The Question

What is the difference between Dark Energy and Dark matter; aren't they considered the same thing?

The Answer

Dark Energy appears to be, based on the brightness of the most distant type-Ia supernovae, a mysterious force that is accelerating the expansion of the universe. These recent discoveries have provided good evidence that there is such an outward force on the universe (variously called the cosmological constant, "quintessence," or "dark energy").

Data about the rotation of galaxies shows us that the outer parts rotate as fast as the inner parts. This only makes sense if there is a spherical distribution of matter in each galaxy, which is not what we see. Therefore we infer that there is a certain amount of Dark Matter in each galaxy. This could be some exotic particles, or just lots of stars too small to have ignited.

Aside from this, there is also the Dark Matter that we think is there, based on theoretical arguments. This is something we can measure by looking at the cosmic microwave background and distant supernovae. These are the measurements (recently made) that imply the existence of both Dark Matter and Dark Energy.

You can read a bit about these recent results at

http://map.gsfc.nasa.gov/news/

And more detail than you probably want at

http://panisse.lbl.gov/public/

Hope that helps.

-Kevin Boyce and Martin Still,
for "Ask an Astrophysicist"

The Question

What is the driving force behind dark energy? Is it the same thing as vacuum energy?

The Answer

We don't know what causes dark energy.

Dark energy and vacuum energy may be related,but they don't appear to be the same thing. Vacuum energy is a manifestation of quantum mechanics and the Heisenberg uncertainty relation that delta-E x delta-t is greater than or equally to Planck's constant divided by 2. Basically everything has a small residual energy - that is delta-E can never be total zero. There can be other contributions as well.

Dark energy is a possible explanation of the observed behavior of the Universe at very large scales. Conceptually it appears to permeate all of space and look like a very slightly repulsive gravitational interaction. A calculation of the vacuum energy gives something too large to be dark energy, but there must be more to the story. We just don't know what that story is.

Here is a good discussion of the issues:
http://blogs.discovermagazine.com/cosmicvariance/2011/10/04/

Jay and Jeff
for Ask an Astrophysicist

The Question

I've read that dark energy is the energy driving the accelerated expansion of the universe. Its density doesn't decrease as space increases. So, does this mean that it has existed with equal density from the beginning of the universe? Have scientists found that the accelerationed expansion of the universe was first decresing, and has started to increase more "recently"?

The Answer

According to current theories dark energy has always been around and has always exerted the same force. However, in the early universe, matter was closer together and the effects of gravity caused the expansion of the universe to slow down. About 5 billion years ago, the density of matter (both normal matter and dark matter), which had been decreasing since the Big bang, became low enough that the dark energy was able to start accelerating the expansion of the universe.

More information can be found at:
http://imagine.gsfc.nasa.gov/science/questions/dark_energy.html
http://physicsworld.com/cws/article/print/19419
http://en.wikipedia.org/wiki/Dark_energy

Hope this helps,
Mike and Georgia
for "Ask an Astrophysicist"

The Question

My question involves the theoretical basis for the existence of Dark Energy. My understanding is that dark energy is theorized because researchers using the Doppler effect believe the galaxies at the edge of the visible universe are accelerating away from us, when it was expected they would begin to slow. But I feel this is incorrect. These galaxies, being 13.5 to 14 billion light years away, would be showing red shift from 13.5 to 14 billion years out of date, and of course they would appear to be accelerating. We are seeing them as the existed shortly (within half a billion years) after the big bang, NOT as they really exist today, and therefore the entire basis for thew existence of 'dark energy' should be called into question. Unless the red shift is instantaneous?

The Answer

You are correct that when we look at very distant objects, we are seeing light that left them a long time ago, so we are seeing them in the past so to speak. What we have is a velocity and a distance from that time. We can also look at somewhat closer objects, but still quite distant, and get a velocity and a distance of those objects (at a later time in universal terms). So we can construct a plot of distance and velocity, where distance is also related to time before present. We expect some slowing of the initial expansion due to gravitational attraction. The observed curve on that plot departs from the expected curve. In the first several billion years, the Universe slowed about as expected, but several billion years ago it apparently began to accelerate. So actually, it is galaxies that are relatively close to us (still billions of light years) that are moving away faster than expected.

Jay and Jeff
for Ask an Astrophysicist

The Question

Which scientists are currently on the forefront of Milky Way study and theories? Where can I find their work?

