An Etymological Dictionary of Astronomy and Astrophysics
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A solution to Einstein's theory of → general relativity that is spatially homogeneous but allows for rotation and/or shear. See also → Bianchi cosmological model.

→ anisotropic; → homogeneous; → cosmological; → model.

A cosmological model based on the theory of → general relativity, which is homogeneous but → anisotropic. There are actually ten dinstinct Bianchi types, classified according to the particular kinds of symmetry they posses.

Luigi Bianchi (1856-1928), Italian mathematician; → cosmological; → model.

Pertaining or relating to → cosmology.

→ cosmology; → -al.

A term introduced by Einstein into his gravitational → field equations in order to allow a solution corresponding to a → static Universe. The cosmological constant is physically interpreted as due to the → vacuum energy of quantized fields. See also → dark energy.

→ cosmological; → constant.

The impressive discrepancy of about 120 orders of magnitude between the theoretical value of the → cosmological constant and its observed value. → Quantum field theory interprets the cosmological constant as the density of the → vacuum energy. This density can be derived from the maximum energy at which the theory is valid, i.e. the → Planck energy scale (10¹⁸ GeV). The theoretical vacuum → energy density is (10¹⁸ GeV)⁴ = (10²⁷ eV)⁴ = 10¹¹² erg cm^-3. On the other hand, the observed vacuum energy density is estimated to be about (10^-3 eV)⁴ = 10^-8 erg cm^-3. There is, therefore, a discrepancy of about 120 orders of magnitude.

→ cosmological; → constant; → problem.

The distance to a remote galaxy based on its redshift assuming that the redshift is caused by the → Doppler effect and reflects the general expansion of the Universe.

→ cosmological; → distance .

→ cosmological; → epoch.

A mathematical description of the Universe, based on observation, which tries to explain its current aspect, and to describe its evolution during time.

→ cosmological; → model.

The → hypothesis that on → large scales the → Universe is → isotropic and → homogeneous, that is, it appears the same at all places and, from any one place, looks the same in all directions. See also → perfect cosmological principle.

Introduced by E.A. Milne in 1933; → cosmological; → principle.

The → redshift of a remote object (galaxy, quasar, supenova) due to the expansion of the Universe.

→ cosmological; → redshift.

The → Universe at the → redshift z = 0.

→ current; → cosmological; → epoch.

Same as → Milne Universe.

→ Milne Universe; → cosmological; → model.

The → assumption, adopted by the → steady-state theory, that all observers, everywhere at all times, would view the same large-scale distribution of matter in the → Universe in all regions and in every direction. In contrast to the → cosmological principle, the perfect cosmological principle adds the assumption that the Universe does not change with time on the large scale.

→ perfect; → cosmological; → principle.