of the 'force'.

Until recently people just put a value of zero in for the (CC), now things may be different. We have yet to see what will hold up with newer instruments such as MAP. http://map.gsfc.nasa.gov/ coming online.

The March 1998 issue of Sky & Telescope magazine also has a report on this same item.

I hope this helps,
Mike Arida and Tim Kallman for the Ask an Astrophysicist Team

Question ID: 980821a

Is the Cosmological Constant real or not? [Follow-up to above] (Submitted September 8, 2008)

The Question

(Submitted September 8, 2008)

In simple-minded engineering terms, the Cosmological Constant term is a 'fudge factor' used to make theory match observations. Einstein put it in to make General Relativity match the static universe model then in vogue. When Hubble's results came in, Einstein disavowed it. Now some Cosmologists are saying that the Cosmological Constant is in fact non-zero. If their observations prove true, that would suggest that GR is missing something now being approximated by the CC term. Has this been noted by anyone in the astrophysics field?

The Answer

Yes, you're right, this is a huge deal for physicists precisely because they want to go beyond the mere description (using the cosmological constant) to a deeper understanding of what it is.

The Cosmological Constant (CC) has long been debated, even since the time Einstein added it. He, indeed, added it to keep the Universe static - without the CC, General Relativity (GR) required that the Universe either be contracting or expanding. When Hubble's results came in, the CC dropped out of GR. However, even after that, the CC remained in people's minds - I remember during my undergraduate days, there was still debate about whether the CC was really equal to zero or not.

In even more recent history, it has been discovered that there is another component to the Universe that had not been observed until 1997 - this is dark energy, which you may have heard of. We can only observe the effects of dark energy on the largest scales, which is one reason it took so long to discover. The remarkable thing about dark energy is that it has the property of negative pressure (think of blowing into a balloon and having it deflate with each breath), which is causing the rate of expansion of the Universe to increase.

One proposed way to account for this dark energy in cosmology is the cosmological constant. You can read more about that here:

http://map.gsfc.nasa.gov/universe/uni_accel.html

We hope this helps!
Barbara & Koji
For the Ask an Astrophysicist team

Question ID: 080908a

The accelearted expansion of Space is not a constant. (Submitted September 04, 2001)

The Question

Recent studies of exploding supernovae show that the Universe' expansion is accelerating. Hubble's law uses Hubble's constant (independent from distance) to calculate the distance for remote objects. Aren't both calculations in a contradiction? If the Universe is accelerating then Hubble's constant must not be a constant too.

The Answer

You are correct. If verified, the observation of an accelerating universe means that Hubble's constant is not in fact a constant after all. This is how science works - Hubble came up with a good idea that fit the observed data of the time. But technology and funding improves as time goes by, and more sensitive data has resulted in the idea being modified.

with regards,

Martin Still & Kevin Boyce
for NASA's "Ask an Astrophysicist"

Question ID: 010904a

Is the Hubble Constant really a constant, if the Universe is expanding? (Submitted May 28, 2010)

The Question

I heard that the Hubble constant varies every year because the universe expands. Is this true? If not why, and how can it be a constant if it varies? I am confused, please help.

The Answer

Thanks for the question. If you are on a train cruising between two stations, does the fact that the train is moving imply that its speed is changing? Of course not --- and the Hubble constant is like the speed of a train. The fact that the universe is expanding does not imply the Hubble "constant" must change. If the train accelerates or brakes, on the other hand, its speed will change, but that's a separate question from whether the train is moving or not.

In fact, the expansion of the universe is accelerating, but not so fast that we can detect the changes in the Hubble constant directly. If we wait 1 billion years, its value will have changed enough for us to be able to measure the difference. For a wait of 1 million years, we probably have no chance, with the accuracy of the techniques we use today. So, for all practical purposes, the Hubble constant is indeed a constant.

Best wishes,

Koji & Georgia
for "Ask an Astrophysicist"

Question ID: 100528a

    The Size and the Mass of the Universe

Does the Universe have a dimension or size? (Submitted March 28, 1997)

The Question

It seems that it doesn't make sense to think of the Universe as having either a center or an edge, but does it then make at all sense to think of it as having any kind of dimension or size? Perhaps the Universe contains all sizes and therefore has no sizes. Perhaps I've been reading too much about Richard Gott and this very bizarre de Sitter space.

The Answer

You are right that it doesn't make sense to think of the Universe as having an edge. It is better to think of the Universe as the surface of a balloon, with the stars and galaxies on the that surface.

However, we can still think of the Universe as having size and dimension. Indeed, the Universe does contain all sizes, but it too has a size, just as the balloon has a size. Physicists raise questions about the number of dimensions, and have theories for the Universe consisting of 11 dimensions. This, plus more familiar concepts such as the curvature of space-time, may make it difficult to think about the Universe having a size. But none the less, we consider that it does.

Jim Lochner
for Imagine the Universe!

Question ID: 970328b

Is the observable Universe not all of the Universe? (Submitted March 20, 2007)

The Question

The acceptance of Big Bang theory assumes the universe is ultimately finite and therefore measurable. But given the physical and temporal constraints that govern all our activities, the observable and measurable (empiric) universe may surely only represent a fraction of what exists across space/time.

How do we know where the universe begins and ends? And therefore - how can the concept of an infinite universe (or even multiverse constructs) be ruled out?

The Answer

You are right that the observable universe may be a fraction of the universe. The total energy and total volume of the universe may be infinite (we do not know), but the energy density (energy per unit volume) of the universe is finite, and this is a more important quantity for our understanding. Various observations, such as the observed expansion of the universe, the nature of the cosmic microwave background (CMB), etc. show that the universe started with the Big Bang (at the beginning, the whole space and time was confined to a point, and then it started expanding).You may see,

http://map.gsfc.nasa.gov/m_uni/uni_101matter.html

http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html

http://en.wikipedia.org/wiki/Big_Bang

The multiverse theory is not ruled out, but it is still in the conceptual and mathematical stage.


Hope this helps,

Sudip & Koji
for "Ask an Astrophysicist".

Question ID: 070320a

Did WMAP results prove an infinite universe? (Submitted August 05, 2010)

The Question

I have heard that WMAP proved that the universe is spatially flat with a very high accuracy. And I know from Freidmann models that the flat universe is spatially infinite. So my question is, can we say that WMAP proved the spatial infinity of the universe? And does this contradict with the fact that the universe is temporally finite; it's age is 13.7 billion years old?

The Answer

No. WMAP showed that inflation must have happened, so the part of the Universe that we can see was driven to flatness by the rapid expansion. There could be much more of the Universe out there beyond what we can see, but we cannot know anything about it because it is not causally connected to us (basically it is too far away). The finite age of the Universe is just our measurement of how much time has elapsed since the start of inflation and the Big Bang and has nothing to do with whether or not the Universe is spatially infinite. It does determine how much of the Universe we can see.

Jay and Jeff
for Ask an Astrophysicist

Question ID: 100805b

How do you measure the size of the Universe? (Submitted November 24, 1997)

The Question

I'm a 13 year old student from Denmark, who wants to know how big the Universe is and how the size of it is measured.

The Answer

The simple answer is that the observable Universe is about 10 billion light years in radius. That number is obtained by multiplying how old we think the Universe is by the speed of light. The reasoning there is quite straightforward: we can only see out to that distance from which light can have reached us since the Universe began. (But see my note marked * below).

