Astronomy Buff

The Cosmic Microwave Background: Strongest Evidence Yet for the Big Bang

by Tony on February 9th, 2007

This is my fifth post for Just Science Week.

There was a time, not long ago, when cosmologists were fighting over whether the Big Bang was the correct model for the universe, or something else like the steady state theory, or perhaps some variation on an oscillating universe.

The fighting stemmed from the fact that we just didn’t have very many observations to support any one idea. They could all be right.

Until the late sixties, when Arno Penzias and Robert Wilson discovered a mysterious hiss through a very large antenna, all we had to go on were the observations by Edwin Hubble which showed distant galaxies flying apart. Unfortunately, Hubble’s data was rather poor and led to some proposals that could be used against the Big Bang idea.

Observations using Type 1a supernovae later confirmed Hubble’s initial conjecture.

What Penzias and Wilson heard through their antenna turned out to be the next big piece in the cosmological puzzle. They were listening to the remnants of the radiation left over from the Big Bang (although they didn’t know it at the time). This became known as the Cosmic Microwave Background (CMB). Cosmic because it’s left over energy from the creation of the cosmos; Microwave because of the frequency the radiation was emitted; and Background because it is the same everywhere.

Now, everyone is quite comfortable with the idea that the Big Bang best describes how everything was created. It fits all available observations we are able to make, and the more spacecraft we launch to study CMB (and now dark matter), the more the Big Bang withstands the scrutiny of science.

A big question I get frequently is, “How Does the CMB Support the Big Bang Theory? What’s so great about it?

Illlustration Credit: NASA/WMAP Science Team

Technorati Tags: cosmic microwave background, cosmology, Just Science Week

So far, we have three big pieces of evidence that support the idea that a Big Bang occurred (from Doug Scott’s CMB Page):

In full, the three cornerstones of the Big Bang model are:
(1) the blackbody nature of the CMB spectrum;
(2) redshifting of distant galaxies (indicating approximately uniform expansion); and
(3) the observed abundances of light elements (in particular helium and heavy hydrogen), indicating that they were “cooked” throughout the Universe at early times.

Because of these three basic facts, all of which have strengthened over the decades since they were discovered, and several supporting pieces of evidence found in the last deacade or two, the Big Bang model has become the standard picture for the evolution of our Universe.

We’ve already talked about the second one above, that’s Hubble’s Law, and number three simply comes from our observations of the available elements in the universe. But its the CMB that’s the real clincher here. This is a direct observation of the actual radiation that was left over from the Big Bang, this is something we can physically measure.
How do we know it’s radiation from the Big Bang? If the Big Bang occurred when we think it did, about 13.5 billion years ago, and if it created temperatures in the universe like the models predict, then by now, the entire universe should have cooled to roughly the temperature that we see in the CMB radiation, about 2.73 degrees Kelvin. That’s precisely what we see.

Further, that radiation field should be in thermal equilibrium, meaning that the radiation from that the Big Bang has had time to settle into an equilibrium state. When an object is in thermal equilibrium, it radiates in a very specific way and it’s called a blackbody radiator. They have a spectrum that looks exactly like the figure above.

A teapot full of hot water is not in thermal equilibrium and is therefore not a blackbody. But let it cool and sit there and then measure it, you find it gets closer to being a blackbody (it’s not a great one, but I needed an analogy).

The Cosmic Background Radiation looks exactly like a blackbody and it’s peak is at the exact spot it should be if the Big Bang occurred when we think it did. In fact, the CMB blackbody curve is better than anything we could create artificially in the lab. Cosmologists take this to mean that it wasn’t produced by normal matter in the universe, but rather the matter-antimatter collisions that drove the universe during the inflationary stage.

The discovery of the CMB was a big deal to Big Bang proponents. Of all evidence found so far, the CMB radiation (and the discovery of dark matter) is the biggest yet to support the Big Bang Theory.

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