In 1964, a pair of radio astronomers, American physicist Robert Wilson (1936-) and German-born physicist Arno Penzias, working for the Bell Telephone Laboratories in Holmdel, New Jersey, stumbled upon the best evidence in existence to support the big bang model of cosmogenesis. Penzias and Wilson were recalibrating a radio antenna originally built to bounce signals off of Echo, the first telecommunications satellite. The same antenna was to both transmit and receive microwave signals from the second such satellite, Telstar. The astronomers-turned-radio engineers were measuring the antenna's gain (its power to amplify faint signals), and were looking out of the plane of the Milky Way. No matter where the antenna was pointed, whether at a radio source or at empty space, they detected a very faint, very uniform signal. The intensity of the radiation they found was identical to that emitted by a body at a temperature of about 3K. Radio astronomers characterize signals by comparing them with a black body distribution. Any body, at any temperature above absolute zero, emits a spectrum of radiation that is independent of the composition of the body, and depends only on its temperature. Wilson and Penzias were perplexed; the signal was clearly of cosmic origin, but no known cosmic source broadcast at the measured wavelength, 7.35 cm, and none known was so energetically uniform.
Meanwhile, a few miles away (and unbeknownst to the Bell labs duo) P. J. E. Peebles, a Princeton University astrophysicist, argued that if the universe had originated in the hot fireball of the big bang, its glow, though cooled by fifteen billion years of subsequent expansion, should be detectable. Peebles had independently rediscovered what had first been predicted in 1949 by cosmologists Ralph Alpher and Robert Hermann, who calculated that the present temperature of the big bang fireball should be about 5K. Inexplicably, their prediction was never acted upon, and by the 1960s was seemingly forgotten.
Wilson and Penzias discovered that relic glow, now known as the cosmic microwave background radiation (CMBR), for which they received the 1978 Nobel prize in physics.
For the first few hundred thousand years after the big bang it was too hot for electrically neutral atoms to exist. The Universe was filled with a soup of particles, electrons, protons, neutrons, neutrinos, and photons. The photons were constantly interacting with the charged particles, the electrons and protons. This scattering, the photon's transformations of energy related to interactions with charged particles, provided the mechanism by which the Universe attained thermal equilibrium (uniform temperature). The photon energies obeyed a black body distribution, reflecting the temperature of the Universe.
The Universe, however, was expanding, and thus cooling down. At some point (anywhere from 300,000 to 700,000 years after the bang, depending upon the details of the model), neutral atoms formed, which do not interact with the now free-streaming photons. The universe became transparent. The CMBR photons have not interacted with matter since that time, and thus provide a picture of the early Universe. The expansion of the Universe, the stretching of space-time, stretches the wavelengths of the CMBR photons and the black body photon distribution is red shifted as the Universe cools to yield its present temperature (which has been measured at all wavelengths and to great accuracy) of 2.73K.
The CMBR is, however, not perfectly uniform. One region of the sky is slightly hotter, while the diametrically opposed point is cooler. This dipole anisotropy reflects Earth's motion through the CMBR (in the general direction of the constellation Hydra) with a speed of about 550 km/s. High precision experiments performed by Earth-orbiting satellites and balloon-borne instruments have detected very tiny random fluctuations in temperature, which mirror density inhomogeneities in the early Universe. These regions of higher than average density seem to be about the size needed to seed structure, galaxy and galactic cluster formation, resulting in the clumpy universe observed today.
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