Alexander's dark band
navâr-e siyâh-e Aleksânder
Fr.: bande noire d'Alexandre
A dark space or band between the primary and secondary rainbows when both are visible. This effect is due to the minimum refraction angle for the → primary rainbow and the maximum for the → secondary rainbow. The only light in the dark region is caused by (a small amount of) scattering, and not the refraction of light in water droplets.
Named for Alexander of Aphrodisias, Greek Peripatetic philosopher and commentator, who first described the effect in 200 AD.
baryonic dark matter
mâde-ye siyâh-e bâriyoni
Fr.: matière noire baryonique
→ Dark matter made up of → baryons that are not luminous enough to produce any detectable radiation. It is generally believed that most dark matter is → non-baryonic. The baryonic dark matter could reside in a number of forms, including cold gas and compact objects.
cold dark matter (CDM)
mâdde-ye sard-e târik (#)
Fr.: matière noire froide
Any → hypothetical → non-baryonic → dark matter that is → non-relativistic at the point of → decoupling in the → early Universe. CDM plays a key role in → cosmic structure formation. See also → CDM model, → lambda cold dark matter, → Meszaros effect, → missing satellites problem.
Cosmic Dark Age
asr-e târik-e keyhâni
Fr.: âge sombre cosmique
The period of time in the early history of the Universe, between the → recombination era and the advent of the → first stars.
Fr.: sombre, obscur, noir
Having very little or no light.
M.E. derk, O.E. deorc, from P.Gmc. *derkaz.
Târik, Mid.Pers. târig "dark," târ "darkness," Av. taθra- "darkness," taθrya- "dark," cf. Skt. támisrâ- "darkness, dark night," L. tenebrae "darkness," Hittite taš(u)uant- "blind," O.H.G. demar "twilight."
niyâveš bé târiki
Fr.: adaptation à l'obscurité
The automatic adjustment of the iris and retina of the eye to allow maximum vision in the dark, following exposure of the eye to a relatively brighter illumination.
→ dark; → adaptation.
abr-e târik (#)
Fr.: nuage sombre
A relatively dense cloud of → interstellar gas, mainly molecular, whose dust particles obscure the light of stars behind it. A famous example is the → Horsehead Nebula silhouetted against the reddish glow of the → H II region IC 434. Individual dark clouds come in a range of sizes from tens of → light-years to tiny → Bok globules of only a few thousands → astronomical units.
Fr.: courant d'obscurité
Current generated in an electronic detector by thermal effects, even in the absence of input signal. In a → CCD detector, the current rises from thermal energy within the silicon lattice comprising the CCD. These electrons are captured by the CCD's potential wells and counted as signal. → dark current noise.
dark current noise
nufe-ye jarayân-e târiki
Fr.: bruit du courant d'obscurité
In a → CCD detector, statistical fluctuation of the → dark current, equal to the square root of the dark current. CCDs can be cooled either with thermoelectric coolers or liquid nitrogen to reduce this effect. Ideally, the dark current noise should be reduced to a point where its contribution is negligible over a typical exposure time.
Fr.: énergie noire
A hypothetical form of energy that fills all the space and tends to increase the rate of expansion of the Universe. Assuming the existence of dark energy is a way to explain recent observations that the Universe appears to be expanding at an increasing rate (→ accelerating Universe). Dark energy seems to be a kind of anti-gravity force and is supposed to be related to → vacuum energy. Where gravity pulls things together at the more local level, dark energy tears them apart on the grander scale. The acceleration equation, one of Einstein's equations for the homogeneous Universe, indicates that if the Universe is accelerating, the pressure of the driving component should be strongly negative. The dark energy density relates to the → cosmological constant via: ρ&Lambda = Λc2/(8πG), where G is the → gravitational constant and c the → speed of light. The first indication of dark energy was provided by the observation of → Type Ia supernovae. Other probes of dark energy are: → baryon acoustic oscillations, → weak gravitational lensing, and clusters of galaxies. In the standard model of cosmology, dark energy currently accounts for almost 74% of the total mass-energy of the Universe. Two proposed forms for dark energy are the cosmological constant and exotic component such as → quintessence.
osneheš-e târiki, nurdâd-e ~
Fr.: pose d'obscurité
A → CCD frame obtained with closed → shutters in the absence of any light source, in order to estimate the → dark current of the → detector.
fariz-e târik (#)
Fr.: frange noire
One of the successive dark and light bands produced by diffraction or interference of light.
