An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

A type of → cosmic defect in the fabric of space-time predicted in some models of the early Universe.

→ cosmic; → texture

The time as measured by a clock that is at rest relative to the expanding space, and that has been set to zero at the very beginning, the time of the hypothetical → Big Bang singularity. The cosmic time is interpreted as the → age of the Universe (Einstein-online).

→ cosmic; → time.

The entire, large-scale structure of the → Universe in which → galaxy clusters are connected by → cosmic filaments (made up of → dark matter and → baryons) in a spongelike geometry, while the low-density → voids are connected to each other by low-density tunnels. The term cosmic web was coined in 1996 by J. Richard Bond et al. (Nature, 380, 603).

→ cosmic; → web.

An intense beam of cosmic rays coming from any direction on the sky, which originates outside the solar system.

→ cosmic; → ray; → burst.

Spurious signals in CCD frames caused by ionizing radiation which appear as a set of pixels with intense values sparsely scattered over the CCD frame. High energy particles generate muons, which deposit around 80 electrons per micron in silicon. With a collection depth of 10-20 microns, a cosmic-ray event is seen on a CCD frame as having a signal of up to a few thousand electrons, usually concentrated in one or two pixels. Although attributed to cosmic-ray hits, they may also be due to background terrestrial radiation.

→ cosmic rays; → event.

The ionization of → interstellar medium (ISM) gas by → cosmic rays. Cosmic rays are a primary source of ionization, competing with stellar → ultraviolet photons and → X-rays produced by embedded → young stellar objects. Cosmic rays play a key role in the chemistry and dynamics of the interstellar medium. The ionization fraction in turn drives the chemistry of → molecular clouds and controls the coupling of the gas with the Galactic → magnetic field. Moreover, cosmic rays represent an important source of → heating for → molecular clouds because the energy of primary and secondary electrons produced by the ionization process is in large part converted into heat by → inelastic collisions with ISM atoms and → molecules (see, e.g., Padovanit et al., 2009, arXiv:0904.4149).

→ cosmic; → ray; → ionization.

An extensive (many kilometres wide) → cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a → primary cosmic rays collides with atmospheric nuclei creating many → secondary cosmic rays. Also known as → air shower.

→ cosmic; → ray; → shower.

Cosmic rays which typically have energies in the range 10¹⁵ to 10²⁰ electron volts. For the most part, they are protons and other atomic nuclei, and come from distant cosmos, perhaps even from outside our own Galaxy.

→ high; → energy; → cosmic; → ray.

The → cosmic rays which arrive on the Earth's → atmosphere from the outer space. The primary cosmic rays are very high energy → protons and to a lesser extent heavier nuclei which rain upon the Earth from all diretions in the outer space. They contain about 90% protons, 7% → alpha particles and about 1% still heavier nuclei of amost all the atoms from Li to Ni ( → mass number< 60). See also: → secondary cosmic rays.

→ primary; → cosmic; → ray.

A burst of secondary charged and neutral particles arising when → primary cosmic rays collide with the atmospheric oxygen or nitrogen nuclei in the upper atmosphere. The collision produces mostly → pions (π), along with some → kaons (K), → antiprotons, and → antineutrons. Neutral pions very quickly decay, usually into two → gamma rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a → muon and a → neutrino. The fragments of the incoming nucleus also interact again, also producing new particles.

→ secondary; → cosmic; → ray.

A particle belonging to the most energetic population of → cosmic rays with an energy above ~ 10²⁰ → electron-volts. The UHECRs constitute a real challenge for theoretical models, because their acceleration requires extreme conditions hardly fulfilled by known astrophysical objects. See also → UHECR puzzle, → Greisen-Zatsepin-Kuzmin cutoff.

→ ultra- + → high-energy cosmic ray.