An Etymological Dictionary of Astronomy and Astrophysics
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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.

→ dark; → cloud.

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.

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.

→ dark; → current; → noise.

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.

→ dark; → energy.

A → CCD frame obtained with closed → shutters in the absence of any light source, in order to estimate the → dark current of the → detector.

→ dark; → exposure.

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."

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.

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.

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; → candidate.

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; → decay.

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.

→ dark; → matter; → halo.

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.

→ dark; → nebula.

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).

→ dark; → star.

To become dark or darker. → darkening.

Verb of → dark.

The act or process of becoming dark or darker. → limb darkening.

Verbal noun of of → darken.

Items of information.

L. data, plural of → datum.

Dâdehâ, plural of dâdé, → datum.

In the → software architecture, the code that deals with reading from or writing to the data store, hiding its nature and complexity.

→ data; → access; → layer.

The process of controlling telescope operations during observation and obtaining data.

→ data; acquisition, from L. acquisitionem, from acquirere "get in addition," from → ad- "extra" + quærere "to search for, obtain."

Alfanješ, verbal noun of alfanjidan (variant alfaqdan) "to acquire, get," Bactrian αλφανζ "to acquire," Sogdian δβ'yz "to acquire, gain, get" (Cheung 2007); → data.

The evaluation of reduced data; → data reduction.

→ data; → analysis.

Any extensive record or collection of data, observational (usually obtained with a particular instrument) or theoretical (grid of models usually regarding a particular branch of astrophysics).

→ data; → archive.