CmbEdit
The Cosmic Microwave Background (CMB) is the afterglow of the hot, early universe, a pervasive field of microwave radiation that provides a snapshot of the cosmos at about 380,000 years after the Big Bang. It is the residual radiation from a time when matter and radiation were tightly coupled in a hot plasma, briefly forming a thermal bath that cooled as the universe expanded. Today the CMB fills all space and is observed with a nearly uniform temperature of about 2.725 kelvin, with minute fluctuations that encode the seeds of all structure in the Universe. Its discovery and subsequent detailed study have solidified the standard model of cosmology and offered a pristine testbed for fundamental physics Cosmic Microwave Background.
Because the CMB is a relic of the early universe, its properties connect directly to the physics of the Big Bang, inflation, and the growth of cosmic structure. The radiation we detect today is a redshifted remnant from the era of recombination, when photons decoupled from matter as electrons and nuclei combined to form neutral atoms. The near-perfect blackbody spectrum and the tiny fluctuations in temperature across the sky provide a wealth of information about the contents and geometry of the universe, including the densities of baryons and dark matter, the rate of expansion, and the characteristics of early-universe physics. The study of the CMB has become a cornerstone of modern cosmology and a proving ground for competing theories about the origin and evolution of the cosmos Cosmic Microwave Background.
History and discovery
The concept of a pervasive, relic radiation field in the universe was anticipated by early 20th-century cosmologists, but the empirical breakthrough came in 1964 when Arno Penzias and Robert Woodrow Wilson detected a steady, isotropic radio signal that could not be explained by known sources. The signal matched the predictions of a uniform, thermal radiation bath from the early universe, a finding later associated with the theoretical work of Ralph Alpher, Robert Herman, and George Gamow on a hot, dense origin for the cosmos. Subsequent missions and analyses identified the radiation as the cosmic microwave background, a discovery that earned recognition as one of the pivotal pillars of the Big Bang model and modern cosmology. The first satellite-scale measurements came from the COBE mission, which confirmed a near-blackbody spectrum and began to map large-scale anisotropies. Later, precision space missions such as WMAP and Planck (spacecraft) provided detailed maps of temperature fluctuations and polarization, refining the parameters of the cosmological model and the history of the universe Cosmic Microwave Background.
Physical characteristics
The CMB is observed as a nearly isotropic microwave glow across the sky, with a spectrum that closely matches a blackbody at a temperature of T0 ≈ 2.725 K. This spectrum is a key fingerprint of a universe that began in a hot, dense state and cooled as it expanded. The distribution of temperature across the sky is not perfectly uniform; it exhibits tiny anisotropies with typical amplitudes of a few parts in 100,000. These fluctuations trace density variations in the early universe that grew under gravity to form the large-scale structures we observe today, such as galaxies and clusters. The angular pattern of these fluctuations is commonly analyzed through its power spectrum, revealing a series of acoustic peaks that reflect the physics of a photon-baryon plasma in the pre-recombination era.
Polarization adds another crucial dimension. The CMB is partially polarized due to Thomson scattering of photons off free electrons during recombination and, to a lesser extent, during later epochs. The polarization pattern can be decomposed into E-modes and B-modes, each carrying different physical information. E-modes are primarily generated by density fluctuations, while B-modes could carry signatures of primordial gravitational waves from inflation or secondary sources such as gravitational lensing by large-scale structure. The study of CMB polarization thus provides complementary constraints on cosmology beyond temperature measurements. For a broader treatment, see Cosmic Microwave Background and CMB polarization.
The CMB also serves as a standard ruler in cosmology through the sound horizon at the time of recombination. The imprint of acoustic oscillations in the primordial plasma appears in the angular power spectrum and constrains the content and geometry of the universe, including the densities of baryons and dark matter and the overall curvature. These measurements are interpreted within the context of the ΛCDM model, sometimes referred to as the standard model of cosmology, and have driven precise determinations of several key parameters, such as the Hubble constant, the matter density, and the spectral index of primordial fluctuations. For related concepts, see Baryon acoustic oscillations and Lambda-CDM model.
Observations and missions
Observational progress has come from successive generations of space-borne and ground-based experiments. The early confirmation of the CMB spectrum by COBE established the thermal nature of the radiation and opened the door to anisotropy measurements. The era-defining results from WMAP delivered full-sky maps of temperature anisotropies and began to measure polarization, yielding tight constraints on cosmological parameters and offering compelling evidence for a flat, accelerating universe dominated by dark energy. The ongoing and planned efforts of missions and observatories, including the high-precision measurements by Planck (spacecraft), as well as ground-based facilities like the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT), continue to refine our understanding of the early universe and the growth of structure. Each instrument has contributed to a more precise view of the primordial fluctuations and the processes that shaped the modern cosmos Cosmic Microwave Background.
Implications for cosmology
The CMB provides a stringent testbed for cosmological theories. Its observed properties support the hot Big Bang paradigm and constrain the total energy budget of the universe, the composition of matter and energy components, and the expansion history. The nearly scale-invariant spectrum of primordial fluctuations is consistent with predictions from cosmic inflation, a period of rapid expansion that set the initial conditions for structure formation. Measurements of the CMB power spectrum, especially when combined with lower-redshift observations, yield tight limits on the densities of baryons and dark matter, the Hubble constant, and the spatial curvature, with current data indicating a spatially flat universe within observational uncertainties. The CMB also probes the epoch of reionization, providing insights into when the universe first formed galaxies and began to ionize the surrounding intergalactic medium. In addition, secondary anisotropies, such as the Sunyaev–Zel'dovich effect from hot gas in galaxy clusters, reveal the interplay between radiation and large-scale structure in the late-time universe. For related topics, see Cosmology, Big Bang, Inflation (cosmology), Recombination (astronomy), and Baryon acoustic oscillations.
Controversies and debates
As with any deep probe of the early universe, the CMB has generated questions and debates. The ample data from missions like Planck (spacecraft) and WMAP have largely reinforced the standard ΛCDM model, but attention remains on a set of large-scale anomalies that some researchers have discussed, including unusual alignments of low multipole moments and other hemispherical asymmetries observed in the temperature maps. The prevailing view is that these anomalies are intriguing but not yet compelling evidence for new physics; they may reflect statistical flukes, residual foregrounds, or unmodeled systematics, rather than a need to abandon the established cosmological framework. Ongoing and future analyses, including improved foreground removal, polarization measurements, and cross-correlations with other probes, aim to determine whether these features point to new physics or are simply statistical artifacts. For context on these discussions, see Axis of evil (cosmology) and CMB anomalies.