The Cosmic Microwave Background (CMB) is the faint afterglow of the Big Bang, now cooled to about 2.73 K. It permeates the universe and represents the oldest light observable, released roughly 380,000 years after the Big Bang when photons decoupled from matter and the cosmos became transparent.
Microwave observations reveal the CMB as a nearly uniform background with tiny temperature irregularities—about one part in 100,000. These fluctuations correspond to primordial density variations that later grew into galaxies, clusters, and the large-scale structure of the universe.
NASA’s COBE mission first measured the CMB’s perfect blackbody spectrum and detected its anisotropies. This was followed by NASA’s WMAP and ESA’s Planck, which produced increasingly precise temperature and polarization maps, enabling determination of the universe’s age, composition, and shape.
Advanced observatories like the Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT) focus on smaller-scale CMB variations, detecting gravitational lensing effects and the Sunyaev–Zel’dovich effect to map galaxy clusters and the cosmic web.
CMB polarization—classified as E-mode and B-mode patterns—offers critical insight into cosmic inflation and potential primordial gravitational waves, making it a key target for experiments like BICEP and future facilities.
As one of the three main pillars supporting the Big Bang model, the CMB provides the most precise measurements of key cosmological parameters, indicating the universe is approximately 13.8 billion years old and made up of ~5% normal matter, ~27% dark matter, and ~68% dark energy.