Cosmic Microwave Background Radiation

Einstein's theory of general relativity tells us, in a very broad sense, how the universe behaves. It does not, however, tell us what it is like now, what it was like in the past or what it will be like in the future. To know these things we must supplement relativity with more information and make some assumptions about what we believe the universe to be like -- what's in it, how it started, etc.

In 1929, Edwin Hubble found that, in general, other galaxies are moving away from us at a rate which is proportional to their distance from us. This was the first experimental support for the so-called Big Bang theory, in which the universe is expanding. Until this era, the evolution of the universe was either assumed to be static.

Until 1965 debate raged over whether or not there was a Big Bang. In that year, Dicke, Peebles, Wilkinson and Roll published a paper predicting that if the universe was indeed the result of a Big Bang, there should be relic radiation left over from its initial stages permeating the universe today. This was similar to conclusions drawn earlier by Alpher and Gamow. In the same issue of the Astrophysical Journal as the Dicke et al. paper, Penzias and Wilson published a paper innocuously titled "A Measurement of Excess Antenna Temperature at 4080 Mc/s", which the Princeton group interpreted as being this relic radiation.

In 1989, NASA launched its first dedicated NASA cosmology mission, the Cosmic Background Explorer (COBE), which demonstrated, among other things, that this radiation has a spectrum precisely given by the Planck distribution for photons at a single temperature (the so-called blackbody function), as radiation from a Big Bang should be, and that it is unlikely to be the result of any other astrophysical mechanism.

Since the universe is expanding, at some earlier time everything in the universe was much closer together than today. Thus, the density of matter and energy was much greater than it is today. And since compressed gases get hotter, any gas in the universe was at significantly higher temperature. Long enough ago, the density and temperature were high enough that Hydrogen atoms would be stripped apart as soon as they formed, so the matter in the universe must have been a soup of protons and electrons. The protons, electrons, and photons were tightly coupled, as each time a Hydrogen atoms formed, a photon would ionize the atom and split it back apart. The epoch where things cooled down enough for Hydrogen atoms to survive is when matter and photons decoupled.

The Cosmic Microwave Background (CMB) is a relic of the epoch of decoupling. After the formation of Hydrogen (often called the epoch of "recombination", even though it was really the epoch of "combination" of protons and electrons), the photons didn't have enough energy to ionize hydrogen. The photons then traveled on their merry way, only dimly aware of the matter. These photons were in the ultraviolet part of the spectrum at the epoch of recombination, but in the meantime, due to the expansion of the universe, they have stretched out into microwaves. Thus, the CMB provides a "snapshot" of the universe just after recombination, since the light we see today (the photons) should still have the imprint of the matter at this early time. The temperature that characterizes the photons today (2.7 degrees Kelvin above absolute zero) tells us when the epoch of recombination occurred (since we know the temperature required to ionize Hydrogen and we know the expansion rate of the universe).

The CMB is very isotropic: we measure nearly the same temperature of diffuse background photons in every direction we look. But now, we see stars, galaxies, clusters of galaxies and more. If the CMB is a snapshot of the structure of the universe at the epoch of recombination, and universe was so smooth in its infancy, how did it become the clumpy mess it is today? To explain the structure in the universe today and the lack of structure early in the universe, physicists and astrophysicists have developed a host of theories to account for observed facts, and these theories have radically changed our notions of the very early evolution of the universe, far before even the epoch of recombination.

To account for the inhomogeneous distribution of mass on some scales today, these theories assume that the clumps of matter had to arise from some general sort of seeds. In the inflationary model, for example, the structure arises from small perturbations present very early in the universe that slowly grew to the distributions we see today. Topological defect models assume that the structure was formed by the interaction of matter with topological defects caused by phase transitions which occurred during the expansion of the universe.

Different theories predict different amounts of structure in the universe today and different levels of homogeneity in the CMB. These predictions have been and will continue to be compared with the measured structure in the universe now, and until recently they simply had to make sure they didn't call for 'too much' structure in the CMB.

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