An Etymological Dictionary of Astronomy and Astrophysics
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The period of time in the early history of the Universe, between the → recombination era and the advent of the → first stars.

→ cosmic; → dark; → age.

Topological irregularities in the → space-time  → continuum, caused by the abrupt cooling of the → early Universe shortly after the → Big Bang, as predicted by some → cosmological models. These regions of immensely high density might have been the seeds of → structure formation through → gravity. Same as → topological defect.

→ cosmic; → defect.

Measurement of the distances to the farthest objects in the Universe based on a bootstrapping series of methods, each applicable to more distant objects, and each dependent on the previous methods.

→ cosmic; → distance; → scale.

Aggregations of matter on the order of a fraction of a micron across, irregularly shaped, and composed of → carbon and/or → silicates found in the → interstellar medium. Dust absorbs stellar light causing large dark patches in regions of the → Milky Way Galaxy and dark bands across other galaxies.

→ cosmic; → dust.

Same as the → Layzer-Irvine equation.

→ cosmic; → energy; → equation.

Same as the → expansion of the Universe.

→ cosmic; → expansion.

A galaxy at a → redshift of z = 2.3259 lying behind a massive → cluster of galaxies and magnified by the → lensing effect of the cluster. It was first discovered in → submillimeter waves. The lensing cluster lies at a redshift z > 1.5 causing an → amplification factor for the background galaxy of 32 (A. M. Swinbank et al. 2010, Nature 464, 733).

→ cosmic; eyelash, from → eye + lash, from M.E. lashe (n.) lashen (v.) "to blow, stroke." Such called because of its narrow and elongated shape.

Možé "eyelash," from Mid.Pers. mec "eyelash," mecitan "to blink;" cf. Skt. mes "to open the eyes;" O.C.S. po-mežiti "to close the eyes;" keyhâni, → cosmic.

A very large-scale structure made of → galaxy clusters threaded like beads on a chain. Cosmic filaments are chiefly made up of → dark matter but also, to a lesser extent, of → baryonic matter. They are the largest entities in the → Universe and can be up to 1 billion → light-years long. They are separated by great → voids.

→ cosmic; → filament.

The → observable region of the → Universe, limited in extent by the distance → light has traveled during the time elapsed since the beginning of the Universe (→ Big Bang). No signal from the objects lying beyond the cosmic horizon can be received because light has not yet had enough time to travel the distance. The cosmic horizon can be defined in two ways:
1) The size of the → observable Universe as derived from ct, where c is the → speed of light and t is the → age of the Universe, 13.8 billion years, hence 13.8 billion → light-years.
2) The → comoving distance. The distance given above corresponds to the size Universe had 13.8 billion years ago. Since then the Universe has been growing at a rate of 3.52c. Therefore, the current radius of the observable Universe is about 48 × 10⁹ light-years. Same as → particle horizon, → Hubble distance, → Hubble radius, and → Hubble length. See also → sound horizon.

→ cosmic; → horizon.

A diffuse radiation which consists of the cumulative infrared emission from all galaxies throughout cosmic history. It is about 50 times weaker than the → cosmic microwave background radiation (CMBR). Since the CIB is produced by the dust within such galaxies, it carries a wealth of information about the processes of star formation therein.

→ cosmic; → infrared; → background.

Tiny fluctuations in the intensity of the → cosmic microwave background radiation (CMBR) as a function of angular position over the sky, first discovered in the → Cosmic Background Explorer (COBE) observations. At a level of 1 part in 100,000, these temperature variations trace the distribution of matter and energy when the Universe was very young, about 380,000 years old. Since the CMB spectrum is described to a high precision by a → blackbody law with temperature T₀, it is usual to express the anisotropies in terms of temperature fluctuations ΔT/T₀ and expand them on the sky in → spherical harmonic series ΔT/T₀ (θ,φ) = Σ a_lmY_lm(θ,φ), where θ and φ are the → spherical polar coordinates, Y_lm is the spherical harmonic functions with → multipole index l, and the sum runs over l = 1, 2, ..., ∞, m = -l, ..., l, giving 2l + 1 values of m for each l, and a_lm is the multipole moment of the decomposition. The power spectrum of the anisotropies is defined as C_l≡ mean | a_lm |² = 1/(2l + 1) Σ mean | a_lm |². See also → CMB angular power spectrum.

→ cosmic; → microwave; → background; → anisotropy.

The polarization of the → cosmic microwave background radiation due to → Thomson scattering by → free electrons during the → recombination era. The polarization can greatly enhance the precision with which the parameters associated with → acoustic oscillations are derived; because it carries directional information on the sky. When an → electromagnetic wave is incident on a free electron, the scattered wave is polarized perpendicular to the incidence direction. If the incident radiation were → isotropic or had only a → dipole variation, the scattered radiation would have no net polarization. However, if the incident radiation from perpendicular directions (separated by 90°) had different intensities, a net → linear polarization would result. Such → anisotropy is called → quadrupole because the poles of anisotropy are 360°/4 = 90° apart.

