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Barycentric Julian Date (BJD) gâhdâd-e žulian-e gerânigâhi Fr.: date julienne barycentrique The → Julian Date referenced to the → barycenter of the → solar system. The BJD is more precise than the → Heliocentric Julian Day because the Sun is not stationary. It moves due to the → gravitational attraction of Jupiter and the other planets. → barycentric; → Julian Date. |
baryogenesis bâriyonzâyi Fr.: baryogénèse The hypothetical mechanism of creating the → baryon asymmetry in the → Universe. Universe. Explaining the observed matter asymmetry is an important open question in physical cosmology. → Sakharov conditions. |
baryon bâriyon (#) Fr.: baryon Any of the class of the heaviest → subatomic particles that includes → protons, → neutrons, as well as a number of short-lived particles whose decay products include protons. Baryons obey the → Fermi-Dirac statistics. They form a subclass of the → hadrons and are further subdivided into → nucleons and → hyperons. Gk. barys "heavy" + → -on, from "fermion." |
baryon acoustic oscillation (BAO) naveš-e sedâyik-e bâryoni Fr.: oscillation acoustique baryonique In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the → recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the → CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating → cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h^{-1}Mpc (h being the → Hubble constant in units of 100 km s^{-1} Mpc^{-1}). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like → Type Ia supernovae, BAOs provide a → standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how → dark energy has affected the expansion of the Universe (see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401, 2148). → baryon; → acoustic; → oscillation. |
baryon asymmetry nâhamâmuni-ye bariyon Fr.: asymmétrie baryonique The observation that in the present → Universe there is → matter but not much → antimatter. Observations do not show the presence of galaxies made of antimatter, nor gamma rays are observed that would be produced if large entities of antimatter would undergo → annihilation with matter. However, the → early Universe could have been baryon symmetric, and for some reason the matter excess has been generated, through some process called → baryogenesis. → Sakharov conditions. |
baryon number adad-e bâriyoni (#) Fr.: nombre baryonique 1) The difference between the total number of → baryons and
the total number of → antibaryons in a system of
→ subatomic particles.
It is a measure of → baryon asymmetry and is
defined by the quantity
η = (n_{b} - n_{b-})/n_{γ},
called the → baryon-photon ratio,
where n_{b} is the → comoving number
density of baryons, n_{b-} is the number of
antibaryons, and n_{γ} is that of photons. The value of η for
the → cosmic microwave background radiation (CMBR)
has been very well determined by the → WMAP satellite to be
η = (6.14 ± 0.25) x 10^{-10}. The baryon number is assumed to be
constant. The photons created in
stars amount to only a small fraction, less than 1%, of those in the CMBR. |
baryon-photon ratio vâbar-e bâriyon-foton Fr.: rapport baryon-photon The → baryon number compared with the number of photons in the → Universe. The baryon-photon ratio can be estimated in a simple way. The → energy density associated with → blackbody radiation of → temperature T is aT^{4}, and the mean energy per photon is ~kT. Therefore, the number density of blackbody photons for T = 2.7 K is: n_{γ} = aT^{4}/kT = 3.7 x 10^{2} photons cm^{-3}, where a = 7.6 x 10^{-15} erg cm^{-3} K^{-4} (→ radiation density constant) and k = 1.38 x 10^{-16} erg K^{-1} (→ Boltzmann's constant). The number density of baryons can be expressed by ρ_{m}/m_{p}, where ρ_{m} is the mass density of the Universe and m_{p} is the mass of the → proton (1.66 x 10^{-24} g). → CMB measurements show that the baryonic mean density is ρ_{m} = 4.2 x 10^{-31} g cm^{-3} (roughly 5% of the → critical density). This leads to the value of ~ 2 x 10^{-7} for the number density of baryons. Thus, the baryon/photon ratio is approximately equal to η = n_{b}/n_{γ} = 2 x 10^{-7}/3.7 x 10^{2} ~ 5 x 10^{-10}. In other words, for each baryon in the Universe there is 10^{10} photons. This estimate is in agreement with the precise value of the baryon-photon ratio 6.14 x 10^{-10} derived with the → WMAP. Since the photon number and the baryon number are conserved, the baryon-photon ratio stays constant as the Universe expands. |
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. |
baryonic matter mâde-ye bâriyoni (#) Fr.: matière baryonique Ordinary matter composed of → baryons, i.e. → protons and → neutrons, as distinct from → non-baryonic, exotic forms. |
