Cosmology and observation

Coded aperture imaging

Coded aperture imaging is a two-step technique that replaces focusing optics with a patterned mask, reconstructing sky images computationally, chiefly in hard X-ray and gamma-ray astronomy.

Coincidence counting

Coincidence counting is an experimental technique that counts events detected simultaneously, or within a set time interval, in two or more detectors, greatly improving the signal-to-noise ratio.

Cold dark matter

Cold dark matter (CDM) is a model of the universe's invisible mass as slow-moving collisionless particles, allowing structure to form hierarchically from the bottom up.

Color–color diagram

A color–color diagram is an astronomy plot graphing two colors of a celestial source, each a magnitude difference between two wavelength bands, used to classify stars, quasars, and galaxies.

Conformal cyclic cosmology

Conformal cyclic cosmology (CCC) is a cosmological model proposed by Roger Penrose in 2010, in which our universe is one aeon in an endless sequence of aeons.

Cooling flow

A cooling flow is the slow inward flow of hot X-ray-emitting gas at the centers of galaxy clusters, predicted in the 1970s but never observed as expected.

The Copernican principle is the foundational cosmological assumption that we are not preferred observers of the universe, named for Nicolaus Copernicus, whose 1543 heliocentrism displaced Earth from the center.

Corona

The corona is the outermost, million-degree layer of a star's atmosphere; the Sun's corona is visible during total solar eclipses and feeds the solar wind.

Coronagraphic imaging

Coronagraphic imaging is an astronomical technique that suppresses a star's light with a coronagraph so much fainter nearby objects, such as exoplanets and debris disks, can be imaged directly.

Coronagraphy

Coronagraphy is an optical technique that blocks a bright object's light so faint material near it, such as the Sun's corona or exoplanets, can be imaged; Bernard Lyot built the first coronagraph in 1931.

Cosmic Background Imager

The Cosmic Background Imager (CBI) was a 13-element radio interferometer built by Caltech that operated on Chile's Chajnantor Plateau from 1999 to 2008, measuring cosmic microwave background temperature and polarization.

Cosmic birefringence

Cosmic birefringence is the rotation of light's polarization plane as it crosses the universe, sought in the cosmic microwave background as a sign of parity-violating physics.

Cosmic chronometers

Cosmic chronometers are passively evolving galaxies whose differential stellar ages measure the expansion rate H(z) directly at each redshift, a method proposed by Jimenez and Loeb in 2002.

Cosmic coincidence problem

The cosmic coincidence problem, also called the "why now?" problem, asks why dark energy and matter have comparable densities today despite evolving differently as the universe expands.

Cosmic dipole

The cosmic dipole is a large-scale sky anisotropy seen in the cosmic microwave background and in distant quasars, whose mismatch, the cosmic dipole anomaly, exceeds 5σ.

Cosmic distance ladder

The cosmic distance ladder, or extragalactic distance scale, is the chain of overlapping methods that measures astronomical distances step by step, from parallax of nearby stars through Cepheid variables to Type Ia supernovae in distant galaxies.

Cosmic dust

Cosmic dust, also called astrophysical dust or extraterrestrial dust, consists of tiny solid particles in interstellar space that dim starlight and form the raw material of planets.

Cosmic Evolution Early Release Science Survey

The Cosmic Evolution Early Release Science Survey (CEERS) is a James Webb Space Telescope survey of the Extended Groth Strip to study early galaxy formation.

Cosmic Evolution Survey

The Cosmic Evolution Survey (COSMOS) is a multi-wavelength astronomical survey of a 2 square degree equatorial field probing how galaxies form and evolve over cosmic time.

Cosmic homogeneity scale

The cosmic homogeneity scale is the distance above which galaxies stop clustering fractally and become statistically uniform, typically measured at about 70 h⁻¹ Mpc in galaxy surveys.

Cosmic inflation

Cosmic inflation is a theory in which space expanded exponentially fast for a fraction of a second around 13.8 billion years ago, before the hot Big Bang.

Cosmic latte

Cosmic latte, or Cappuccino Cosmico, is the average color of light from galaxies in the observable universe, a pale beige computed by astronomers in 2002.

Cosmic microwave background

The cosmic microwave background (CMB), also called relic radiation, is leftover light from the universe at 380,000 years old, detected as microwaves near 2.725 K.

Cosmic microwave background spectral distortions

Cosmic microwave background spectral distortions are departures of the CMB's spectrum from a perfect blackbody, caused by energy released after thermalization fails in the early universe.

Cosmic shear

Cosmic shear is the correlated distortion of distant galaxies' apparent shapes by weak gravitational lensing, used to map dark matter and first detected independently by four groups in 2000.

Cosmic variance

Cosmic variance is the fundamental sampling uncertainty in cosmology that comes from observing only one Universe, limiting measurements like the cosmic microwave background power spectrum.

CosmicFlows

CosmicFlows is a series of catalogs of galaxy distances and peculiar velocities, led by R. Brent Tully at the University of Hawaiʻi, running from Cosmicflows-1 (2008) to Cosmicflows-4 (2023).

Cosmogony

Cosmogony is the study or account of the origin of the universe, in mythic, theological, philosophical, or scientific form, distinct from cosmology, which studies structure and evolution.

Cosmological constant

The cosmological constant, written Λ, is a term in Einstein's general relativity, introduced in 1917 and now the simplest model of the universe's accelerating expansion.