We determine the age of the Universe in a number of ways. One is to estimate the age of the oldest stars we see. Our knowledge of how stars of a given size evolve with time is very good (based on what we know about atomic and nuclear physics) so the major uncertainty here is usually measuring how far away (and so how big) such stars are. The standard method is to look for very small changes in the apparent positions of the stars as the Earth moves around the Sun. (This effect is called parallax). A second way to get an age for the Universe is to try to figure out the time of the big bang itself. Here the method is to use a series of techniques (based on how bright things appear to be - like Cepheid variable stars - that we think we know the true brightness of) to determine first the distance of the nearby galaxies, then increasingly distant galaxies, until we have estimated distances for many galaxies for which relative velocity measurements have been made (using the Doppler red shift of features in their spectra). The relative velocities we observe for distant galaxies have been largely determined by the expansion of the Universe begun with the 'big bang'. So, once we've determined how expansion velocity correlates with distance for some range of distances, it's possible to extrapolate back (with some assumptions) to calculate the instant of the big bang, when all the matter in the Universe was at a single point.

(If any of these terms like 'parallax', 'Cepheid' and 'red shift' are unfamiliar, try entering them in the search window on our home page).

The determination of greater and greater distances is one of the great themes of astronomy. Most introductory books will give you an outline of the story, which you can then fill in to any level of detail with further reading.

Our website has a lot of material on recent developments. For instance, there are already several answers in the 'Ask an Astrophysicist' archive which deal with the size and age of the Universe. If you enter things like 'size of the Universe', 'age of the Universe', or 'distance scale' in our search window you will get lists of links to many of the most relevant discussions.

Paul Butterworth
for the Ask an Astrophysicist team

* Note: The observable Universe may be only a small part of the physical Universe. In some theories, the Universe may have expanded very fast just after the 'big bang', and only a little bit may have remained within range of detection.

Question ID: 971124x

Is my conclusion about the Cosmic Particle Horizon correct? (Submitted March 28, 1997)

The Question

I understand that the cosmic particle horizon is located about 20 billion light years away, in all directions, from me. Light coming from objects located past the horizon cannot be seen because the light has not had enough time to reach me yet. But doesn't the cosmic particle horizon expand with the expansion of the Universe. Therefore, light that I couldn't see before can be seen in the future. Is this conclusion sound?

The Answer

The cosmic particle horizon is related to the age of the Universe. Its size certainly grows, but with the age of the Universe, not with the expansion of the Universe. As the expansion slows, we certainly would see more in the future than in the past. But, of course, the time scales for this to happen are very large.

Jim Lochner
for Imagine the Universe!

Question ID: 970328c

Can two areas of the Universe be so far away from each other that the light from area A can not reach area B? (Submitted February 12, 1999)

The Question

Can two areas of the Universe be so far away from each other that the light from area A can not reach area B?

The Answer

It is now believed that the Universe is large enough that there can be areas A and B that have not been able to exchange light.

This is a result of recent observations that indicate that the rate of the Universe's expansion is speeding up, contrary to what astronomers were expecting a couple of years ago.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

Question ID: 990212a

How can one estimate the size of the Universe if any part of it past this critical distance is forever cut off from our measurement? (Submitted June 30, 1997)

The Question

I'm a college graduate with a degree in computer science. However, my favorite pastime has always been reading about astronomy, quantum mechanics, etc. that's my background. My question is:

When astronomers speak of the estimated size of the "known Universe", are they setting this distance (from us) based upon the furthest visible object, or upon calculation? This is in reference to the fact that quasars (as far as I know) are the furthest observable objects. Yet they travel at speeds approaching that of light away from us. Obviously, if there was anything further than the distance at which the expansion of the Universe = c, it would be impossible for us to detect it, now or ever. To sum up the question: how can one estimate the size of the Universe if any part of it past this critical distance is forever cut off from our measurement? One could argue that since we cannot ever reach these locations, for us they do not exist, but I think that's a horrible cop-out.

The Answer

What astronomers mean when they speak of the "known Universe" depends on the astronomer. Most often it refers to the region of the Universe from which light could travel to us since shortly after the Big Bang.

The farthest observable discrete objects are the quasars (visible at such great distances because they are so bright). However, the cosmic microwave background radiation, at 3 degrees Kelvin, comes from even further away. It has a redshift of about 1000, and comes from the time when the Universe was much smaller, and filled with hot ionized gas (plasma) at 3000 Kelvin, as hot as the surface of some stars. Dense plasma blocks light, and so we cannot see anything beyond that distance.

If the theory known as "inflation" is true, the size of the "known Universe" is much smaller than that of the Universe as a whole. If you look at the "known Universe", every part of it looks about the same, as far as we can tell. As an analogy, if you look at a typical cornfield in Kansas, it all looks the same as far as the eye can see. For there to be as much variety as you would expect in a world, the world has to be much larger than the size of a Kansas cornfield. Likewise, inflation says that the Universe is much larger than the known Universe.

How much larger is hard to determine, and theories are untrustworthy since we can never confirm them by observations. (Actually, 'never' is a bit of an overstatement. If you waited long enough, the Universe would slow its expansion and you may be able to see a bit further. But that would take billions of years.)

For more information on inflation, look at the references on http://imagine.gsfc.nasa.gov/ask_astro/cosmology.html#970202

David Palmer
for Ask an Astrophysicist

Question ID: 970630c

How do astronomers measure the mass of the cosmos? (Submitted January 14, 1998)

The Question

I am a high school senior. I have little familiarity with astronomy. I recently saw a program on PBS, where they discussed a way of measuring the mass of the entire cosmos. I am wondering how this could possibly be done, and how long it would take. I also wonder what effect knowing the mass will have on science.

The Answer

There are a variety of ways of attempting to measure the mass of the universe. All are based on assumptions which are somewhat analogous to the assumptions used in public opinion polling. That is, we measure the mass in some region of the universe which we think is typical, and then assume (!!) that this can be applied to the universe as a whole. Clearly this is a tricky thing to do, and the answers become believable only when it is done by a variety of people using a variety of techniques which all give consistent answers. For example, we can measure the density of material in the earth's interior, and if we assumed that this were typical of the universe as a whole we would go very wrong. More plausible estimates come from measures of the mass in galaxies (by measuring the speeds of stars and then using Newton's laws to infer the mass), and from measuring the mass in clusters of galaxies (by measuring the temperature and density of hot gas from its X-ray emission and then assuming that it is confined by the gravity of the cluster). You can read more about this at:

http://imagine.gsfc.nasa.gov/science/objects/dark_matter1.html

I hope this helps,

Tim Kallman
for the Ask an Astrophysicist Team

Question ID: 980114d

How do we find the mass of the visible Universe by finding the number of galaxies in the Universe and the mass of an average galaxy? (Submitted November 15, 1997)

The Question

Hi, we were wondering how you would determine the mass of the visible universe by finding

a)the number of galaxies in the universe

and

b)the mass of an average galaxy.

This is a real poser for us; nobody seems to have an answer more accurate than 'a lot.' So could you give us the best estimate we have today of the number of galaxies in the universe? and the average mass of a galaxy? or at least give us a clue as to how to find or determine these things?

The Answer

The Hubble Deep Field is 2.7 arcminutes on a side "with nearly 3000" detected galaxies.

Astrophysical Formulae by Kenneth Lang discusses how galaxy mass is measured, and gives a list of 106 galaxies and their masses. These range from 0.25 to 2080 billion solar masses, with most within a factor of a few of ~10 billion. These are the masses determined by looking at how rapidly they turn, so not all of this mass is 'visible mass', it includes intragalactic dark matter.

