Cosmic microwave background (CMB or CMBR, CBR, MBR)
radiation
is weak microwaveelectromagnetic 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.
They are what is left of existing photons constantly being 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.