cosmic microwave background
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cosmic microwave background is the thermal electromagnetic radiation left over from the early universe, a remnant of the hot, dense state following the Big Bang. It fills the entire observable universe and is a cornerstone of modern cosmology, providing a snapshot of the universe when it was only 380,000 years old. Its detection and study have profoundly shaped our understanding of the universe's composition, geometry, and evolution.
Discovery and observation
The existence of a relic radiation field was first predicted in the 1940s by George Gamow, Ralph Alpher, and Robert Herman based on Big Bang nucleosynthesis models. Its accidental discovery in 1965 by Arno Penzias and Robert Wilson of Bell Labs using a Holmdel Horn Antenna earned them the Nobel Prize in Physics. This discovery provided decisive evidence against the rival steady-state theory championed by Fred Hoyle. Subsequent observations by instruments like the Cosmic Background Explorer (COBE) satellite, which precisely measured its black-body spectrum, and later missions such as the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite have mapped its minute temperature variations with extraordinary precision, revolutionizing the field.
Physical characteristics
The radiation is remarkably uniform, with an average temperature of approximately 2.725 kelvins, corresponding to a peak wavelength in the microwave region of the electromagnetic spectrum. Its spectrum is that of an almost perfect black body, the most precise such spectrum ever measured in nature, as confirmed by the Far-Infrared Absolute Spectrophotometer on COBE. This thermal radiation is highly isotropic, meaning it appears nearly the same in all directions, with only tiny fluctuations of about one part in 100,000. The photons have been redshifted by a factor of over 1000 since their emission due to the expansion of the universe.
Origin and cosmological significance
It originated during the epoch of recombination, when the expanding universe cooled sufficiently for protons and electrons to combine into neutral hydrogen atoms. This event made the universe transparent to radiation, allowing photons to travel freely through space; these are the photons observed today. This surface of last scattering provides a direct image of the infant universe. Its existence and properties are considered the strongest evidence for the hot Big Bang model, ruling out alternative theories. It serves as a backlight for studying the formation of large-scale structures like galaxy clusters through effects such as the Sunyaev–Zel'dovich effect.
Anisotropies and polarization
While highly isotropic, it contains tiny temperature anisotropies, or fluctuations, which were first detected by COBE and mapped in detail by WMAP and Planck. These anisotropies are categorized into different angular scales: primary anisotropies imprinted at recombination, and secondary anisotropies caused by interactions with intervening structures, like the integrated Sachs–Wolfe effect. The radiation also exhibits polarization, caused by Thomson scattering of photons off electrons in the early universe. This polarization contains two types: E-modes, generated by density fluctuations, and B-modes, which can be a signature of primordial gravitational waves from the epoch of cosmic inflation, as investigated by experiments like BICEP and Keck Array.
Implications for cosmology
Precise measurements of its anisotropies have allowed cosmologists to determine key parameters of the Lambda-CDM model with great accuracy. These include the age of the universe (about 13.8 billion years), its geometry (very close to flat), and its composition: ordinary baryonic matter, cold dark matter, and dark energy. Data from Planck have tightly constrained models of cosmic inflation and provided limits on the number of neutrino species. It also informs studies of reionization and helps test fundamental physics, including potential variations in the fine-structure constant and the cosmological principle.