HOWEVER, the sample of galaxies that have had their masses measured is a VERY biased sample of relatively nearby galaxies, and the Hubble Deep Field looks largely at young galaxies as they were a long time ago, before a lot of evolution (e.g. galaxies gobbling up other galaxies) occurred.

David Palmer and Jeff Silvis
for Ask an Astrophysicist

Question ID: 971115d

    The Density and the Fate of the Universe

What is the present accepted value of the density of the Universe? (Submitted November 30, 1996)

The Question

What is the present accepted value of Omega,the density of the Universe and does this value include the missing mass recently discovered by astrophysicists ?

The Answer

Many theorists prefer Omega=1.0. Observationally, many research groups are trying to measure the value of Omega; although this is difficult and there are many sources of uncertainties (including the current uncertainty over the value of the Hubble constant), most recent published values fall far short of 1.0. For example, a recent paper by a couple of scientists here at Goddard (Loewenstein & Mushotzky 1996, Astrophysical Journal Letters, vol 471, L83) quotes the plausible range as 0.1-0.4. This particular measurement is based on X-ray observations of clusters of galaxies, which is one of the most powerful techniques available for the study of Omega, since the distribution of X-ray emitting gas in clusters is believed to trace their total gravitational potential (stars, gas, and dark matter), and the clusters are believed to contain a major fraction of the mass of the Universe.

Dark matter, by the way, is the preferred name for what you are calling the missing mass: it is not missing, we can detect its gravitational influences (that's how astrophysicists can detect its presence), it just doesn't shine like the stars do.

[Note added on 2003 March 4: There have been exciting developments in this regard over the last several years. There appears to be a mysterious substance called the dark energy, in addition to normal and dark matters; together, they make up a total Omega of 1.0. See http://imagine.gsfc.nasa.gov/ask_astro/answes/990210c.html,
http://imagine.gsfc.nasa.gov/ask_astro/answes/010104a.html, and
http://map.gsfc.nasa.gov/m_mm/mr_content.html.]

Koji Mukai
with helps from Drs. Chen, Loewenstein and Snowden
for Imagine the Universe!

Question ID: 961130b3

Can you tell me about the end of time? (Submitted June 13, 1997)

The Question

Can you tell me about the end of time?

The Answer

Thanks for your question about the end of time. In order to arrive at an answer, astronomers use their knowledge of gravity together with the Big Bang.

We observe all distant galaxies to be receding from us, and from this we conclude that the Universe is expanding uniformly. In fact, the current picture of the evolution of the cosmos is that since the birth of the Universe in the explosive Big Bang, the Universe has continued to expand.

We also observe that massive objects attract each other through the gravitational force. This force tends to contract matter locally (for example, a gas cloud condenses to form a star). On the large scale you can think of the expansion of the Universe acting to separate galaxies from one another, and the gravitational force acting to attract them toward one another.

The "end of time" depends on just how much mass there is in the Universe. We talk about this in terms of the density of the Universe, and compare densities to the critical density. If the density is greater than the critical density, then eventually gravity will overtake the expansion. The expansion will slow down and eventually reverse, so that the Universe will be contracting. Eventually it will end in a collapse (or a bounce) called the Big Crunch. If the density is less than the critical density then the Universe will continue to expand forever, with the gravitational force never overtaking the expansion. An ongoing area of research is to measure the density of the Universe. Currently, some observations (and some theories) indicate that the density of the Universe is very close to the critical density. In this case the expansion will slow down so that it is approaching zero expansion as time approaches infinity.

If you are curious about this topic, you might want to check out the book "Cosmic Questions: Galactic Halos, Cold Dark Matter, and the End of Time" by Richard Morris (1993).

Regards,

Padi Boyd,
for the Ask an Astrophysicist

Question ID: 970613d

Is there enough matter in the Universe for a "Big Crunch"? (Submitted January 09, 1998)

The Question

I was confronted today at school with a wild possibility: is it true that some astrophysicists have concluded that there is not enough mass in space for a Big Crunch? I know it is possible for this to happen, I'm just curious as to how space can always be expanding, but can never collapse.

The Answer

It may seem like a wild possibility, but current observations say that there is NOT enough matter in the universe to reverse the expansion and head us towards a "Big Crunch". There certainly isn't enough "visible" matter (stars and gas and stuff), but there is evidence that there is matter that we can't see, so called "Dark Matter". See http://imagine.gsfc.nasa.gov/science/objects/dark_matter1.html for more information on this. Current measurements give a value for "Omega" (the density of the universe divided by the density required to halt the expansion) of 10 - 40% (including dark matter). Many theorists still prefer an Omega of one, but if the matter is out there, we haven't seen it yet.

Thanks for your question.

Eric Christian
for Ask an Astrophysicist

Question ID: 980109a

Does the discovery of intergalactic hydrogen imply a Big Crunch? (Submitted May 30, 2000)

The Question

Does the discovery of the intergalactic "hydrogen fog," which now accounts for the missing matter in the universe, mean that the "big crunch" model is now viable or is the universe nevertheless going to expand endlessly?

The Answer

You (or local news broadcasts for that matter) may have gotten confused by the similar terminology. What they have recently announced:

http://oposite.stsci.edu/pubinfo/pr/2000/18/index.html

is the discovery of "missing hydrogen."

To back up a step: the fate of the universe is determined by the density of the universe. In the usual cosmological units, Omega=1.0 is the critical value (Omega>1 implies Big Crunch). We can see the gravitational influence of about Omega~0.4 of matter, but the visible stars and galaxies only amount to Omega~0.05 --- the remainder, Omega~0.35 or so, is called dark matter (they are dark, not missing).

Some of the dark matter is probably plain ordinary matter: gas that never became stars, dead stars, planets, bricks, ... However, the observed ratio of helium (and several other elements) to hydrogen, mostly created in the big bang, is thought to imply that ordinary ("baryonic") matter only amounts to Omega~0.1. The rest is the exotic ("non-baryonic") dark matter.

As the press release says:

"Astronomers believe at least 90 percent of the matter in the universe is hidden in exotic dark form that has not yet been seen directly. But more embarrassing is that, until now, they have not been able to see most of the universe's ordinary, or baryonic, matter (normal protons, electrons, and neutrons)."

In summary, the news story is that scientists have at last directly seen some of the ordinary matter that they had previously detected only through indirect means. It therefore does not change our view of the fate of the universe. Other recent discoveries suggest that the universe will expand forever, however.

Best wishes,

Koji Mukai & Bram Boroson
for "Ask an Astrophysicist"

Question ID: 000530a

Will the Universe keep collapsing and re-expanding? (Submitted January 14, 1998)

The Question

I have seen programs and read books about the Universe. Mainly they speak of it either being constant, never-ending or ever-expanding with the end of the universe happening when it finally collapses upon itself. Could the "Big Bang" be a continuous cycle of collapse and expansion, collapse and expansion....? It goes against everything in me to believe that one day it could end. I don't know how well I have stated my question, but I have always understood that matter is a constant. That paper can be burned and its composition changed, but the atoms are all still present only in a different form. I also understand that change is the only certainty in the universe as we know it, so an end does not seem possible. Has someone who has a respected opinion in this regard discussed it or disregarded this idea?