The Answer

Boy. That is a very BIG question, because there is a tremendous amount of research involving the Milky Way galaxy, and it involves almost every aspect of theoretical and observational astronomy. Computational astrophysicists study how and why spiral structure forms and endures, and the dynamics of the globular clusters that surround the disk of the Milky Way in a halo. Observers study star forming regions, map spiral arms, measure abundances of elements in the interstellar medium, search for dark matter, etc. So, I'm just going to pick my favorite at the moment, which is the search for MAssive Compact Halo Objects (MACHOS), one possible explanation for the dark matter that must exist in galaxies, including the Milky Way.

Basically, according to careful observations, we know that up to 90% of the matter in galaxies must be in the form of "dark matter" to account for the dynamics we observe. On top of that, the dark matter appears to be distributed in a spherical halo around the Milky Way, while the luminous matter is located largely in the flat disk.

The basic idea behind the MACHO project is something called "microlensing:" if a MACHO were to pass between us and a distant star, the light would be bent around the MACHO by it's gravitational field passing almost directly in front of the star (in other words, it would act as a gravitational lens). The light curve that results would be independent of the wavelength of the radiation (as opposed to many intrinsic brightness changes), would not change the polarization, would be symmetric, and would have a very distinctive shape. In searching for such microlensing events, the MACHO project is taking long baseline, large field photometric data, and analyzing each star. An early conclusion of the project was that up to 50% of the dark matter in the Milky Way may be MACHOS in the halo with masses ranging from the mass of Jupiter to one tenth the mass of the Sun. An added benefit the large collection of data on variable stars in the Large Magellanic Cloud (one target). For updates on the project, and to learn about the science team, check out:

http://www.macho.mcmaster.ca/

As I said, there are many branches of study of the Milky Way. If your interest is different, I suggest reading the material in our learning center:

https://imagine.gsfc.nasa.gov/

that relates to your interest. We also include links to other learning centers and information sources.

Regards,
Padi Boyd,
for the "Ask an Astrophysicist" Team

The Question

If neutrinos are massless what else could make the galaxies spin like that? I am 18 years old and next year I be a student in physics at University level.

The Answer

The context of your question is lost, but I presume by 'spin like that' you are referring to me fact that the speed at which galaxies spin is too fast to be held together by the gravity of all the stars that we can see.

The galactic missing mass, which provides additional gravity, is probably produced by some sort of 'dark matter' (things we do not see). This can be in the form of Massive Compact Halo Objects (MACHOs) which may be stars too small to glow brightly (or other, more bizarre objects) or it can be heavy particles, or shadow matter, or primordial black holes, or any of a number of other things.

MACHOs have been detected through their gravitational effect on light, although there is no definitive knowledge of exactly what they are.

http://www.macho.mcmaster.ca/

David Palmer
for Ask an Astrophysicist

The Question

I have taken several high energy physics courses in College and know some astronomy. Isn't Dark matter theory just a weak attempt to mesh theory with observation or has there been any research with empirical results proving the existence of 'Dark Matter?' Any additional references to information would be helpful.

The Answer

We have a brief explanation on dark matter at:

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

To supplement:

Astrophysicists have accumulated a large body of evidence for dark matter. In this context, 'dark matter' means just that --- matter of whatever type that does not shine brightly (in visual light, X-rays or at any other wavelengths). Even though we do not see dark matter directly, its gravitational influence can be seen in the motion of gas and stars in galaxies, and in the motion of hot gas and galaxies within clusters of galaxies. There is recent evidence from microlensing observations that at least some of the dark matter in our own galaxy is in the form of MACHOS, or MAssive Compact Halo ObjectS --- these are planets or stars, made up of ordinary (baryonic) matter, that are too faint to be observed directly, but can act as a gravitational lens and magnify the brightness of brighter stars in the background. There is nothing 'weak' in the observational proof for dark matter in this sense.

In a cluster of galaxies, we can estimate the masses of stars in the galaxies and the hot gas that fill the cluster. We can also infer the total mass of the cluster that is needed to keep it gravitationally bound. The latter is typically found to be ~5 times the combined mass of the stars and the hot gas; an analogy with our Galaxy suggest that only some of the dark matter can be MACHOS. Although circumstantial, such results point strongly to the presence of non-baryonic dark matter in the clusters of galaxies.

When it comes to deciding what kind of exotic particles may make up the non-baryonic dark matter, however, there may be a hint of 'weakness', in that different particle physicists favor different exotic particles. Moreover, as far as I know, there has not been a direct detection of these exotic particles.

Best wishes,

Koji Mukai for
Ask an Astrophysicist
with help from Dr. Mushotzky

The Question

Does a ratio of baryonic vs. non-baryonic matter have any part in the balance of the universe?

