An Etymological Dictionary of Astronomy and Astrophysics
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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 g_ab, but an equivalence class of metrics where the metrics ğ_ab and g_ab are considered to be equivalent if there is a smooth positive scalar field Ω for which ğ_ab = Ω g_ab (R. Penrose, 2012, The Basic Ideas of Conformal Cyclic Cosmology).

→ conformal; → cyclic; → cosmology.

Of, pertaining to, or constituting a cycle or cycles.

→ cycle; → -ic.

Same as → ignorable coordinate.

→ cyclic; → coordinate.

A group in which each element can be expressed as a power of any other element.

→ cyclic; → group.

Any sequence of changes in a → thermodynamic system that returns the system into its → initial  → state.

→ cyclic; → process.

A quadrilateral in which all four vertices lie on the circumference of a circle.

→ cyclic; → quadrilateral.

Of or pertaining to an → epicycle.

→ epicycle; → -ic.

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.

→ epicyclic; → frequency.

In a → disk galaxy, the motion of a star about the orbital → guiding center when it is displaced radially. See also → epicyclic frequency, → epicyclic theory.

→ epicyclic; → oscillation.

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.

→ epicyclic; → theory.

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 cause cancer.
2) Astro.: Strong → infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 microns, which are attributed to PAH molecules in the form of → dust grains with radii smaller than 10 Å. PAHs are common toward various sources characterized by a strong → ultraviolet radiation field (such as → active galaxies, → reflexion nebulae, → planetary nebulae, → H II regions). In particular, they trace the presence of → massive stars. The PAH features are thought to result from the fluorescent emission process of PAH molecules and ions excited by ultraviolet radiation of nearby stars. PAHs are likely produced by → photoevaporation of → very small grains.

→ poly-; → cyclic; → aromatic; → hydrocarbon.