The Answer

I think this is a question that has occurred to many people contemplating the Big Bang. Historically, cosmologists have fallen into 2 fairly clear categories: those who are very reluctant to contemplate a 'one-shot' universe (we might call them recyclers), and those who are not. Right now there is a preponderance of evidence that the universe will not collapse, based on measurements of the rate of expansion and the mass density. I agree with you that this is somewhat less appealing than recycling. However, I also am interested in how science often challenges my preconceived ideas, and so I am personally interested in keeping my mind open on this subject. Die-hard recyclers will undoubtedly be able to suggest ways of bringing about collapse. You can read more about the Big Bang on our site, for example at:

http://imagine.gsfc.nasa.gov/science/questions/age.html

I hope this helps,

Tim Kallman
for the Ask an Astrophysicist Team

Question ID: 980114c

Is a vacuum causing the acceleration of the universe edge? (Submitted September 02, 2001)

The Question

It has been said that because our universe creates its own "space and time" it is expanding into pure nothing. Is there a possibility that this "nothings"' main attribute is that of a perfect vacuum pulling the universe apart like a balloon inside a bell jar when the air is removed?

The Answer

Interesting idea, which may help explain why the universe appears to be expanding at an ever increasing rate. If I can extend your logic - an infinitely dense point of matter appears in an otherwise perfect vacuum state. An event of some sort causes that point to begin to expand very rapidly, overcoming whatever initial gravitation pull would keep the point of mass together. As the new universe continues to expand, there is less and less gravitational pull to bring all of this mass back to its origin. If the pulling force outward is constant and the gravitational pull continues to decrease, the expansion rate will continue to increase. This is a valid line of thought, however, let's say one inserts a puff of gas into an evacuated bell jar. The gas will quickly expand, but the more volume it fills, the slower the expansion rate (at least in terms of the radius of the expansion; maybe the change in volume per unit time is constant or increases?).

Apart from that potentially damaging argument, there is the issue of the definition of "universe." Or universe, by one definition, is everything. There is nothing beyond or outside of it, not even the empty space-time we can conceive of as perfect space, so there would be no vacuum into which the universe could expand. This may seem a bit of a paradox, as we can always imagine something outside of our house or our solar system, but then it really becomes a question of philosophy as much as science.

Thanks for the question, and keep reading and thinking,
Scott & Laura
for Ask an Astrophysicist

Question ID: 010902a

    Matter and Anti-Matter in the Universe

What is the known percentage of hydrogen in the Universe and where is it? (Submitted November 13, 1997)

The Question

I know that hydrogen is the most common element in the Universe. Can I know from you the known percentage of hydrogen in the Universe? And where it is precisely?

The Answer

When the Universe was formed in the Big Bang, the resulting elemental matter was about three quarters hydrogen, one quarter helium, and a few parts-per-billion of lithium (by weight). http://imagine.gsfc.nasa.gov/science/questions/composition.html

(When I say 'elemental matter', I am referring to matter made of the common chemical elements we see around us. However, one of the great mysteries of astrophysics is, we don't know what most of the Universe is made of. Between 90% and 99% of the mass of the Universe seems to be completely unknown. This 'dark matter' has so far been detected only gravitationally: galaxies and clusters of galaxies seem to be heavier (or at least have more gravitational attraction) than can be explained by summing up all the stars and gas clouds we see.

http://imagine.gsfc.nasa.gov/science/objects/dark_matter2.html

Some of this matter coalesced into galaxies and stars, although much remained as gas. As time went on, stars burned some of their hydrogen to heavier elements. These stars occasionally released their material (as winds, in supernova explosions, and in other events) so that it combined with the remaining gas, and formed new stars, and so on. All material on Earth except for the hydrogen (such as that in water), helium, and lithium is this burned material. Our Sun, and most of the stars you can see in the night sky, are later-generation stars, and we can see the material burned by earlier stars in their atmospheres. http://imagine.gsfc.nasa.gov/ask_astro/stars.html#961112a

Only a few percent of the original hydrogen and helium in the Universe has been burned this way. Most of it is still around, and so the elemental matter of the Universe is still about three quarters hydrogen, which is primarily in the form of clouds of gas and stars.

David Palmer
for Ask an Astrophysicist

Question ID: 971113i

If the Big Bang was pure energy, what made the matter? (Submitted July 10, 2009)

The Question

This is a simple queston. everything is made in stars from mostly hydrogen and helium becuase of gravity. but then where did the hydrogen come from? Please do not say the big bang did it that is what the teacher says and then that everything comes from nothing. that is silly. only nothing comes from nothing. and I am here and I'm not nothing

The Answer

We hate to tell you this, but your teacher is pretty close. All the hydrogen in the Universe was created in the Big Bang. But we wouldn't say that it came from "nothing." Right after the Big Bang (which wasn't really an explosion, but rather the start of the expansion of the Universe), the Universe was in a very hot, dense state. In fact, it was entirely dominated by radiation (no matter!). Now, you may have heard that matter can be converted into energy, but the reverse is also true.

Energy can be converted into matter. In the early Universe (we're talking the very first second!), this is exactly what happened. The radiation was converted into the first protons (hydrogen). A few minutes later, there was a short period where some of the hydrogen fused into helium and a teensy bit of lithium. The very first stars formed from this material. The fusion that takes place in their own cores gave us all the heavier elements that we see today.

Now, where the radiation that was converted into the hydrogen came from and what came "before" the Big Bang, these are questions astronomers don't know the answer to right now and may never know.

For more check out these links:
http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang
http://www.astro.ucla.edu/~wright/cosmology_faq.html

Amy C. Fredericks and Michael Loewenstein
for Ask an Astrophysicist

Question ID: 090710

How serious is the hypothesis about anti-stars? (Submitted May 16, 1997)

The Question

How serious is the hypothesis about anti-stars? (Are there some experiments, theories, etc.?)

I study physics and a year ago I did a kind of homework concerning CAPRICE experiment (balloon flight) who's aim is to measure the flux of positrons, antiprotons and possibly, search for lighter anti-nuclei. As far as I understood, one of the reasons for this kind of experiments is the fact that previous data of the flux of antiparticles showed higher flux than predicted from several models. There was a hint, that antistars may exist which emit anti-protons and positrons as Sun emits solar wind. Well, when I mentioned this to my professor, he smiled and acted as it was an astrology I was talking about, not astrophysics.

Are the results of CAPRICE experiments already known?

The Answer

We don't know for sure that we live in a matter Universe, only that individual superclusters of galaxies are each made of either matter or antimatter. (Otherwise anti-matter atoms in the gas that pervades a supercluster would interact with the matter parts of it, or vice-versa, giving an easily identifiable energy emission).

We may live in a matter supercluster within a segregated matter-antimatter Universe. (Most people don't think that it's likely, but we don't have enough data to rule it out. That's why people are still looking.) If we find any anti-atoms, they must come from anti-stars from a antimatter supercluster a long way away.

Your can find information and results from CAPRICE on a number of web sites:

http://ida1.physik.uni-siegen.de/caprice.html
- has references to papers

http://ida1.physik.uni-siegen.de/caprice2.html
- describes the next generation CAPRICE

David Palmer, Jim Lochner, and Karen Smale
for the Ask an Astrophysicist

Question ID: 970516c

Did the Big Bang also create anti-matter galaxies? (Submitted March 05, 1998)

The Question

I read in a text somewhere that a theory exists that at the moment of the Big Bang, there was a powerful expulsion of matter and antimatter in opposite directions at enormous speeds. Doesn't this mean that, also in theory, there are antimatter galaxies somewhere? How well respected is this theory in the scientific community, and what do you personally think of it?