The Answer

According to the recent wmap satellite results non-baryonic matter occupies as much as ~74% (this is called "dark energy") while visible baryonic matter occupies only ~4%. The rest of the 22% is called "dark matter", but the vast majority of the the dark matter in the universe is thought to be non-baryonic.

Keigo Fukumura
for "Ask an Astrophysicist"

The Question

There has been a lot of debate over the nature of missing matter in the universe and people have been putting forward many candidates for this missing matter. My question is simply given the equivalence of mass & energy couldn't the so called dark matter simply be the energy left and floating around from the big bang ?

I am a banker by profession however have a keen interest in relativity and astrophysics through reading books without any formal education.

The Answer

You are absolutely correct about the contribution of the energy left from the big bang to the energy of the universe. Unfortunately, this cannot be the missing mass. It is certainly taken into account and in fact it makes the dominant contribution to the energy density of the universe at early times. However, since its energy density drops faster than that of ordinary matter it is not important today. It simply constitutes the 2.7 degree K cosmic background radiation. Furthermore, because it consists totally of photons, i.e. it moves at the speed of light it cannot be clustered in the gravitational potential of a galaxy, which cannot trap particle traveling faster than about 300 km/sec. For that we indeed need particles with non-zero inertial mass.

Sincerely,
Demos Kazanas
for the Ask an Astrophysicist Team

The Question

Neutrino oscillations are real, ergo they have rest mass. Can they be slowed down, to be stationary relative to galaxy clusters? If so, could they comprise at least a significant fraction of (cold) dark matter (cf the hot dark matter they were always considered to be)? If not, why not?

The Answer

All non-baryonic dark matter candidates interact very weakly with each other and with ordinary matter: They cannot have strong nuclear or electromagnetic interaction, by definition. They can interact via weak nuclear force, which is the case with neutrinos. However, this force is so weak a neutrino can go through a chunk of lead a light year thick without being stopped. So, non-baryonic dark matter particles cannot be slowed down, except by gravity. (The general expansion of the universe can slow them down, too, but this is another manifestation of gravity.)

The measurements indicate the neutrinos can only have a small rest mass. This makes it rather hard for gravity to slow them down. In fact, cosmologists believe neutrinos (even assuming they are massive) are moving very fast, not much below the speed of light. In general, particles with near-zero rest masses (including neutrinos) can only be a constituent of hot dark matter:
http://csep10.phys.utk.edu/astr162/lect/cosmology/darkmatter.html

See also:
http://www.astro.princeton.edu/~dns/MAP/Bahcall/node6.html

Best wishes,

Koji & Scott
for "Ask an Astrophysicist"

The Question

Could WIMPs be virtual in nature? If virtual particles can melt away a black hole (ala Stephen Hawking), couldn't they also have momentary mass?

This would solve the WIMP problem in that they would send out gravitons, but they wouldn't react much with ordinary matter as they disappear rapidly and mostly inhabit empty space.

Therefore, couldn't empty space have its own gravity?

I wonder if this'll have applications in solving the cosmological constant discrepancy. It'd surely explain the acceleration of universal expansion.

Could this also have value to the quantum theory/gravity conundrum?

The Answer

This is a nice thought, and one that theoretical physics have thought about in the past; the best brains in the world are struggling over the detailed maths to see if a similar idea can actually be made to work, though.

Virtual particles certainly aren't the explanation for WIMPs. For one thing, we need something that stay billions of years, long enough to cluster into galaxies and the like. Virtual particles disappear too quickly to do so. [By the way, the key part of Hawking radiation is not the virtual particles, but the fact that virtual particles can turn into real particles in the extreme conditions around a small black hole. If they stayed virtual, they won't do anything to the black hole.] For another, theorists have plenty of ideas as to what real particles can make up WIMPs --- the problem is that we don't have big enough accelerators (or other similarly expensive experiments) capable of determining which of the various possibilities is correct.

On the other hand, theorists do think that the Cosmological Constant has to do with vacuum energy, which is basically what you are suggesting. As we mentioned, the problem is working out the math --- naive calculations come out 120 orders of magnitude off, which is enough to embarrass even theoretical cosmologists.

Hope this helps,

Koji & Scott
for "Ask an Astrophysicist"

The Question

I am a real novice (advance apology for stupid question) but I first heard about dark matter last week and have been transfixed by this mystery. I basically understand (1) that particle physicists are trying to establish the existence of WIMP's in cryogenic crystals and in the ice shelf and (2) that part of the key to the dark matter theory is that as you move out from the center of a galaxy, the speed of the rotation stabilizes which implies that there is mass there that does not emit light. My question is do particle physicists believe that WIMP's are dispersed throughout the galaxy? And if so, why wouldn't WIMP's affect the orbit of other bodies within the galaxy (i.e. our own solar system)?

