Physical cosmologyPhysical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood.
ParsecLe parsec (), de symbole pc, est une unité de longueur utilisée en astronomie. Il est défini comme valant exactement , soit environ . Le nom parsec est la contraction de « parallaxe-seconde », une expression se rapportant à sa définition historique, désormais obsolète ( figure). est emprunté à l'anglais parsec, mot-valise proposé par l'astronome britannique Herbert Hall Turner à partir de par[allax] (en français, « parallaxe ») et de sec[ond] (« seconde »).
Recessional velocityRecessional velocity is the rate at which an extragalactic astronomical object recedes (becomes more distant) from an observer as a result of the expansion of the universe. It can be measured by observing the wavelength shifts of spectral lines emitted by the object, known as the object's cosmological redshift. Hubble's law is the relationship between a galaxy's distance and its recessional velocity, which is approximately linear for galaxies at distances of up to a few hundred megaparsecs.
PhotoionizationPhotoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Not every interaction between a photon and an atom, or molecule, will result in photoionization. The probability of photoionization is related to the photoionization cross section of the species – the probability of an ionization event conceptualized as a hypothetical cross-sectional area. This cross section depends on the energy of the photon (proportional to its wavenumber) and the species being considered i.