The Answer

You are correct in that we do believe that equal amounts of matter and anti-matter were created in the big bang. However today we see no strong evidence for anti-stars or anti-galaxies. When matter and anti-matter meet they turn into energy and we know what range that energy this energy should be seen. Although some anti-matter events are seen, they are not enough to assume that half the cosmos is anti-matter. The amount of anti-matter observed can be explained by processes that have occurred since the big bang.

So where is the anti-matter? There is no reason to think that they could have/would have separated at the time of the Big Bang, like you suggested. One theory states that anti-matter decays slightly faster than matter. Before the matter and anti-matter had a chance to recombine, some of the anti-matter decayed. So when they recombined, there was some matter left over which formed our universe.

Hope this helps,

Jeff Silvis
For Ask an Astrophysicist

Question ID: 980305a

If the Big Bang made equal amounts of matter and antimatter, where is all the antimatter? (Submitted March 03, 1999)

The Question

My question is, since the amount of matter and anti-matter is equal, why there is a such big different in the ratio of these two things?

The Answer

You are correct in that we do believe that equal amounts of matter and anti-matter were created in the big bang. However today we see no strong evidence for anti-stars or anti-galaxies. When matter and anti-matter meet they turn into energy, and we know what range of energy this energy should be seen. Although some anti-matter events are seen, they are not enough to assume that half the cosmos is anti-matter. The amount of anti-matter observed can be explained by processes that have occurred since the big bang.

So where is the anti-matter? There is no reason to think that they could have/would have separated at the time of the Big Bang, like you suggested. One theory states that anti-matter decays slightly faster than matter. Before the matter and anti-matter had a chance to recombine, some of the anti-matter decayed. So when they recombined, there was some matter left over which formed our universe.

Hope this helps,

Jeff Silvis, Allie Hajian and John Cannizzo
for Ask an Astrophysicist

Question ID: 990303a

    The Cosmic Microwave Background

Can light be contained and how do we observe light from the Big Bang? (Submitted February 20, 1997)

The Question

In light of your answer regarding light life, would it be theoretically possible to construct a container that would contain and preserve light energy for use later? As a follow-up to the question regarding the big bang, if the energy that was emitted in the form of light at the time of the big bang has a direction away from the center, how is it possible to observe it? Are we observing the cosmic energy that it leaves behind as it passes each point in space or is that energy emitted by the expanding sphere in a direction opposite to its expansion?

The Answer

Yes, it would be theoretically possible to construct a container to preserve light energy. It would have to be made of a material that was perfectly reflecting so that when the light hit the walls, it would bounce off without losing any energy.

In reference to the Cosmic Microwave Background (CMB), when we called it a gas, it was not just an analogy. The air in a room has no "direction" or "source", and the same is true for the photons that make up CMB. The cooling of the CMB since the Big Bang is determined from essentially the same equations that you would use to figure out how much any other gas cools when it expands.

No, the cosmic background should not be thought of as emanating from a single central location in space. Remember to think of it as "gas" of photons that has no "direction" or "source", just as the air in a room. Hence, the cosmic background permeates the entire Universe uniformly In essence, the radiation expanded outward with the Universe as the Universe expanded. The background radiation observable to infrared detectors by looking at any empty region of space (i.e. a region without a star or a galaxy).

Jim Lochner
for Imagine the Universe!

Question ID: 970220a

Why is the CMB seen only in the microwave range? (Submitted May 01, 2009)

The Question

What causes the CMB to vibrate exclusively in the microwave frequency range? Why not some other frequency range, or at a variety of frequencies? What does this tell us about the nature of space, or the conditions at the time of the big bang, or both?

The Answer

The CMB follows a blackbody, or Planck spectrum, which is uniquely determined by the temperature of the emitting object - in this case the Universe . The temperature we see today is 2.7 kelvin, and this reflects how much the Universe has expanded since the time when the Universe became transparent to radiation. The universe became transparent when atoms could form, about half a million years after the Big Bang, at a temperature of about 3,000 degrees. The ratio of the temperatures then and now (3,000 degrees to ~ 3 degrees) is the ratio of the size of the universe then to the size now - a factor of 1,000.

This is also the ratio of the wavelength of light scattered then and now.

There is a huge body of literature that explains the CMB in great detail, including websites for COBE and WMAP, two satellites that made many measurements of the CMB and the uniformity of the CMB.

Here is one place to get started:
http://imagine.gsfc.nasa.gov/features/satellites/archive/map_foggy.html

Jay and Jeff
for Ask an Astrophysicist

Question ID: 090501a

Does the CMB imply that the universe is not infinite? (Submitted October 01, 2003)

The Question

I have read and heard in many discussions about the CMB that it firsts started out as a Gamma Ray radiation and eventually should pass all the electromagnetic spectrum down to the Radio Wave portion as the Universe continue it's accelerated expansion. My question is: Was there a time in the Universe where it was bathed in Visible Light? Is this an era of the Olbers' Paradox? Can we measure the precise age of the Universe using the "rate of change" of this background radiations as it evolves with the expansion of the Universe? And lastly, The presence of CMB at the temperature of about 3.0 Kelvin, doesn't it imply a "bounded" Universe. The Universe is not infinite?

The Answer

First, the presence of the CMB does not mean that universe is necessarily bounded, in fact current measurements in its anisotropy show that the universe is unbounded now.

You are correct, at a certain period in the past the CMB was at visible wavelengths, and before that was at gamma-ray temperatures, though at that point it wasn't background radiation as the mater and energy were strongly interacting.

The CMB takes its origin around 380,000 years after the big bang when matter and energy decoupled. (energy became free to travel through matter and not get constantly absorbed and reemitted) When it first became free to travel, the CMB was at ~4000K or in the orange band of the spectrum. If anyone with sensory organs sensitive in the visible band was around to see, the night sky would have glowed faintly in orange.

More details can be found at:
http://isaac.exploratorium.edu/~pauld/activities/astronomy/expandinguniverselecture.html
http://map.gsfc.nasa.gov/

Hope this helps,
Michael Arida for Ask an Astrophysicist

Question ID: 031001b

How did COBE measure the temperature of the Cosmic Background? (Submitted July 06, 1996)

The Question

Data collected by COBE (Cosmic Background Explorer) is very useful. Can you tell me how was it used to calculate the temperature of the cosmic background? Where can I find more information about COBE's explorations? (and what about data collected by COBE?)

