conformal cyclic cosmology (CCC)
keyhânšenâsi-ye carxe-yi-ye hamdis
Fr.: cosmologie cyclique conforme
A cosmological model developped by Roger Penrose and colleagues according which the Universe undergoes repeated cycles of expansion. Each cycle, referred to an aeon, starts from its own "→ big bang" and finally comes to a stage of accelerated expansion which continues indefinitely. There is no stage of contraction (to a "→ big crunch") in this model. Instead, each aeon of the universe, in a sense "forgets" how big it is, both at its big bang and in its very remote future where it becomes physically identical with the big bang of the next aeon, despite there being an infinite scale change involved, on passing from one aeon to the next. This model considers a conformal structure rather than a metric structure. Conformal structure may be viewed as family of metrics that are equivalent to one another via a scale change, which may vary from place to place. Thus, in conformal space-time geometry, there is not a particular metric gab, but an equivalence class of metrics where the metrics ğab and gab are considered to be equivalent if there is a smooth positive scalar field Ω for which ğab = Ω gab (R. Penrose, 2012, The Basic Ideas of Conformal Cyclic Cosmology).
Of, pertaining to, or constituting a cycle or cycles.
hamârâ-ye nâdidé engâshté
Fr.: coordonnée ignorée
Same as → ignorable coordinate.
Fr.: groupe cyclic
A group in which each element can be expressed as a power of any other element.
Fr.: processus cyclique
Fr.: quadrilatère cyclique
A quadrilateral in which all four vertices lie on the circumference of a circle.
Of or pertaining to an → epicycle.
Fr.: fréquence épicyclique
In the → epicyclic theory of Galactic rotation, the frequency at which a star in the → Galactic disk describes an ellipse around its mean circular orbit. The epicyclic frequency relates to the → Oort's constants. In the solar neighborhood the epicyclic frequency is about 32 km s-1 kpc-1.
Fr.: oscillation épicyclique
Fr.: théorie épicyclique
The theory that describes the Galactic dynamics, that is the orbits of stars and gas clouds in the → Galactic disk, as well as the spiral → density wave. Formulated by Bertil Lindblad (1895-1965), the epicyclic theory assumes that orbits are circular with small deviations. Star orbits are described by the superposition of two motions: i) a rotation of the star (epicenter) around the Galactic center at the circular angular velocity, Ω, and ii) a retrograde elliptical motion at → epicyclic frequency, κ. The epicyclic motion in the Galactic plane occurs in a retrograde sense to conserve → angular momentum. In general Ω and κ are different and, therefore, orbits do not close. However, seen by an observer who rotates with the epicenter, orbits are closed ellipses.
polycyclic aromatic hydrocarbons (PAHs)
hidrocarburhâ-ye aromâtik-e polisiklik, ~ ~ bol-carxe-yi
Fr.: hydrocarbures aromatiques polycycliques
1) Chemistry: A family of → organic molecules composed
of carbon and hydrogen atoms (→ hydrocarbons) in which
→ carbon atoms appear in multiple loops (polycyclic)
with strong chemical → bonds that exist between them (aromatic).
PAHs are formed during the incomplete burning of coal, oil and gas, garbage, or other
organic substances like tobacco or charbroiled meat. As a pollutant, they are of
concern because some compounds (benzo(a)pyrene) have been identified as tending to