cosmic microwave background

Cosmic microwave background (CMB or CMBR, CBR, MBR) radiation is weak microwave electromagnetic radiation detectable in all directions, presumed to be a result of the Big Bang. It is quite uniform (showing just 0.04% wavelength-variation in its peak strength, throughout the celestial sphere) but the variation that exists (CMB anisotropies) is studied as a means of studying the early universe. The CMB was discovered by accident in 1964 by Arno Penzias and Robert Wilson, earning them the 1978 Nobel Prize, having been predicted in 1948 by Ralph Alpher and Robert Herman.

The photons we see have been traveling toward us since the time of recombination, representing a "picture" of that time, at about redshift 1090, before that having been constantly scattered since the time of Big Bang itself. At recombination, the universe became transparent because cooling electrons and protons throughout space settled down enough to combine into neutral hydrogen, which is much less likely to scatter photons. Those reaching us are those that happened to be coming our way and happened to be within the portion of the universe surrounding us at the distance light can travel since recombination, a spherical shell-shaped region surrounding us termed the surface of last scattering. Their spectrum is basically a redshifted 3000 K black-body spectrum (the temperature at the time of recombination), appearing now as a 2.725 K black body spectrum.

Other types of radiation from every direction (from other causes), together known as the cosmic background radiation (CBR), have also been found, though the phrase cosmic background radiation is sometimes used to mean specifically the CMB.

The phrase CMB foreground refers to microwave emission from nearer sources at the same frequencies. Such nearer sources are of interest to CMB researchers so they can be accounted for, which is necessary to work out an accurate picture of the CMB.

In planetary science, the initials CMB are also used to abbreviate an entirely different phrase: core-mantle boundary, i.e., the border of a planet's core.