Fr.: époque cosmologique
current cosmological epoch
zime-ye keyhânšenâxti-ye konuni
Fr.: époque cosmologique actuelle
Fr.: époque électrofaible
A period in the early history of the Universe lasting from 10-36 to 10-12 seconds after the → Big Bang. The electroweak epoch begins at the same time as cosmic → inflation is triggered. This is also the time when the → strong force breaks from the → grand unified force and ends with another → phase transition will occur in which the → weak interaction breaks from the → electroweak force.
1) The date for which → orbital elements or
the positions of celestial objects are calculated. Specifying the
epoch is important because the apparent positions of objects in the
sky change gradually due to → precession and
→ nutation, while orbital elements change due
to the gravitational effects of the → planets.
The → standard epoch used in ephemerides
(→ ephemeris) and stellar catalogues at present
is January 1, 2000, 12h (written also as 2000.0).
See also: → Julian epoch.
From M.L. epocha, from Gk. epokhe "pause, cessation, fixed point," from epekhein "to pause, take up a position," from epi- "on" + ekhein "to hold, to have;" cf. Av. hazah- "power, violence, superiority;" Skt. sahate "he masters," sáhas- "power, violence, might;" Goth. sigis; O.H.G. sigu; O.E. sige "victory;" PIE base *segh- "to hold."
Zime, from Mid.Pers. zim "time, year, winter," from Av. zyam-, zayan- "winter," probably related to zaman "time" + nuance suffix -é.
Fr.: angle de phase initial
Same as the → initial phase angle.
epoch of reionization (EoR)
Fr.: époque de réionisation
epoch of thermalization
Fr.: époque de thermalisation
The period during the → early Universe before the → recombination era when the photons were hot enough to ionize hydrogen. The density was so high that the interactions between → matter and → radiation were very numerous. Therefore, matter and photons were in constant contact and their → temperatures were the same. As a result, the radiation became → thermalized, i.e. the → electromagnetic spectrum of the radiation became that of a → blackbody, a process called → thermalization. Since the time of recombination the photons of → cosmic background radiation have been free to travel uninhibited by interactions with matter. Thus, their distribution of energy is a perfect → blackbody curve, as predicted by the → Big Bang theory and shown by several observations, such as → Cosmic Background Explorer (COBE), → Wilkinson Microwave Anisotropy Probe (WMAP), and → Planck Satellite.
Fr.: époch julienne
A way of specifying the date as a year with a decimal based on the Julian year of 365.25 days and the Barycentric Dynamical Time (TDB). The standard epoch currently in use is J2000.0, which corresponds to January 1, 2000 12:00 Terrestrial Time.
Fr.: époque de polarité
The time during which the Earth's magnetic field was of a single polarity; an interval of time between reversals of Earth's magnetic field.
Fr.: époque actuelle
Fr.: époque de recombinaison
Same as → recombination era.
Fr.: époque de réionisation
An early epoch in the → Universe's history, but after the → recombination epoch, when the → first stars formed and their → ultraviolet light began to ionize the → neutral hydrogen gas that filled the Universe. The epoch of reionization is estimated to last between → redshifts of 12 to 6 (or when the Universe had between 2 and 5% of its age). Reionization marks the end of the → Dark Age in cosmic history.
Fr.: époque de référence
A particular date and time that specifies the reference system to which celestial coordinates are referred. From 1984 the → Julian year is used, as denoted by the prefix J, e.g. J2000.0.