CMB

The cosmic microwave background (CMB) is the thermal radiation that permeates the universe, interpreted as the afterglow of the hot, dense early cosmos. It fills space almost uniformly and is most prominent in the microwave part of the spectrum. The CMB was accidentally discovered in 1965 by Arno Penzias and Robert Wilson, who detected a persistent isotropic microwave signal later identified as the remnant radiation from the Big Bang.

The CMB spectrum is an almost perfect blackbody with a present-day average temperature of about 2.725 kelvin. The photons come from the surface of last scattering, when the universe cooled enough for protons and electrons to combine into neutral hydrogen, about 380,000 years after the Big Bang (redshift z ≈ 1100). Before that, the photons were tightly coupled to matter and could not travel freely.

While nearly uniform, the CMB exhibits tiny temperature fluctuations of roughly one part in 100,000. These anisotropies reflect primordial density variations and have a characteristic angular power spectrum with a series of acoustic peaks, caused by oscillations in the photon-baryon fluid before recombination. The CMB is also polarized, providing additional information about early-universe physics and potential signatures of gravitational waves from inflation in the B-mode component.

Space missions such as COBE, WMAP, and Planck have measured the CMB with increasing precision, yielding tight constraints on cosmological parameters—such as the Hubble constant, the densities of baryons and dark matter, and the properties of dark energy—and supporting the standard Lambda-CDM model. Ongoing research aims to detect primordial B-modes and to refine our understanding of the early universe and inflation.