The 1965 discovery of cosmic background radiation by Arno A. Penzias and Robert W. Wilson provided important evidence in support of big bang based cosmological models first proposed after American astronomer Edwin Hubble's (1889-1953) discovery that the universe was expanding. Big bang models assert that the universe started as an extremely small and highly energetic plasma of all kinds of particles. As this matter expanded, the form of the matter changed as it underwent many transitions. Cosmic background radiation is a relic of one of these cosmological transitions, and it remains the best evidence to verify the big bang theory. In addition, precise measurements of cosmic background radiation allow physicists to study the formation of structure in the early universe.
The early stages of the universe were radiation dominated, because all particle energies were so high that they could be considered without mass, as in photons. Scattering reactions between charged particles were constantly taking place and photons were continuously absorbed and emitted. As the mean free path of a photon was extremely short, the universe was essentially opaque. As the universe expanded, it cooled, and particle energies decreased. As energies decreased, certain scattering reactions could no longer take place, and in other cases, bound states of particles formed.
At a temperature of about 3,500K, the energies of free nuclei and electrons became low enough that they began to form atoms. Once the atoms had formed, interactions between light and matter became increasingly infrequent, and the universe became transparent to light. The radiation, mostly in the visible and nearby frequency range, could suddenly propagate over long distances without being absorbed. Because the radiation was emitted long ago, and the universe has undergone much expansion, the light has been redshifted to a lower frequency. The redshift is similar to the Doppler shift; because every point of the universe is expanding away from us, the frequency of the light emitted is decreased. The redshift also can tell us when radiation was emitted by what measure it has been shifted. In the case of the background radiation, the red-shift is significant enough that the visible frequencies are shifted down to the microwave region, indicating that the atoms formed when the universe was about a million to ten million years old (the universe is approximately 10 to 15 billion years years old.)
The spectrum of radiation matches the spectrum of a blackbody at about 3K. It is extremely uniform; the temperature difference between any two points in the sky is found to be less than 0.01%. This temperature uniformity is one of the hallmarks of the big bang theory. The atoms forming at the time the background radiation was emitted began to clump together due to gravity, eventually leading to galaxies. Some differences in the spectrum at different points in the sky, therefore, can be expected due to structure formation. Satellites observing the cosmic background radiation continue to provide data to be used in new theories of how these structures formed.
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