1) For any system that has an → input and
the return of a fraction of the output to the input for the next action.
Feedback process allows a system to regulate itself by monitoring its own
output. It is of prime importance to
the working of all regulatory mechanisms found in
living and non-living nature, as well as in social systems such
as education and economy.
Fr.: boucle de rétroaction
zamân-e negâh bé gozašté
Fr.: temps de regard en arrière
The time that has elapsed since the light was emitted from a distant object (of → redshift z). Because → light moves at a → constant → speed, it takes a finite time to travel from distant objects. Hence, we "see" distant objects at a point in time in their past. In other words, look-back time is the difference between the age of the Universe now and the age of the Universe at the time the photons were emitted from the object. See also → comoving distance.
Zamân, → time; negâh, → look; gozašté "past, passed" (from gozaštan "to pass, proceed, go on," variant gozâštan "to put, to place, let, allow;" Mid.Pers. widardan, widâštan "to pass, to let pass (by);" O.Pers. vitar- "to pass across," viyatarayam "I put across;" Av. vi-tar- "to pass across," from vi- "apart, away from" (O.Pers. viy- "apart, away;" Av. vi- "apart, away;" cf. Skt. vi- "apart, asunder, away, out;" L. vitare "to avoid, turn aside") + O.Pers./Av. tar- "to cross over").
microwave background radiation
tâbeš-e paszamine-ye rizmowj
Fr.: rayonnement micro-onde du fond cosmique
Thermal radiation with a temperature of 2.73 K that is apparently uniformly distributed in the Universe. It is believed to be a redshifted remnant of the hot radiation that was in thermal equilibrium with matter during the first hundred thousand years after the Big Bang. Same as → cosmic microwave background (CMB) radiation.
Fr.: rétroaction negative
Fr.: effet Paschen-Back
An effect on spectral lines obtained when the light source is located in a strong magnetic field. The strong field disrupts the coupling between the orbital and spin angular momenta, resulting in a different pattern of splitting.
Named for the German physicists Friedrich Paschen (1865-1947) and Ernst E. A. Back (1881-1959); → effect.
Fr.: rétroaction positive
Fr.: rétroaction radiative
The radiative energy put back to the environment through an astrophysical process. For example, in the process of → star formation → accretion disks form around → protostars. The material in the disk spirals inward and on to the protostar, provided that there is an efficient mechanism to redistribute → angular momentum outward in the disk. During this process → gravitational energy is transformed into radiation due to → viscous dissipation in the disk and at the → accretion shock around the protostar. This radiation heats the region around the protostar and may → suppress subsequent → fragmentation and further star formation. Thus, radiative feedback plays a critical role in regulating the stellar → initial mass function.
Fr.: fond du ciel
The emission of a part of the night sky that does not contain any detectable objects. Sky background results from the combined radiation from faint, unresolved stars and other emitting astronomical objects. The mean brightness of night sky background measured at the → Cerro Tololo Inter-American Observatory (CTIO) for the period 1992 to 2006 are: U = 22.12, B = 22.82, V = 21.79, R = 21.19, and I = 19.85 mag arcsec-2 ( → rms ~ 0.2 mag arcsec-2). See also → sky brightness.
Fr.: rétroaction stellaire
The process whereby large quantities of → energy and → momentum are released into the gas surrounding → star formation regions in galaxies. More specifically, → massive stars inject → energy, → mass, and → metals back to the → interstellar medium through → stellar winds and → supernova explosions. Feedback inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars. Observations reveal feedback in the form of → galactic-scale outflows of gas in galaxies with high → star formation rates, especially in the → early Universe. Feedback in faint, low-mass galaxies (→ low-mass galaxy) probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized, a process called → reionization (Dawn K. Erb, 2015, Nature, 9 July).
Fr.: rétroaction des supenovae
1) The process whereby the energy and matter contained in a → supernova
are injected into the → interstellar medium after the
→ supernova explosion.
The → thermal energy injected into the ISM serves to
→ suppress → star formation, while
→ heavy elements → nucleosynthesized
inside SNe tend to enhance star formation.
pas-zamine-ye iks, zamine-ye ~ (#)
Fr.: fond de rayons X
A diffuse background radiation in X-ray wavelengths which has several origins. At very low energies it is due to hot gas in the → Local Bubble. In the → soft X-ray energy band it comes from active galaxies at moderate redshifts. In → hard X-ray range the background is thought to be due to integrated emission from many → quasars at various redshifts.