The Answer

(This answer was supplied to us by Dr. Al Kogut, of the Infrared Astrophysics Branch in the Laboratory for Astronomy and Solar Physics at NASA-Goddard)

COBE measured the temperature of the cosmic microwave background using one of the three instruments on board, the Far Infrared Absolute Spectrophotometer (FIRAS). FIRAS measures the frequency spectrum (change in intensity with respect to observing frequency) of radiation in deep space, and compares this spectrum to observations of an on-board blackbody target whose temperature is well known. There are three ways to determine the absolute temperature:

  1. Vary the target temperature until the target produces the same spectrum as the sky. Read off the temperature from thermometers buried inside of the target.
  2. Observe the frequency at which the cosmic microwave background (CMB) is brightest. The CMB follows a Planck spectrum, for which there is a well-known relation between frequency and intensity. By measuring the peak frequency, the "color temperature" can be calculated.
  3. Observe the Doppler shift induced by the motion of the Earth about the Sun. This creates a characteristic dipole pattern on the sky, whose spectrum (the derivative of a Planck spectrum with respect to frequency) depends on the temperature of the CMB. FIRAS measured this dipole and was thus able to calculate the CMB temperature.
The final value for the CMB temperature (T = 2.728 +/- 0.004 K) is a weighted average of the 3 methods. Further information on COBE and the cosmic microwave background is available on the COBE home page,

http://www.gsfc.nasa.gov/astro/cobe/cobe_home.html

COBE data are publicly available via anonymous ftp from the National Space Science Data Center (NSSDC), or check their web page at http://nssdc.gsfc.nasa.gov/

Question ID: 960706

What does it mean for the universe to become transparent? (Submitted April 04, 2000)

The Question

I am an adult who recently visited the Hayden Planetarium at the Museum of Natural History and was totally blown away by what I learned. In trying to learn even more about the universe, I've picked up several books but am stuck on the theory of recombination and how it relates to transparency. I just don't get what it means that the universe became transparent to background radiation at the time of recombination (300,000 years into the universes lifespan). I think I understand the idea of recombination well enough, but what does it mean that at that point the universe became transparent to the radiation? Does it mean that the universe separated from the radiation and began to form the beginning of galaxies? Or is the word transparency used in the more generally understood meaning; that at the point of recombination the matter could seen through.

Your time on this is much appreciated. I am simply fascinated by this and want to gain a better understanding of what's 'out there' and why it's there.

The Answer

You're asking a very good question. What "transparent" means is that after the recombination period the photons can actually "escape" because they're not constantly colliding and being re-emitted. "Recombination" is a sort of absolute limit for us to see.

I assume you know that looking away from the Earth is the same as looking back in time. For example, the Sun is at 8 light minutes from us, so when we look at the Sun, we are seeing it the way it was 8 minutes ago. Now suppose that we could built an incredible powerful telescope to peer at objects very, very, very far. The "recombination" limit tells us that even if we could do such thing, we will never see beyond that time (because before no photon could "escape").

I hope this answers your question.

Ilana Harrus for

the "Ask an Astrophysicist" team.

Question ID: 000404a

Why isn't the universe opaque after reionization? [Follow-up to above] (Submitted October 29, 2004)

The Question

Before recombination at 380,000 yrs ABB the universe was opaque to radiation due to Compton scattering.

Then why isn't it opaque today afer the reionization that occurrred between 500 million and 1 billion years ago.?

I don't believe it is because there is not enough hydrogen ions and free electrons to do the Compton scattering, because when the Hydrogen was a neutron before reionization it caused the Lyman alpha forest.

But I know that the IGM is not opaque because we can clearly see distant quasars and galaxies as in the Hubbble Deep Field. Hence my confusion.

The Answer

This is a very interesting and important question (and in researching it I certainly learned a lot more about reionization and cosmology than I knew before). First of all, a brief history of the universe up to the time of reionization (restating some of the points you make, but in chronological order). In the earliest universe, up to around 300,000 years after the Big Bang (or z~1000), the universe was completely ionized, filled with a hot (T > 104 K) and very dense plasma. Any light emitted was immediately scattered by a free electron. At the time of recombination, neutral hydrogen atoms formed and the universe became more transparent. However, neutral hydrogen does absorb light at particular wavelengths, most prominently at the Lyman alpha transition of 121.6 nm. Since this wavelength is redshifted by the expansion of the universe, the absorption feature is spread out over a wide part of the electromagnetic spectrum to form the Lyman alpha forest as seen at
http://antwrp.gsfc.nasa.gov/apod/ap030126.html
and explained in
http://astron.berkeley.edu/~jcohn/lya.html
. Then in the epoch starting between 0.5 and 1 billion years after the Big Bang (z~6), the neutral hydrogen was reionized probably by quasars and hot stars. It is only after this time that the universe has been truly transparent. You can see an excellent graphic of this process at
http://www.astro.caltech.edu/~george/reion/
and links therein. The epoch of reionization is of great interest currently in astronomy with many questions including when and how it all happened. See for example
http://www.ast.cam.ac.uk/~rtnigm/reion/RTNreionization.html.

Now on to your question. The primary reason that the reionized universe is transparent is that because of the expansion of the universe the intergalactic medium (IGM) is much less dense on average than the plasma before recombination. There are some localized regions of dense ionized hydrogen (or H-II regions) such as the Orion Nebula and in galaxy clusters where temperatures are very high (T > 108 K) and X rays are emitted. Everywhere else the IGM is cold and extremely tenuous, so there just aren't very many electrons to scatter light from distant galaxies and quasars.

Cheers,

Hans Krimm and Jay Norris
for "Ask an Astrophysicist"

Question ID: 041029a

Shouldn't the CMB be uniform? (Submitted February 14, 2013)

The Question

In the early universe, during inflation, would not all matter/energy be uniformly distributed and uniformly expanding? If not, why not and at what point during inflation would it no longer be uniform? Would temperature be uniform throughout the early expanding universe? I ask all this to understand how matter would break apart in the first place and form separate stars or objects. I suppose all things being equal, why didn't everything either clump together in one big mass or its opposite, just continue spreading out so that all space with matter be uniform in density & temperature.

The Answer

Excellent question! In the most simple description of the Big Bang and the creation of the universe, you would expect that quantities like temperature, density, etc. would be constant at all points in the universe at a given time. However, if that were really the case, then the universe would be a perfectly smooth distribution of matter and we wouldn't have galaxies, stars, planets, or astronomers to ask such questions. What we think happened is that the early universe had some small pertubations in density and temperature as a result of quantum mechanics. Quantum mechanics treats physics probabilistically and places limits on things like how uniform a distribution of temperature can be. As the universe expanded, these initially tiny fluctuations gave rise to the structure we see today.

What is really fascinating is that we can see the imprint of these fluctuations on the cosmic microwave background (CMB), a signal that fills our universe and is left over from the time that the universe first became transparent to light, around three hundred and eighty thousand years after the big bang. Detailed maps of the CMB by NASA and others provided strong experimental support of the Big Bang model and have allowed us to better understand the early universe. For more, check out this link

http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html

Hope that helps,
-Ira & Bernard
for "Ask an Astrophysicist"

Question ID: 130214a

Can we map the Universe before inflation? (Submitted December 22, 2010)

The Question

If the universe is symmetrical, is it possible to use the WMAP spacecraft to map the universe before or after inflation?

The Answer

To be clear, it is only the observable universe that is seen to be roughly symmetrical, and the Universe as a whole (which is beyond the observable realm) may not be symmetric in its own right. WMAP can observe a region of the Universe as it was roughly 400,000 years after the Big Bang, which is also thought to be after the inflation era. The WMAP results can therefore give us the conditions of the Universe at 400,000 years old and we can use other observations (e.g., observations of distant galaxies and large-scale structure) to form a theoretical mapping of how the Universe has evolved from the Big Bang into its present state. We recommend viewing http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation, which has some nice descriptions of the cosmic microwave background and its relation to the evolution of the Universe.

Bret & Antara
for Ask an Astrophysicist

Question ID: 101222a

    Stars and Galaxies - their Formation and Clustering

What is the structure of the Universe? (Submitted June 14, 1997)

The Question

What is the structure of the Universe?

The Answer

The structure of the local Universe has been mapped recently by Margaret Geller and her colleagues. Their results show that matter is distributed with large clusters of galaxies describing nearly flat sheets, with voids in between, almost like water is distributed in soap bubbles. One result of their work is a "portrait" of the structure of the local Universe referred to as the "stick man".

