The Cosmic Microwave Background Radiation

Perhaps the most conclusive, and certainly among the most carefully examined, piece of evidence for the Big Bang is the existence of an isotropic radiation bath that permeates the entirety of the Universe known as the "cosmic microwave background" (CMB). In 1965, two young radioastronomers, Arno Penzias and Robert Wilson, almost accidentally discovered the CMB using a small, well-calibrated horn antenna. It was soon determined that the radiation was diffuse, emanated unifromly from all directions in the sky, and had a temperature of approximately 2.7 Kelvin. Initially, the two young scientists were bereft of a satisfactory explanation for their observations, and considered the possibility that the CMB may have been due to some undetermined systematic noise. However, it soon came to their attention through Robert Dicke and Jim Peebles of Princeton that this background radiation had in fact been predicted years earlier by George Gamow as a relic of the evolution of the early Universe.

If the universe was once very hot and dense the photons and baryons would have formed a plasma. As the universe expanded and cooled there came a point when the radiation (photons) decoupled from the matter. The radiation cooled and is now at 2.7 Kelvin. The fact that the spectrum (see figure) of the radiation is almost exactly that of a black body implies that it could not have had its origin through any prosaic means. This has led to the death of the steady state theory.

Further investigations, including more recent ones by the COBE satellite (Smoot et. al.), confirmed the virtual isotropy of the CMB to better than one part in ten-thousand.

A map of the sky at microwave frequencies, showing that the CMB is almost completely the same in all directions.

Given this qualification, any attempt to interpret the origin of the CMB as due to present astrophysical phenomena (i.e. stars, radio galaxies, etc.) is discredited. Therefore, the only satisfactory explanation for the existence of the CMB lies in the physics of the early Universe.

While the CMB is predicted to be very smooth, the lack of features cannot be perfect. At some level one expects to see irregularities, or anisotropies, in the temperature of the radiation. These temperature fluctuations are the imprints of ...

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