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The Question

(Submitted September 22, 1997)

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

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