Many astronomy textbooks include a diagram of it, for example Astronomy Today (Eric Chaisson and Steve McMillan, Prentice Hall). The "stick man" is only clearly evident in the first slice of the survey (Figure 26.23 of the book). Other textbooks usually include the image in a chapter on galaxies.

Regards,

Padi Boyd
for the Ask an Astrophysicist

Question ID: 970614b

Were there seeds for galaxy formation before atoms formed? (Submitted November 07, 2007)

The Question

According to WMAP results, galaxies were seeded in the very early universe. Is there no problem in explaining how these seeds could survive the universal maelstrom before matter became atomic?

The Answer

The most likely seeds of galaxies are density fluctuations, which are a fundamental property of the quantum mechanics that we believe, at least to a good approximation, describe the Universe. The early Universe was hot enough to ionize atoms, but not hot enough to get rid of these fluctuations.

The bigger problem is actually how to make these fluctuations grow large enough and fast enough to explain the large scale structure that we see today. Most theories use the idea of mergers to grow things quickly - the idea is that relatively small regions of matter self-gravitate into clumps, and then these clumps aggregate into bigger clumps, and so on.

Jay and Jeff
for Ask an Astrophysicist

Question ID: 071107a

What is the Great Attractor? (Submitted September 24, 1999)

The Question

The great attractor has been described as an agglomeration of matter. What would it look like, a galaxy? What are its proportions and what are its effects on the Milky Way and other local galaxies.

The Answer

The Great Attractor is far bigger than a galaxy. In the terminology of astronomers, there are clusters of galaxies containing maybe hundreds of galaxies, and superclusters containing many clusters. The Great Attractor is a supercluster, or something even bigger (the terminology becomes a bit fuzzy when it comes to the largest scale structures in the universe!).

The gravity of the Great Attractor has been pulling the Milky Way in its direction --- the motion of local galaxies indicated there was something massive out there that are pulling the Milky Way, the Andromeda Galaxy, and other nearby galaxies towards it. For a while, nobody could see what it was, because it lies behind the plane of our Galaxy --- that means the gas and dust in our Galaxy obscures the light from the Great Attractor, and it is outshone by the stars and other objects in our Galaxy.

X-ray observations with the ROSAT satellite then revealed that Abell 3627, a previously known cluster of galaxies, was much more massive than originally suspected, containing many more galaxies. Optical astronomers had missed a great number of galaxies, because of the obscuration, but with hindsight (and with better observations), could spot many more galaxies. It is now thought that the Great Attractor is probably a supercluster, with Abell 3627 near its center.

There is an optical image of Abell 3627 at:

http://antwrp.gsfc.nasa.gov/apod/ap960218.html

Hope this helps.

Koji Mukai, Rich Mushotzky & Maggie Masetti
for Ask an Astrophysicist

Question ID: 990924a

Which formed first in the universe - galaxies or stars? (Submitted January 08, 1998)

The Question

Which came first in the formation of the universe as we see it today? Stars or galaxies? I've heard that many books have discussed this topic, but many say that it could have been either way.

The Answer

You've asked a very interesting question that I'm afraid I can't give you a good answer to. This is a sort of chicken-and-egg question that has not been scientifically resolved yet. People who work on the dynamics of galaxy formation say that the galaxy-mass gas clouds should form first. But in the "earliest" galaxies we see, the gas contains heavy elements that seem to have come from a first generation of stars. The big bang (by current theory) should have produced only hydrogen, helium, and a trace amount of lithium. So where did these first generation stars form? We don't have the answer, but it's one of the things that astrophysicists are trying to resolve.

Thanks for your question.

Eric Christian
for Ask an Astrophysicist

Question ID: 980108a

What came first, black holes or galaxies? (Submitted September 25, 2013)

The Question

In the time frame of 500 million to 750 million years after it all started, I mean the expansion, is there any evidence of what came first, black holes or galaxies?

The Answer

This is a question that is still unanswered. There is strong evidence that the central black holes influence galaxy evolution by pulling in gas and energizing the local environments, but it is not clear whether the black holes formed first from the primordial gas clouds, or whether they came only after the stars were formed. In any case, there is apparently the need for some sort of "seed" to start the formation of a galaxy, perhaps dark matter.

Cheers,

Hans Krimm
for Ask an Astrophysicist

Question ID: 130925a

Did bigger stars in the early universe make more black holes? (Submitted February 16, 2011)

The Question

When the Universe had cooled after the Big Bang to the point where the first stars could form and given the sheer plentude of star forming materials that would have been available then, how massive would the first stars have been (compared to recently formed new massive stars) and what would the state today of the black holes they formed when they nova'ed ~12-13 billion years ago? How many black holes would have been formed in the first 100 million years after the first star formation?

The Answer

Thanks for your question. One key aspect to the early Universe is that the gas available to make stars did not have metals (which, to astronomers, mean every element except hydrogen and helium), since metals came later as a result of nuclear fusion in the stars themselves. This effectively made cooling of gas slow and is thought to have lead to a comparitively larger proportion of massive stars; some of which exceded several hundred times the mass of the sun. By comparison, we are only aware of stars reaching ~150 times the mass of the sun in the nearby Universe. The question about how many black holes would have formed is of significant debate and currently a topic of theoretical astronomy. It is likely, however, that in the early Universe, when the size of the Universe was small compared to today, many of these first black holes merged to give rise to increasingly more massive black holes. These massive black holes could then quickly "sink" to the centers of what would become the galaxies. This is a leading theory for how the supermassive black holes, commonly found at the centers of galaxies, formed over cosmic history.

(see, e.g., http://en.wikipedia.org/wiki/Supermassive_black_hole).

Bret & Antara
for Ask an Astrophysicist

Question ID: 110216a

    Advanced Cosmology Questions

Can you explain Olbers' Paradox? (Submitted August 15, 1998)

The Question

I'm a middle school geography teacher with no formal expertise in, but a lifelong fascination with, astronomy and space in general. I seem to remember from a long ago college astronomy course a discussion of Oblers' Paradox that explains why we don't have perpetual daylight despite the billions of bright stars that presumably send their light to all parts of the earth. In trying to explain this concept to my eight-year old daughter, I get tongue-tied by all the technical jargon involved. Can you help me put my explanation in layman's terms?

The Answer

In an infinite universe, which has existed forever, we shouldn't have night. Imagine a universe divided into shells, with stars of a single brightness distributed evenly --- if you look at a shell twice as far, each star is only a quarter as bright, but there are four times as many stars, so each shell is equally bright. If you have an infinite number of shells, you end up with infinite brightness!

The big bang cosmology solves this, mainly by the implied age of the universe. We only see light emitted within the last 12 billion years (or whatever the age of the universe might be). This is a long time, but certainly not infinite, and not enough to make the night sky bright.

Koji Mukai & Maggie Masetti
for Ask an Astrophysicist


Question ID: 980815b

Will a closed universe violate the 2nd law of thermodynamics? (Submitted December 23, 1996)

The Question

Correct me if I'm wrong, but the 2nd law of thermodynamics states that Entropy i.e. the disorder in the Universe, must increase with time. Does this mean that a closed Universe will violate the 2nd law of thermodynamics ? Or, would a better definition of entropy be - Increase in the homogeneity of the energy/mass distribution of the Universe.

If this alternative definition is adopted then a closed Universe would not violate the 2nd law of thermodynamics.

I could of course be entirely wrong , but I would be interested in your comments.

