Cosmic microwave background

The cosmic microwave background (CMB, CMBR), or relic radiation, is <a href="/facts/Microwave_radiation/HmVSN31Y">microwave radiation</a> that fills all space in the <a href="/facts/Observable_universe/CdTGMdsp">observable universe</a>. With a standard <a href="/facts/Optical_telescope/dVfBR10I">optical telescope</a>, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive <a href="/facts/Radio_telescope/SxIXUowL">radio telescope</a> detects a faint background glow that is almost <a href="/facts/Isotropic/mc3QEY2x">uniform</a> and is not associated with any star, galaxy, or other <a href="/facts/Astronomical_object/BCHLa0Q2">object</a>. This glow is strongest in the <a href="/facts/Microwave/HmVSN31Y">microwave</a> region of the electromagnetic spectrum. The accidental <a href="/facts/Discovery_of_cosmic_microwave_background_radiation/Lsh1IDUF">discovery of the CMB</a> in 1965 by American radio astronomers <a href="/facts/Arno_Allan_Penzias/MEautAvP">Arno Allan Penzias</a> and <a href="/facts/Robert_Woodrow_Wilson/CsmuvVqa">Robert Woodrow Wilson</a> was the culmination of work initiated in the 1940s.
The CMB is landmark evidence of the <a href="/facts/Big_Bang/I5T1vTnv">Big Bang</a> <a href="/facts/Scientific_theory/moMsLrUn">theory</a> for the origin of the universe. In the Big Bang <a href="/facts/Cosmological_model/ww4ChbEq">cosmological models</a>, during the earliest periods, the universe was filled with an <a href="/facts/Opacity_(optics)/l90q0sAV">opaque</a> fog of dense, hot <a href="/facts/Plasma_(physics)/1d3btMVU">plasma</a> of <a href="/facts/Sub-atomic_particle/tMEzcTWQ">sub-atomic particles</a>. As the universe expanded, this plasma cooled to the point where protons and electrons combined to form neutral atoms of mostly hydrogen. Unlike the plasma, these atoms could not scatter thermal radiation by <a href="/facts/Thomson_scattering/hUj6gSgB">Thomson scattering</a>, and so the universe became transparent. Known as the <a href="/facts/Recombination_(cosmology)/cUGysC0q">recombination</a> epoch, this <a href="/facts/Decoupling_(cosmology)/Kj6FbgLf">decoupling</a> event released <a href="/facts/Photons/mFNhAEzA">photons</a> to travel freely through space. However, the photons have grown less <a href="/facts/Photon_energy/Gy4zEAaE">energetic</a> due to the <a href="/facts/Cosmological_redshift/FJYfELCP">cosmological redshift</a> associated with the <a href="/facts/Expansion_of_the_universe/f6Uvt79B">expansion of the universe</a>. The surface of last scattering refers to a shell at the right distance in space so photons are now received that were originally emitted at the time of decoupling.
The CMB is very smooth and uniform, but maps by sensitive detectors detect small but important temperature variations. Ground and space-based experiments such as <a href="/facts/Cosmic_Background_Explorer/U3QfJ3Al">COBE</a>, <a href="/facts/Wilkinson_Microwave_Anisotropy_Probe/UUJWfy0U">WMAP</a> and <a href="/facts/Planck_(spacecraft)/cECmML33">Planck</a> have been used to measure these temperature inhomogeneities. The anisotropy structure is caused by various interactions of matter and photons up to the point of decoupling, which results in a characteristic pattern of tiny ripples that varies with angular scale. The <a href="/facts/Distribution_(mathematics)/yfaJ4mUp">distribution</a> of the anisotropy across the sky has <a href="/facts/Frequency/QUSbPaW8">frequency</a> components that can be represented by a <a href="/facts/Power_spectrum/q3Rfs7b0">power spectrum</a> displaying a sequence of peaks and valleys. The peak values of this spectrum hold important information about the physical properties of the early universe: the first peak determines the overall <a href="/facts/Curvature_of_the_universe/EJFEm9bw">curvature of the universe</a>, while the second and third peak detail the density of normal matter and so-called <a href="/facts/Dark_matter/y5HgX55N">dark matter</a>, respectively. Extracting fine details from the CMB data can be challenging, since the emission has undergone modification by foreground features such as <a href="/facts/Galaxy_cluster/BhIEgbx8">galaxy clusters</a>.

Cosmic microwave background open-in-new

Cosmic microwave background