→ cosmic; → microwave; → background; → polarization.

The diffuse → electromagnetic radiation in the → microwave band, coming from all directions in the sky, which consists of relic photons left over from the very hot, early phase of the → Big Bang. More specifically, the CMBR belong to the → recombination era, when the → Universe was about 380,000 years old and had a temperature of about 3,000 K, or a → redshift of about 1,100. The photons that last scattered at this epoch have now cooled down to a temperature of 2.73 K. They have a pure → blackbody spectrum as they were at → thermal equilibrium before → decoupling. The CMB was discovered serendipitously in 1965 by Penzias and Wilson (ApJ L 142, 419) and was immediately interpreted as a relic radiation of the Big Bang by Dicke et al. (1965, ApJL 142, 383). Such a radiation had been predicted before by Gamow (1948, Nature 162, 680) and by Alpher and Herman (1948, Nature 162, 774). This discovery was a major argument in favor of the Big Bang theory. In 1992, the satellite → Cosmic Background Explorer (COBE) discovered the first anisotropies in the temperature of the CMB with an amplitude of about 30 µK. See also: → cosmic microwave background anisotropy, → dipole anisotropy, → CMB lensing, → CMB angular power spectrum, → acoustic peak, → baryon acoustic oscillation, → WMAP.

→ cosmic; → microwave; → background; → radiation.

The theoretical → low-energy neutrinos that decoupled from the rest of matter about two seconds after the → Big Bang when the temperature dropped to approximately 2.5 MeV (redshift of z ~ 6 ×10⁹). The CNB is similar to the → cosmic microwave background (CMB), but older. It is estimated that today the CNB has a temperature of T_ν = (4/11)^1/3T_γ, ~ 1.95 K (or 1.67 × 10^-4 eV), where T_γ is the CMB temperature of 2.728 K. Also called the relic neutrinos.

→ cosmic; → neutrino; → background.

Radio waves emanating from extraterrestrial sources.

→ cosmic; → radio; → noise.

Extremely energetic atomic nuclei which travel through the Universe at practically the speed of light and strike the Earth from all direction. Almost 90% of all the incoming → primary cosmic rays are → protons, about 9% are helium nuclei (→ alpha particles) and about 1% are → electrons (beta minus particles). Some cosmic rays come from the Sun (mainly due to → solar flares), most come from galactic → supernovae, and a few with the highest energy are suspected to originate from outside the → Milky Way. As for their flux, about 1 charged particle per second per cm² impacts the Earth. The typical kinetic energy of these particles is about 10 MeV/nucleon to several GeV/nucleon, although there are some at higher energies. In fact, the cosmic ray with the highest energy has been measured above × 10²⁰ eV. These → ultra-high energy cosmic rays are suspected to be extragalactic, as there is no plausible mechanism of acceleration to these energies by a supernova, for example. Again, compare these energies to those of solar neutrinos that have only 0.26 MeV. Cosmic rays may be divided into → primary cosmic rays and → secondary cosmic rays. Their energy ranges from 10⁹ to 10²⁰  → electron-volts.

→ cosmic; → ray; The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.

A quantity, denoted a(t), which describes how the distances between any two galaxies change with time. The physical distance d(t) between two points in the Universe can be expressed as d(t) = R(t).x, where R(t) is the → scale factor and x the → comoving distance between the points. The cosmic scale factor is related to the → redshift, z, by: 1 + z = R(t₀)/R(t₁), where t₀ is the present time and t₁ is the time at emission of the radiation. The quantity (1 + z) gives the factor by which the → Universe has expanded in size between t₁ and t₀. It is also related to the → Hubble parameter by H(t) = R^.(t)/R(t), where R^.(t) is the time → derivative of the scale factor. In an → expanding Universe the scale factor increases with time. See also the → Friedmann equation.

→ cosmic; → scale; → factor.

The distortion of images of distant galaxies due to → weak gravitational lensing by → large-scale structures in the → Universe (see, e.g., Kilbinger, M., 2015, arXiv:1411.0115).

→ cosmic; → shear.

A crucial period in the history of the → Universe, when the bulk of stars in massive galaxies were likely formed. Observations of young stars in distant galaxies at different times in the past have indicated that the → star formation rate peaked at the → redshift of z ~ 2, some 10 billion years ago, before declining by a factor of around ten to its present value (P. Madau & Dickinson, 2014, arXiv:1403.0007).

→ cosmic; → star; → formation; → peak.

A hypothetical → cosmic defect predicted to be infinitesimally small in cross section but enormously long and massive. Cosmic strings should not be confounded with → subatomic strings predicted by → string theory.

→ cosmic; → string.