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Cosmological parameters
Cosmological parameters are the numerical quantities that specify a cosmological model, fixing the universe's composition, expansion history, geometry, and structure growth; the standard six-parameter ΛCDM model is measured at percent precision.
Cosmological perturbation theory
Cosmological perturbation theory is the mathematical framework describing small deviations from a uniform expanding universe, linking early-universe models to the cosmic microwave background and the large-scale distribution of galaxies.
Cosmological principle
The cosmological principle is the assumption that the Universe is homogeneous and isotropic on large scales, so no observer occupies a preferred position, underpinning the standard model of cosmology.
Cosmological redshift
Cosmological redshift is the increase in light's wavelength as it travels through an expanding universe, reported as the quantity z and used both as a cosmic clock and a distance indicator for mapping galaxies.
Cosmological simulation
A cosmological simulation is a computer model evolving a volume of the Universe from shortly after the Big Bang to the present, tracking the gravitational growth of cosmic structure.
Cosmology
Cosmology is the branch of astrophysics concerned with the origin, development, and structure of the universe, from its start 13.8 billion years ago to its fate.
Cosmology Large Angular Scale Surveyor
The Cosmology Large Angular Scale Surveyor (CLASS) is an array of four microwave telescopes in Chile's Atacama Desert, led by Johns Hopkins University, that measures cosmic microwave background polarization.
Cosmos
The cosmos (Κόσμος) is the universe as a whole: all space, time, matter, energy, stars, and galaxies, about 13.8 billion years old, studied through cosmology.
COSMOS-Web
COSMOS-Web is a 255-hour wide-field survey conducted with the James Webb Space Telescope in its first cycle, mapping the COSMOS field and expected to contain about a million galaxies.
CRACO
CRACO (Commensal Realtime ASKAP Fast Transient COherent) is a backend on the ASKAP radio telescope that detects fast radio bursts, pulsars, and ultra-long period objects.
Crimean Astrophysical Observatory
The Crimean Astrophysical Observatory is an astronomical research institute at Nauchny in the Crimean Mountains, founded in 1945, whose main 2.6-m Shajn Telescope was once the largest in Europe.
Critical density (cosmology)
Critical density is the average mass-energy density a universe expanding at a given rate needs for no spatial curvature; today it is about 10⁻²⁶ kg/m³.
Cross-correlation spectroscopy
Cross-correlation spectroscopy is an astronomical data-analysis technique that measures radial velocities by shifting a reference spectrum against an observed one, standard since Tonry and Davis formalized it in 1979.
Curvature perturbation
The curvature perturbation is a gauge-invariant quantity in relativistic cosmology measuring the spatial curvature of time slices of the perturbed universe, generated during inflation and observed in the cosmic microwave background.
Cusp–core problem
The cusp–core problem is the disagreement between the dense central cusps predicted by cold dark matter simulations and the shallower, constant-density cores observed in dwarf and low-surface-brightness galaxies.
DAMPE
DAMPE, the Dark Matter Particle Explorer nicknamed Wukong, is a Chinese satellite launched in 2015 to search for dark matter by observing high-energy gamma rays and cosmic rays.
Damped Lyman-alpha systems
Damped Lyman-alpha systems (DLAs) are quasar absorption-line systems with neutral hydrogen column densities above 2 × 10²⁰ cm⁻², used to probe galaxy formation and chemical evolution.
Daniel K. Inouye Solar Telescope
The Daniel K. Inouye Solar Telescope (DKIST) is the world's largest solar telescope, a 4-meter instrument on Haleakalā in Maui, Hawaii that measures the Sun's magnetic fields.
Dark energy
Dark energy is the unknown component causing the universe's expansion to accelerate, discovered in 1998 through distant supernovae and making up about 68% of the universe's energy.
Dark Energy Camera
The Dark Energy Camera (DECam) is a 570-megapixel imager on the Victor M. Blanco Telescope at Cerro Tololo, Chile, the primary instrument of the Dark Energy Survey.
Dark Energy Spectroscopic Instrument
The Dark Energy Spectroscopic Instrument is an optical spectrograph on the 4-meter Mayall Telescope at Kitt Peak, Arizona, built to measure dark energy's effect on cosmic expansion.
Dark flow
Dark flow is a disputed claim that galaxy clusters move together in a single direction, announced in 2008 from WMAP data, but Planck analyses found no detection.
Dark fluid
Dark fluid is a hypothetical single cosmic component acting as dark matter at high density and dark energy at low density, replacing ΛCDM's two dark sectors.
Dark matter
Dark matter is invisible matter inferred only from its gravitational effects, outweighing ordinary matter about five to one; hypothesized by Fritz Zwicky in 1933, it remains unidentified.
Dark matter halo
A dark matter halo, also called a dark halo, is a roughly spherical envelope of dark matter that surrounds galaxies and galaxy clusters and contains most of their mass.
Dark matter substructure
Dark matter substructure is the population of small dark matter clumps, called subhalos, within a galaxy's larger dark matter halo, tested via lensing, stellar streams, and gamma rays.
DASI
DASI, the Degree Angular Scale Interferometer, was a 13-element radio interferometer at the Amundsen-Scott South Pole station that measured cosmic microwave background anisotropy and its angular power spectrum.
DataLink
DataLink is a standard of the International Virtual Observatory Alliance, adopted in 2015, whose {links} service connects discovered astronomy data to related files, previews, and services like cutout generation.
Deceleration parameter
The deceleration parameter is a dimensionless number, written q, that measures whether the universe's expansion slows or speeds up; its present negative value confirms accelerating expansion.
Declination
Declination, also called the declination angle, is the celestial equivalent of latitude, measuring an object's angular distance north or south of the celestial equator, from 0° to ±90°.