From O.Fr. frange, from V.L. *frimbia alteration by metathesis of L. fimbria "fringe, edge of garment."
Fariz, variants farviz, farâviz "fringe, edge of garment."
mâdde-ye târik (#)
Fr.: matière noire
Matter that has no radiation and therefore cannot be detected directly, but whose presence can be inferred from dynamical phenomena produced by its gravitational influence. The existence of dark matter is deduced mainly from the rotational speeds of galaxies, velocities of galaxies in clusters, gravitational lensing by galaxy clusters, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter plays also a central role in cosmic structure formation. There exists a large number of → non-baryonic dark matter candidates. They include, the hypothetical stable particles → WIMPs, → neutralinos, → axions, → gravitinos, etc. Among unstable candidates are gravitinos with mild R-parity violation and sterile neutrinos. See also → baryonic dark matter, → dark matter candidate.
The concept of dark matter was first introduced by J.H. Oort (1932, Bull. Astron. Inst. Netherlands, 6, 249), who studied the vertical motions of the stars in the solar neighborhood and found that the visible matter could account for at most 50% of the derived surface density. → dark; → matter.
dark matter annihilation
nâbudi-ye mâde-ye târik
Fr.: annihilation de la matière noire
A hypothetical process whereby hypothetical → non-baryonic dark matter particles undergo → annihilation interactions with themselves. The process results in observable by-products such as high-energy photons, neutrinos, and other detectable particles. See also → dark matter decay.
→ dark; → matter; → annihilation.
dark matter candidate
nâmzad-e mâdde-ye târik (#)
Fr.: candidat matière noire
A hypothetical physical entity capable of accounting for the corresponding observed phenomena involving → dark matter. → Non-baryonic dark matter candidates include → WIMPs, → neutralinos, → axions, → gravitinos. Among → baryonic dark matter candidates can be noted ordinary and heavy → neutrinos, clouds of → neutral hydrogen gas, and compact objects.
dark matter decay
tabâhi-ye mâde-ye târik
Fr.: désintégration de la matière noire
In theoretical models, the hypothetical transformation of a → non-baryonic dark matter particle when symmetry is violated at special physical conditions. Dark matter decay and → dark matter annihilation are expected to produce enormous amounts of energy in the form of gamma-rays, cosmic rays, etc.
dark matter halo
hâle-ye mâde-ye târik (#)
Fr.: halo de matière sombre
A vast region surrounding a galaxy where dynamical tracers reveal a large amount of → hidden mass. The halo has considerable mass but relatively low luminosity, suggesting the presence of a lot of → dark matter.
Fr.: nébuleuse sombre
An interstellar cloud of absorbing matter whose dust particles obscure the light from stars beyond it and give the cloud the appearance of a dark, starless region.
Fr.: étoile noire, ~ sombre
A hypothetical stellar object whose structure or evolution has been affected by → dark matter. Dark matter models predict that in the → early Universe dark matter (in the form of → WIMPs) should congregate and annihilate in the cores of → Population III stars. The result would be dark stars with properties very different from ordinary stars. The reason is that the presence of large amounts of dark matter during the formation of a star inhibits the collapse and can partially prevent further cooling of the gas beyond a certain critical point, well before a → main sequence object has formed. As a consequence, dark matter stars should be more massive, more luminous, and live longer than Pop. III stars, but would be cooler. The formation of dark stars could have had an impact on the → reionization history of the Universe (see, e.g., P. Scott, 2011, astro-ph/1101.1029, and references therein).
To become dark or darker. → darkening.
Verb of → dark.