The Answer

Two likely possibilities for the dark matter in our own galaxies are MACHOs (MAssive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). MACHOs are low mass stars, brown dwarfs, neutron stars and white dwarfs. If MACHOs make up most of the dark matter, the distribution is not smooth on the scale of the Solar system, but it is smooth on a much larger scale.

If the Galactic dark matter consists of WIMPs, then they are dispersed throughout the Galaxy, with a distribution somewhat different from that of the stars that we can see. Since gravitational pulls of WIMPs from different directions tend to cancel out, the orbit of planets in our solar system is not affected by the presence of WIMPs. However, since there are more WIMPs towards the center of our Galaxy than away from it, the motion of the Solar system (and other stars) in the Galaxy is strongly affected --- this is how astrophysicists infer the presence of the dark matter.

Best wishes,

Koji Mukai
for Ask an Astrophysicist

The Question

Do the concepts of solitons and instantons help or play a role in explaining dark matter or the missing mass of the universe?

The Answer

Dark matter research requires the combined efforts of astrophysicists, cosmologists and particle physicists. We at the "Ask an Astrophysicist" service can tell you a lot about the astrophysical aspects of dark matter research; particle physics is somewhat outside our areas of expertise, however.

We do belive that much of the dark matter is non-baryonic.

A part of this is likely to be massive neutrinos. Recent research on neutrino oscillations (apparent mutation of neutrinos from one type to another) strongly suggests they do have rest masses. They appear to be insufficient to account for all the astrophysically inferred dark matter mass in the Universe, though.

The rest are thought to be more exotic particles, with names like axions, photinos, and so on. Our understanding is that there are plenty of theoretical candidates for what the dark matter may consist of, but no experimental evidence of a specific type of exotic particles. We do not know if the concepts of solitons and instantons are relevant, or useful, to particle physicists working on the dark matter: you may want to check particle physics-oriented web sites, such as:

http://www.superstringtheory.com/

Best wishes,

Koji Mukai & Bram Boroson
for "Ask an Astrophysicist"

The Question

Could dark matter be composed of mini-blackholes formed at the time of the Big bang?

The Answer

This is a very good question. We at "Ask an Astrophysicist" know that it's currently not considered to be a major constituent of dark matter, but we're not sure if we know all the reasoning behind this.

Here is what we do know.

Stephen Hawking proposed primordial (i.e., created just after the Big Bang) black holes in connection with his work on black hole evaporation:

http://imagine.gsfc.nasa.gov/ask_astro/black_holes.html#010703a

This process (Hawking radiation) should have destroyed the lowest mass primordial black holes by now, and should currently be destroying ones that started out with the mass of a small asteroid. However, no mini black holes have been discovered to date, through Hawking radiation:

http://imagine.gsfc.nasa.gov/ask_astro/grb.html#970519a

or through other means.

Note that the number and the masses of primordial black holes depend sensitively on the nature of density fluctuation in the early universe. We have learned a lot on this topic since Hawking made the original suggestion: the Cosmic microwave Background as measured by COBE is extraordinarily smooth on the one hand, while we now know of large-scale clustering of galaxies, on the other hand. As we understand it, current models of density fluctuations that account for these observations do not predict a large number of primordial black holes. Therefore, they are probably not a major component of dark matter.

You might want to fallow this up with the Dark Matter FAQ site at Berkeley:

http://cdms.berkeley.edu/Education/

Hope this helps,

Koji & Ilana
for "Ask an Astrophysicist"

The Question

Since string theory implies up to 11 dimensions, could dark matter be gravitons leaking from other dimensions into ours?

The Answer

Actually, that's exactly one possibility that's being explored by brane-world theorists. Of course, the jury is still out on brane-world theories....

From the Cornell Chronicle:

http://www.news.cornell.edu/stories/March06/Tye.brane.ws.html

"In brane-world theory, the ends of strings are anchored in our brane, so the particles we see can only move within the brane. But the particles that carry the gravitational force, known as gravitons, are closed strings -- little Cheerios -- and can "leak" out of the brane. This explains why gravity is much weaker than the electromagnetic force and the strong and weak nuclear forces. It also offers a possible explanation for the "dark matter" that astronomers need to explain why the mass of the universe doesn't agree with the observed objects. Dark matter could be in an adjacent brane, with its gravitons leaking into ours."

We hope this helps!

Barbara & Ilana
For Ask an Astrophysicist