microwave background radiation
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microwave background radiation(mÿ -kroh-wayv) The remnant of the radiation content of the primordial fireball associated with the hot Big Bang. It is the dominant radiation of deep intergalactic space. A microwave background was first considered by George Gamow in the 1940s, and its existence with a temperature close to 5 K was predicted in 1948 by Ralph Alpher and Robert Herman. The concept was generally neglected and the prediction forgotten until the radiation was discovered in 1965 by Arno Penzias and Robert Wilson.
The microwave radiation is highly isotropic, showing that it must be at cosmological distances when compared to the highly clumpy nearby Universe. The Earth's atmosphere complicates detailed observation of the microwave radiation at millimeter wavelengths where it is most intense (see illustration), and it is best studied by satellite-borne instruments such as were carried by NASA's Cosmic Background Explorer, COBE.
COBE has measured the spectrum of the microwave background to be a perfect black-body curve characteristic of a temperature of 2.735 ± 0.06 K. The microwave background is not completely uniform, showing a slight dipole anisotropy with an enhancement of one part in a thousand in the direction of the Galaxy's motion and an equal and opposite deficit in the opposite part of the sky. This is due to aberration from the real motion of the Galaxy relative to the fixed background.
COBE has also revealed even weaker temperature fluctuations of one part in one hundred thousand on scales of ten degrees and larger. These are the imprints of quantum fluctuations in the early Universe. Further study of such fluctuations, in particular those due to acoustic oscillation of matter before recombination, will constrain cosmological models that determine how the early inhomogeneities collapse to form the large-scale structure we see in today's Universe. These can be directly related to the cosmic power spectrum.
The cosmological distance of the microwave background radiation implies that its photons were more energetic in the past. When the Universe was scarcely a few hundred thousand years old, they were energetic enough to photoionize hydrogen (see Big Bang theory) and their mean free path was drastically reduced by scattering with the free electrons.