The Cosmic Microwave Background is the relic electromagnetic radiation left over from the hot early Universe and now observed as an almost uniform microwave background across the entire sky. It was produced when the expanding Universe cooled sufficiently for electrons and nuclei to combine into neutral atoms, greatly reducing the scattering of photons by free charged particles. Before this transition, photons were repeatedly scattered by the dense ionised plasma and could not travel freely over cosmological distances.
Once neutral atoms formed, photons decoupled from matter and began travelling largely unhindered. Cosmic expansion subsequently stretched their wavelengths, transforming the radiation from its original much higher temperature state into the microwave spectrum observed today. The present blackbody temperature is approximately 2.725 kelvin. The Cosmic Microwave Background therefore provides a direct observational record of the Universe at an early stage and is one of the principal foundations of modern observational cosmology.
The physical origin of the Cosmic Microwave Background is closely connected with the thermal history of the early Universe. At sufficiently high temperatures, matter existed as a plasma in which electrons and nuclei were not bound into stable neutral atoms. Photons interacted frequently with charged particles through Thomson scattering, maintaining approximate thermal equilibrium. As the Universe expanded and cooled, hydrogen recombination occurred and the number of free electrons dropped sharply. Photons then travelled with much greater freedom.
The surface from which the photons last scattered is commonly called the last scattering surface, although it is not a physical shell located at one particular place in space. Because the Universe has expanded since then, the radiation has been redshifted. The observed microwave photons therefore contain information about conditions roughly 380000 years after the beginning of the hot early expansion phase described by the Big Bang Theory.
The Cosmic Microwave Background has an exceptionally close blackbody spectrum. Measurements show that its frequency distribution is extremely well described by thermal radiation with a temperature near 2.725 kelvin, with deviations constrained to very small levels. This spectrum is important because it indicates that the early Universe achieved a high degree of thermal equilibrium before photon decoupling.
The nearly perfect blackbody character would be difficult to explain if the background were simply a collection of unrelated radiation sources produced by stars or galaxies. The radiation has been stretched by cosmic expansion, but redshifting a blackbody spectrum preserves its blackbody form while reducing its effective temperature. The CMB is therefore not merely faint microwave light. Its detailed spectrum records the thermal state of the early Universe and provides evidence that the observable cosmos evolved from a much hotter, denser condition.
The tiny temperature fluctuations in the Cosmic Microwave Background are among its most scientifically valuable features. The average temperature is remarkably uniform, but anisotropies occur at approximately the tens of microkelvin level. These fluctuations correspond to small variations in density, gravitational potential and photon temperature at the epoch of last scattering. Acoustic oscillations in the tightly coupled photon baryon plasma produced characteristic peaks in the angular power spectrum.
The locations and amplitudes of these peaks contain information about the geometry of space, the baryon density, the total matter density, the primordial fluctuation spectrum and other cosmological parameters. The first acoustic peak is especially important for constraining spatial curvature, while higher peaks provide information about baryonic effects and matter content. The CMB therefore functions as a precision measurement of the early Universe rather than simply a historical background glow.
Polarisation provides another major source of information in the Cosmic Microwave Background. Thomson scattering can polarise radiation when the incoming radiation field has an appropriate quadrupole anisotropy. CMB polarisation is commonly separated into E mode and B mode patterns. E modes have been detected extensively and provide information about density perturbations, reionisation and gravitational effects. B modes can be generated by gravitational lensing of E modes and, in some early Universe models, could contain a contribution from primordial gravitational waves.
Measuring primordial B modes is extremely challenging because foreground emission from the Milky Way and lensing signals must be carefully modelled and separated. CMB polarisation also helps constrain the optical depth to reionisation and improves estimates of cosmological parameters. It therefore adds independent information beyond the temperature anisotropies.
The Cosmic Microwave Background is observed using instruments placed on the ground, on balloons and in space. Satellite missions have been particularly important because the Earth's atmosphere absorbs and emits at microwave wavelengths and because full sky measurements require highly stable instrumentation. Major experiments have progressively improved angular resolution, frequency coverage and sensitivity, allowing increasingly precise maps of temperature and polarisation.
Multiple frequency channels are necessary because Galactic dust, synchrotron emission, free free emission and extragalactic foregrounds contaminate the cosmological signal. Data analysis therefore involves calibration, foreground removal, map making, statistical estimation and detailed comparison with theoretical models. CMB observations are also affected by secondary processes occurring after recombination, including gravitational lensing, scattering by hot electrons and interactions with intervening structures. Modern CMB cosmology consequently requires both precise measurement of the primary signal and careful modelling of later astrophysical modifications.
The Cosmic Microwave Background transformed cosmology from a largely qualitative field into a precision quantitative science. Its discovery in 1965 by Arno Penzias and Robert Wilson provided decisive observational support for a hot early Universe and helped establish the Big Bang Theory over competing steady state interpretations. Subsequent measurements demonstrated the near perfect blackbody spectrum and eventually detected small anisotropies.
The COBE mission measured the spectrum with exceptional precision and detected temperature fluctuations, while later missions such as WMAP and Planck provided increasingly detailed maps. These observations constrained cosmological parameters and established a standard framework involving ordinary matter, dark matter and dark energy. The historical progression from discovery to precision measurement illustrates how a faint all sky background became one of the most informative datasets in modern science.
The Cosmic Microwave Background should be distinguished from the general cosmic background radiation produced by later astrophysical sources. The CMB specifically refers to relic radiation from the early Universe, whereas the total microwave sky also contains Galactic and extragalactic foregrounds. It is also incorrect to describe the CMB as radiation coming from the centre of the Universe. Because the observable Universe is approximately homogeneous and isotropic on large scales, every observer would see a corresponding last scattering surface surrounding them.
The CMB does not mark a conventional physical edge of space. Another important distinction is between the CMB temperature and the temperature of ordinary matter today. The CMB photons have cooled through cosmic expansion, while matter in galaxies and clusters can reach much higher temperatures through gravitational and astrophysical processes. The CMB is therefore best understood as a relic radiation field whose detailed properties encode the early conditions and subsequent evolution of the Universe.