The Answer

I asked Demos Kazanas, a theorist the Astrophysics Science Division, to take a look at your question. I attach his response.

Paul Butterworth
Imagine the Universe!


The issue of entropy in gravitating systems is indeed one issue which has not yet been resolved satisfactorily. Let me see if I get your train of thought: you think that a closed Universe, by requiring all matter in it to revert to a state of contraction would violate the second law because we learn from gases that they generally tend to expand if given the volume? If this is indeed your argument, it is not totally correct, as it is based on the notions of entropy developed from the study of non-interacting gases. Once interactions become important, then while entropy may still represent disorder, that notion of disorder is different from that obtained by looking at a non-interacting gas. You can think of the intricate snowflake patterns as such an example. Their formation represents a higher entropy situation for the given conditions of temperature and density and yet they are not disordered at all. The difference comes about because of the intermolecular interactions.

Gravity acts in a similar fashion. The difference is that these interactions are long range and affect the evolution of the gas at all times and not only when the temperature becomes small enough. So the recontraction of the Universe does not decrease its entropy; it takes place precisely because of the action of the gravitational force. In fact the evolution of the Universe is totally adiabatic, as far as the matter content is concerned (if one assumes that it remains homogeneous and isotropic) since the gravitational field does work on the gas to contract it and heat it up . Now, Penrose has argued that that as far as the gravitational field is concerned, homogeneity and isotropy are very unlikely conditions. The gravitational field wants matter to be clumped and therefore, as you have guessed, as the Universe progresses (whether it recontracts or not) it should become increasingly inhomogeneous (which is of course observed). In the recontracting phase these inhomogeneities get greatly amplified and would push the entire matter of the Universe into a large number of black holes, i.e. the collapse will be very inhomogeneous. This inhomogeneity provides a great increase in gravitational entropy, much more than that one gets by the (possible) decrease due to the compression of the gas. So the second law remains valid. I hope this answers your question.

Question ID: 961223a

Will entropy increase with time as the Universe evolves? (Submitted February 10, 1999)

The Question

I have been pondering this question now for an inordinate amount of time. The second law of thermodynamics states, if I can remember correctly, that entropy will increase with time, where entropy = the amount of disorder in a system. With increasing disorder, there is inherently less energy that can be used to do useful work. With this inherent lack of useful energy, is it then feasible that in some point in time, the universe will reach a state of thermal equilibrium, where there is nothing more than a collection of protons evenly spaced apart and all moving at the same speed?

The Answer

Basically yes. That state is called 'the heat death of the Universe'. (The protons will all have decayed, but the Universe will consist of smaller particles all drifting at random and getting more and more distant from each other as the Universe expands).

See http://xxx.lanl.gov/abs/astro-ph/9902189 for a more recent view of this.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

Question ID: 990210a

Could we compare the early Universe to a black hole? (Submitted April 08, 1997)

The Question

If the total mass/energy of Universe was at its beginning in such a minimum sphere of 10E-33 or more, why does it exist? Such an extraordinary condition and diameter is the same as it is valid for black holes. Why does the Universe exist, although its total mass was within the Schwarzschild radius?

The Answer

This is a very good question, and I will do my best answering it.

In the case of a black hole, the Schwarzschild radius is mathematically the radius in at which one would have to be moving at the speed of light in order to escape. Or, the radius that nothing can escape. This is only a function of the mass -- not the density.

Putting the numbers in, the Schwarzschild radius (Rsch) is given by:

Rsch = 3km x Mass; Where the Mass is measured in solar masses.

Now, when do we get a black hole? Answer, when the Schwarzschild radius is bigger than the object we think might make a black hole.

The Sun (Mass = 1 solar mass) is not a black hole because it is bigger than 3 km. If we magically shrunk in down to 3km in radius, then it would become a black hole.

Now back to your question: Why then was the early Universe not a black hole? Well, lets figure out its Schwarzschild radius to get a basic rule of thumb idea of what is going on.

Rsch = 3km x Mass of the whole Universe in solar masses
= about 10 to 100 billion light years
= about the current size of the whole Universe

So, in the basic definition of a black hole I used above (where the size of the object is smaller than the Schwarzschild radius) the whole Universe is one big black hole with us on the inside.

Therefore, the simple answer is that we are inside the event horizon of the whole Universe, and there is no way that we can escape the Universe's grasp.

The more complicated answer is a little different. The Schwarzschild approximation for a black hole is simply that -- an approximation. In general, the most correct description of gravity everywhere is Albert Einstein's General Theory of Relativity. This a very mathematically complicated theory in practice, so we only use it when we have to. Even then we make many simple assumptions, just so that we can solve the equations.

When does general relativity become extremely important? Answer, when the sizes of whatever we are studying become approximately the Schwarzschild radius. So, general relativity becomes important in compact objects and it becomes important when we start trying to understand the Universe as a whole. In the middle, like most things we deal with, we do not have to worry about it because there are simpler theories that will give us the same answers.

I hope this helps. Thanks again for your insightful question.

Jonathan Keohane
-- for Imagine the Universe!

Question ID: 970408c

Could the Laws of Physics be different in another Universe? (Submitted March 08, 1998)

The Question

My question: On PBS, an astrophysicist commented that other universes might have different physics and natural laws. Is this possible and what are some differences that there might be? I think that the formulas used to determine properties might just be different depending on the amount of matter in the universe, but what do the REAL astrophysicists think?

The Answer

There are a handful of constants that shape physics. The three main fundamental constants that we measure but at this point cannot be determined are:

c - this is the speed of light, it is important in electricity, magnetism and the conversion of matter to energy.

h - this is Planck's constant, it is important in atomic and nuclear physics

G - this is the universe gravitational constant, it holds planets in their orbits and determines the large scale structure of the universe.

We have no theory of why the values of c, h, and G are what they are. This begs the question of why they have the values they do, and what the universe might have looked like were they (and other constants like the mass of an electron) different. It turns out you can't change these values much without making life-as-we-know-it impossible. Such consideration has led to several variations of "Anthropic Principle."

These constants reflect fundamental characteristics of space and the quantum mechanical vacuum. In the General Theory of Relativity, for example, G represents the response of space to mass, and therefore is intimately connected with the fabric of space. While G reflects the global, large-scale character of space, Planck's constant, h, reflects the nature of space on atomic scales as it is expressed in the parameters characterizing subatomic particles and photons (energy levels, angular momentum, linear momentum) and the waves associated with matter. The speed of light in a vacuum, c, is the limiting velocity of matter through space and the characteristic velocity of energy through space, and thus reflects, on the one hand, the resistance of space to acceleration (the drag of the vacuum), and, on the other hand, the density of the vacuum, in the same sense that the speed of sound reflects the density of the medium through which acoustical waves travel.

In short, the only way these constants could really be different would be if the characteristics of space were different. I've addressed space and not time here, but it is likely that, based on the same theories mentioned above, the fundamental nature of the time component of spacetime would have to be different if these constants were different. Since spacetime and matter-energy come to us together courtesy of the Big Bang and are in some fundamental sense inseparable, we can assume that G, h, and c could not be changed without re-engineering the birth of the universe which, if you buy the theories that point to near-critical density of matter-energy in the universe, all indicate that space, time, matter, and energy as we know them are inseparable from the three fundamental constants that characterize them. Although we cannot, at this point, alter these constants, one might wonder about how our universe would be different if these values were changed.

Jeff Silvis and Mark Kowitt
For Ask an Astrophysicist

Question ID: 980308a


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