Cosmic Background Radiation Anisotropy
Published on Oct 4, 2006 at 3:37 pm.
2 Comments.
Filed under cosmology.
The winners of the 2006 Nobel Prize in physics have been announced. They are John C. Mather (NASA Goddard Space Flight Center) and George F. Smoot (Lawrence Berkeley National Laboratory). The award was “for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiaton.”
Rather than discussing the people themselves, I thought that I’d say a word about the cosmic background radiation. Reading about the awarding of the prize in the local newspaper, and hearing about it on TV, it is clear that the media are again falling down on the job when it comes to reporting.Â
First of all, Mather and Smoot did not discover the cosmic backround radiation (CBR). That was done by Arno Penzias and Robert Wilson in 1964, and for which they were awarded the 1978 Nobel Prize in physics. George Gamow had predicted the CBR years earlier in a model of the origin of the universe that is generally called the Big Bang (This is not to be confused with an earlier model, also called the Big Bang, which replaced the first model called the Big Bang. Hmm. Cosmologists aren’t really very original it seems when it comes to naming these things.) What Mather and Smoot did was to demonstrate the blackbody nature of the CBR and to measure tiny variations in it.
Let me explain. According to the Big Bang theory, the universe started as a singularity — all of the universe in one point. The universe then expanded, and as it did so, the fundamental forces that make the universe act the way it does started to “freeze out” in the form that they are today. Matter began to form (at first it was just energy and light, and then matter). The universe was soon a seething mass of gasses (almost entirely hydrogen and helium). As the universe expanded, then the gas cooled. When too hot and dense, these gasses did not allow light to travel very far. Eventually, though, after about 300,000 years, things cooled to the point that light began to travel through the gasses. At that time, the gasses were at a temperature of about 3000 K. The universe was awash in a vast light everywhere. This light was formed by hot gasses shining, and this is a very distinctive form of light that we call blackbody radiation. It has specific spectral characteristics.
Now, at this point, it is important to realize that this expansion of the universe is NOT the stuff in the universe expanding into space. Rather everything is sort of sitting around stationary, but getting farther apart, because it is space that is expanding. All the matter is just along for the ride. The universe is not expanding into anything. It is expanding. Space itself is expanding, and all from a single point. This is what became known as the “Hot Big Bang” model.
Gamow predicted that the universe would be awash in this light produced by 3000 K gasses. But, and here is where the bit about space itself expanding becomes important, the nature of this radiation changes over time. As space itself expands, the distance between the peaks of these light waves gets farther apart. This has the effect of making the wavelengths longer. But, the hotter an object, the more short wavelength light it emits. The cooler, the more that it emits predominantly longer wavelengths. With the universe many billions of years old, Gamow proposed that this blackbody radiation would have its wavelengths lengthed to the point that it would now look like light being emited from something much cooler than 3000 K. He figured a few Kelvin. Yakov Zel’dovich computed that the blackbody radiation permeating space might look as though it were coming from something of temperature about 3 K (that is 3 degrees Celsius above absolute zero). This would mean that the blackbody radiation would be mostly microwaves.
When Penzias and Wilson found microwaves coming from all directions in the universe, they were detecting cosmic background radiation. But, they were only detecting the “tail” of the blackbody spectrum. The all important peak of the radiation didn’t penetrate the Earth’s atmosphere, so they could infer that what they found looked like what you’d expect at the tail of a blackbody spectrum, but they didn’t have proof.
Along comes the Cosmic Background Explorer (COBE) satellite. Orbiting Earth above our atmosphere, it could study the CBR without interference. Finally, it was able to deduce that the CBR perfectly matched the theoretical blackbody spectrum of a 2.73 K object. Remember, it was originally 3000 K, but the wavelengths were stretched to mimic the blackbody radiation of a much cooler body. So, now we had definite data behind Penzias and Wilson’s tantalizing findings decades earlier. This was Mather’s primary work.
But that isn’t the whole story. It turns out that there are tiny fluctuations in the background radiation, on the order of one part in 100,000. The image to the left (click on it to see it full size) shows plots of the background radiation as effective temperature. The top plot shows it in gross form, and you see that it is all the same — the universe is the same temperature in all directions. The second plot shows tiny variations in temperature due to the Doppler effect of Earth’s motion through space. The bottom plot takes that effect away and shows the 1 part in 100,000 fluctuations. These fluctuations are essential, bacause the universe is simply not old enough for gravity by itself to have caused the gasses to collapse to form galaxies, galaxy clusters, and galaxy superclusters. There had to have been a tiny seed of density fluctuations long ago. These fluctuations, discovered with the COBE DMR (Differential Microwave Radiometer) were the work of Smoot.
These fluctuations are important in another way. One problem that everyone had with the CBR was that it was the same in all directions. That clearly can not be unless something rather strange were going on in the early universe. That is because opposite sides of the universe are so far away from one another that the time it would take for them to reach the same temperature, unless they started at the same temperature, exceeds the age of the universe … by a LOT. One way around that is a theory called cosmological inflation, first proposed by Alan Guth, and subsequently modified by Andrei Linde and others. According to this model, when the universe was about 10-34 years old, it suddenly expanded in a wild spurt. Space was expanding so fast that things appeared to be moving away from each other at speeds vastly in excess of the speed of light. This does not violate relativity since nothing was really moving. Space was expanding, and nothing in relativity prevents space from expanding as quickly as it can, only that nothing can move through space faster than light. So, this allowed the farthest sides of the universe to be the same temperature, since before the expansion they were in thermal contact and equilibrium. Also, tiny density fluctuations predicted by quantum theory expanded. These fluctuations that were originally so tiny as to
be smaller than subatomic particles were suddenly the size of galaxy superclusters. This is where the seeds of such large scale structures may have come from. Otherwise, cosmologists are at a bit of a loss to explain how such large structures could form in the time since the beginning of the universe.
Mather and Smoot’s findings are consistent with predictions of the Big Bang theory — the most successful theory that we have as to how the universe got from its creation to its present form.
-AstroprofÂ
(Image Credits:Â LBL, NASA, COBE)
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thulasidas on March 11, 2007 at 1:47 am: 1
the explanation of blackbody form and anisotropy of microwave backform radiation is not given clearly
Ritika on February 8, 2008 at 1:26 am: 2
Hi, you have explained the work of the scientists very well. But I feel the antimatter and cosmic background aspects haven’t been covered too well. Although, very nicely written.