Cosmic microwave background

The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology, and it provides a snapshot of the infant universe when it first became transparent to photons approximately 380,000 years after the Big Bang.

Discovery and Historical Significance

The existence of a cosmic background radiation was a key prediction of the Big Bang theory, originally postulated by George Gamow, Ralph Alpher, and Robert Herman in the mid-20th century. It was serendipitously discovered in 1965 by Arno Penzias and Robert Wilson at Bell Telephone Laboratories. While calibrating a sensitive horn antenna, they detected an persistent, isotropic microwave signal that corresponded to a blackbody temperature of approximately 3.5 Kelvin. This discovery, for which they received the Nobel Prize in Physics in 1978, provided overwhelming evidence for the Big Bang model over its then rival, the steady-state theory. Subsequent measurements, particularly by the Cosmic Background Explorer (COBE) satellite in the early 1990s, confirmed with exquisite precision that the CMB spectrum is that of a near-perfect blackbody with a temperature of 2.72548 ± 0.00057 Kelvin.

Physical Origin and Properties

The CMB originates from the epoch of recombination, when the expanding and cooling universe had cooled sufficiently (to around 3000 K) for electrons and protons to combine and form neutral hydrogen atoms. Prior to this epoch, the universe was an opaque plasma of photons, electrons, and baryons, where photons were constantly scattered by free electrons via Thomson scattering. Once neutral atoms formed, photons could travel freely, "decoupling" from matter. These photons, last scattered some 13.8 billion years ago, have been traveling through the expanding universe ever since, undergoing a cosmological redshift by a factor of about 1100. This redshift transforms the original near-infrared and optical radiation into the microwave part of the electromagnetic spectrum observed today. The CMB is remarkably isotropic and homogeneous, with a nearly perfect Planck blackbody spectrum, indicating its origin in a hot, dense, and uniform early state.

Anisotropies and Cosmological Information

While the CMB is extraordinarily uniform, precise measurements reveal tiny temperature fluctuations, or anisotropies, at the level of about one part in 100,000. These anisotropies are of paramount importance to cosmology. They are categorized into primary and secondary anisotropies. Primary anisotropies are imprinted on the CMB at the surface of last scattering and during its journey to us. They arise from several physical processes:

• Sachs-Wolfe Effect: Photons climbing out of gravitational potential wells (dense regions) or descending from hills (underdense regions) at the last scattering surface gain or lose energy, creating temperature variations linked to the underlying density fluctuations.
• Acoustic Oscillations: Prior to recombination, the photon-baryon fluid underwent sound waves driven by the competition between gravitational collapse and radiation pressure. These oscillations were frozen in at recombination, creating a characteristic pattern of peaks and troughs in the angular power spectrum of the CMB temperature map.
• Doppler Shifts: The bulk motion of the photon-baryon fluid at the last scattering surface imparts a Doppler shift to the photons.

The precise angular scale and amplitude of these acoustic peaks provide a wealth of information about the fundamental parameters of the universe, including its geometry (flat, open, or closed), the density of ordinary matter (baryons) and dark matter, the Hubble constant, and the initial conditions for structure formation as described by inflationary theory.

Secondary anisotropies are imposed on the CMB after recombination, such as the Sunyaev–Zel'dovich effect (inverse Compton scattering by hot electrons in galaxy clusters) and gravitational lensing by large-scale structure.

Observations and Major Experiments

Ground-based, balloon-borne, and space-based experiments have progressively mapped the CMB with increasing resolution and sensitivity.

• COBE (1989-1993): Confirmed the perfect blackbody spectrum and detected the large-scale anisotropies.
• Wilkinson Microwave Anisotropy Probe (WMAP, 2001-2010): Provided a detailed full-sky map, precisely measuring the temperature and polarization anisotropies and establishing the current standard model of cosmology (Lambda-CDM model) with unprecedented accuracy.
• Planck satellite (2009-2013): The most sensitive mission to date, Planck mapped temperature and polarization anisotropies with the highest resolution, further constraining cosmological parameters and providing stringent tests of inflation and the nature of dark matter and dark energy.

Current and future experiments focus on measuring the CMB's polarization, particularly the faint "B-mode" polarization patterns, which could carry the imprint of primordial gravitational waves from the inflationary epoch.

Significance in Modern Cosmology

The CMB is the oldest and most distant observable light in the universe. Its study has revolutionized cosmology, transforming it from a speculative field into a precise observational science. The temperature anisotropies are the seeds from which all cosmic structure—galaxies, clusters, and the cosmic web—eventually grew under the influence of gravity. The CMB provides the most compelling evidence for the Big Bang, constrains the composition and evolution of the universe, and offers a direct probe of physics at energies far beyond the reach of terrestrial particle accelerators, from the first fraction of a second after the Big Bang (inflation) to the formation of the first atoms. It remains the cornerstone of the Lambda-CDM model, the prevailing standard model of cosmology.