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

1) General: Related to or located at the back "the side or surface opposite the front or face."
2) The side of the → planispheric astrolabe opposite the hollow of the → mater, → tympanum, and → rete. At its center is hinged the → alidade, whose rim slides on a set of graduated circular scales engraved on the mater. The number of scales varies from one instrument to another. On this face the user can perform observations and read the data needed to set the front side of the instrument in the proper position (online museo galileo, VirtualMuseum).

Back, from M.E., from O.E. bæc; akin to O.H.G. bah "back."

Pas-, from pas "behind" (e.g.: pas-e pardé "behind the curtain"); Mid.Pers. pas "behind, before, after;" O.Pers. pasā "after;" Av. pasca "behind (of space); then, afterwards (of time);" cf. Skt. paścā "behind, after, later;" L. post "behind, in the rear; after, afterwards;" O.C.S. po "behind, after;" Lith. pas "at, by;" PIE *pos-, *posko-.
Pošt-, from pošt, variant of pas.

In a radiotelescope, the unit forming the end part of the reception chain. It generally consists of a spectrometer and performs frequency analysis of the signals. → front-end.

Back-end, from → back + end, from M.E., O.E. ende (cf. Du. einde, O.H.G. enti "top, forehead, end," Ger. ende, Goth. andeis "end"), originally "the opposite side," from PIE *antjo "end, boundary," from base *anta-/*anti- "opposite, in front of, before."

Pas-tah, from pas, → back, + tah "end," Mid.Pers. tah "bottom." The origin of this term is not clear. It may be related to Gk. tenagos "bottom, swamp," Latvian tigas<*tingas < *tenegos "depth," PIE *tenegos "water bottom."

General: That part of a view or scene that serves as a setting for the main objects, persons, etc.
Astro.: The part of an observed field which is not physically related to the objects of interest and lies behind them.

Background, from → back + ground, from M.E., from O.E. grund; akin to O.H.G. grunt "ground."

Paszaminé, from pas-, → back, + zaminé "ground," from zamin "ground," → earth.

→ foreground.

An unwanted signal in a system which is producing or recording a signal. For instance, a randomly fluctuating signal superimposed on the signal from a cosmic radio source.

→ background; → noise.

The isotropic residual microwave radiation in space left from the primordial → Big Bang. Same as → cosmic microwave background (CMB) radiation.

→ background; → radiation.

1) (v.tr.) To deflect photons or particles in a direction opposite to their initial path.
2) Same as → backscattering.
3) The radiation or particles so deflected.

From → back + → scattering.

The light that has undergone → backscattering.

→ backscatter; → backscattering.

Scattering of radiation or particles through angles greater than 90° with respect to the original direction of motion.

→ scattering.

A copy of computer files that is stored separately from the original in order to protect against loss of data.

Backup "substitute, support," from → back + up.

Poštvân "prop, support, help" from pošt, → back, + -vân suffix denting protection, variant of -bân.

An auxiliary observing program to be carried out at telescope in case the atmospheric conditions make the main program unfeasible.

Backup, from → back + up; → program.

Barnâmé, → program; yadaki "reserve, substitute," from yadak "a led horse."

1) Toward the back or rear.
2) Directed toward the back or past.

→ back; → -ward.

A sort of → greenhouse effect in → stellar atmospheres where the deeper layers heat up due to overlying → opacity. The presence of numerous → bound-bound opacities of → metals amplifies the → scattering of → photons, in particular their → backscattering, forcing the → temperature to increase in order to conserve the radiation flux and the transport of energy from the interior to the outer parts of the atmosphere.

→ back; → warming; → effect.

NASA's satellite, designed to measure the diffuse infrared and → cosmic microwave background radiation from the early → Universe. It was launched on November 18, 1989 and carried three instruments: DIRBE (the Diffuse InfraRed Experiment), DMR (Differential Microwave Radiometers), and FIRAS (Far-InfraRed Absolute Spectrophotometer). The COBE observations showed that the cosmic microwave background spectrum matches that of a → blackbody of temperature 2.725 ± 0.002 K. COBE also found anisotropies in the cosmic microwave background at a level of a part in 100,000 (→ cosmic microwave background anisotropy). These tiny variations in the intensity of the CMB over the sky show how matter and energy was distributed when the Universe was still very young. Later, through a process still poorly understood, the early structures developed into galaxies, galaxy clusters, and the large scale structure that we see in the Universe today. Two of COBE's principal investigators, George Smoot and John Mather, received the Nobel Prize in Physics in 2006 for their work on the project.

→ cosmic; → background; → explorer.

→ cosmic microwave background radiation (CMBR).

→ cosmic; → background; → radiation.

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.

The integrated intensity of all of the light emitted throughout the history of the Universe across the whole of the → electromagnetic spectrum, including those which are not individually detected. The EBL spectrum includes cosmological backgrounds associated with either primordial phenomena, such as the → cosmic microwave background radiation (CMBR), or photons emitted by stars, galaxies and → active galactic nuclei (AGN) due to → nucleosynthesis or other → radiative processes, including → dust scattering, → absorption and reradiation. The EBL may also contain signals that are diffuse and extended, including high-energy photons associated with dark matter particle decays or annihilation.

→ extragalactic; → background; → light.