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column density cagâli-ye sotun Fr.: densité de colonne Density of the interstellar matter lying between an object and the Earth in a cylinder with a unity base. |
composite hamnehâdé (#) Fr.: composite Made up of distinct components. The components may retain part of their identities. → compound, → combination. From O.Fr. composite, from L. compositus, p.p. of componere "to put together," → compose. Hamnehâdé, p.p. of hamnehâdan→ compose. |
composite number adad-e hamnehâdé Fr.: nombre composite A whole number which is the product of whole numbers other than itself and 1. The opposite of prime number. → compound number. |
composite population porineš-e hamnehâdé Fr.: population composite A stellar population consisting of more than one → simple population. Also called complex population. → composite; → population. |
composite spectrum binâb-e hamnehâdé Fr.: spectre composite A stellar spectrum displaying lines characterising two stars of different types. |
composition hamneheš (#) Fr.: composition 1) General: The act or manner of composing; the result or product of composing. Hamnehesh, from ham-, → com-, + neheš verbal noun from nehâdan "to put, place," → compose. |
compound proposition gozâre-ye hamnât Fr.: proposition composée A statement formed from simple statements by the use of words such as "and," "or," "not," "implies," or their corresponding symbols. → compound; → proposition. |
conditional proposition gozâre-ye butâri Fr.: proposition conditionelle A compound → proposition in which one → clause asserts something as true provided that the other clause is true. A conditional statement consists of two parts, a hypothesis in the "if" clause and a conclusion in the "then"clause. For instance, "If it rains, then they cancel school." It rains is the hypothesis. "They cancel school" is the conclusion. The clause following if is traditionally called the → antecedent, whereas the clause following then is called the → consequent. → conditional; → proposition. |
critical density cagâli-ye paržani Fr.: densité critique 1) Cosmology: The average density of matter in the Universe
that would be needed to eventually halt the
→ cosmic expansion.
In a spatially → flat Universe,
the critical density is expressed by
ρ_{c} = (3c^{2}/8πG)H_{t}^{2},
where c is the → speed of light,
G is the → gravitational constant, and
H_{t} the → Hubble parameter.
The critical density is currently 9.3 × 10^{-30}g cm^{-3},
about 6 hydrogen atoms per cubic meter
(for H_{0} = 70 km s^{-1} Mpc^{-1}). |
current density cagâli-ye jarayân Fr.: densité de courant The electric current per unit of cross-sectional area perpendicular to the direction of current flow. It is a vector quantity and represented by symbol J. Electric current density is usually expressed in amperes per square meter. |
data acquisition alfanješ-e dâdehâ Fr.: acquisition de données 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. |
de Sitter Universe giti-ye de Sitter Fr.: Univers de de Sitter A solution to → Einstein's field equations of → general relativity which contains no ordinary matter (Ω_{M} = 0) or radiation (Ω_{R} = 0), is → Euclidean (k = 0), but has a → cosmological constant (Ω_{Λ} > 0). The Universe expands exponentially forever. This solution was the first model expanding of → expanding Universe. See also → empty Universe, → Milne Universe. After the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → universe. |
decomposition vâhamneheš Fr.: décomposition The process or the state of breaking down a physical entity or an organic material. From → de- + → composition. |
density cagâli (#) Fr.: densité The amount of any quantity per unit volume. The mass density is the
mass per unit volume. The energy density is the energy per unit
volume; particle density is the number of particles per unit volume. Noun form of → dense. |
density cusp tize-ye cagâli Fr.: cuspide de densité A localized increase in number of → stellar black holes near a → supermassive black hole predicted by models of galactic → stellar dynamics (Bahcall, Wolf, 1976, ApJ, 209, 214). Same as → stellar cusp. |
density fluctuation oftâxizhâ-ye cagâli Fr.: fluctuations de densité In the early Universe, localized enhancements in the density of either matter alone or matter and radiation. According to models, very small initial fluctuations (less than 1 percent) can lead to subsequent formation of galaxies. → density; → fluctuation. |
density of an element cagâli-ye bonpâr Fr.: densité d'élément The number of units of mass of the → chemical element that are present in a certain volume of a medium. The density of an element depends on temperature and pressure. The element Osmium has the highest known density: 22.61 g/cc; in comparison gold is 19.32 g/cc and lead 11.35 g/cc. |
density parameter pârâmun-e cagâli Fr.: paramètre de densité One of the four terms that describe an arranged version of the
→ Friedmann equations. They are all time dependent. |
density profile farâpâl-e cagâli Fr.: profile de densité 1) A → profile representing the
→ density of a quantity. |
density wave mowj-e cagâli (#) Fr.: onde de densité A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory. |
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