base 1, 2, 3, 4) pâyé (#), 5) pâygâh (#), 6) bâz (#) Fr.: base 1) The bottom support of anything; a fundamental principle or groundwork. M.E., from O.Fr. bas, from L. basis "foundation," from Gk. basis "step, pedestal," from bainein "to step." Pâyé "base," from pâ, pây "foot," "from Mid.Pers. pâd, pây;
Av. pad-, cf. Skt. pat: Gk. pos, genitive podos;
L. pes; PIE *pod-/*ped-. |
baseline pâye-xatt Fr.: ligne de base 1) In radio interferometry, the separation between the electrical,
or phase centers of two interferometer elements. |
basic bâzi (#) Fr.: basique Chemistry: Of or denoting or of the nature of or containing a → base. Same as → alkaline. |
basin howzé (#) Fr.: bassin A large impact crater on a planet or moon, typically several hundred kilometers across, flooded with basaltic lava and surrounded by concentric rings of faulted cliffs. From O.Fr. bacin, from V.L. *baccinum, from L. bacca "water vessel," perhaps originally Gaulish. Howzé, from howz "pond, a large reservoir of water" (from Ar. hauz) + -é noun suffix. |
battery bâtri (#) Fr.: batterie A combination of → cells connected together so as to produce useful electrical energy. M.Fr. batterie "a grouping of artillery pieces for tactical purposes," from O.Fr. baterie "beatng, thrashing, assault," from battre "to beat," from L. battuere "to beat." Bâtri, loanword from Fr., as above. |
bay bâhé Fr.: baie A body of water forming an indentation of the shoreline, larger than a cove but smaller than a → gulf (Dictionary.com). M.E. baye, from M.Fr. baie, from L.L. bâia, perhaps ultimately from Iberian bahia. Bâhé, loan from Sp. bahia. |
Bayer designation nâmgozini-ye bayer Fr.: designation de Bayer A stellar designation system in which a specific star is identified by a Greek letter, followed by the genitive form of its hosting → constellation's Latin name. For example, Alpha Eridani, Delta Cephei, Lambda Bootis. The Greek alphabet has only 24 letters. In case a single constellation contained a larger number of stars, Bayer amended with Latin letters: upper case A, followed by lower case b through z (omitting j and v), for a total of another 24 letters. Bayer did not go beyond z, but later astronomers added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include, for Vela: a Vel (Velorum), z Vel, A Vel, Q Vel; for Scorpius: d Sco (Scorpii), A Sco; for Leo: b Leo (Leonis), o Leo, A Leo, → c Orionis. Compare with the → Flamsteed designation. First introduced by Johann Bayer (1572-1625) in his atlas Uranometria, published in 1603 at Augsburg, Germany; → designation. |
Bayes' theorem farbin-e Bayes Fr.: théorème de Bayes A theorem in probability theory concerned with determining the → conditional probability of an event when another event has occurred. Bayes' theorem allows revision of the original probability with new information. Its simplest form is: P(A|B) = P(B|A) P(A)/P(B), where P(A): independent probability of A, also called prior probability; P(B): independent probability of B; P(B|A): conditional probability of B given A has occurred; P(A|B): conditional probability of A given B has occurred, also called posterior probability. Same as Bayes' rule. Named after its proponent, the British mathematician Reverend Thomas Bayes (1702-1761). However, Bayes did not publish the theorem during his lifetime; instead, it was presented two years after his death to the Royal Society of London. |
Bayesian model model-e Bayesi Fr.: modèle bayésien A mathematical framework described by the prior distribution of a random parameter and by the likelihood of the observations. In this framework, all information on the random parameter based on the observations is included in the posterior distribution which can be obtained using → Bayes' theorem (see, e.g., Andrieu et al., 2001, "An Introduction to Monte Carlo Methods for Bayesian Data Analysis," in Nonlinear Dynamics and Statistics, ed. A. I. Mees, Boston: Birkhäuser). Bayes' theorem; → model. |
Bayesian model averaging (BMA) miyângin-giri-ye Bayesi-e model Fr.: An approach to model selection in which one bases inference on an average of all possible models instead of a single best model. The BMA is largely used in various branches of knowledge to properly account for model uncertainty in performing predictions. Bayesian, after Thomas Bayes (1702-1761), → Bayes's theorem; → model; → average. |
Be phenomenon padide-ye Be Fr.: phénomène Be The episodic occurrence of abrupt → mass loss in → Be stars resulting in → Balmer lines in emission and → infrared excess. The Be phenomenon results from a combination of a long-term secular effect and short-term instabilities, such as pulsation. The secular evolution brings the star close enough to the critical → break-up velocity, so that the additional velocity field due to the instability may allow some mass ejection (Maeder 2011). → Be star; → phenomenon. |
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