|Annu. Rev. Astron. Astrophys. 1984. 22: Copyright © 1984 by .
All rights reserved 2.2. Basic Equations The standard results
rest on three pillars: the conservation equations, the
Raychaudhuri equation, and the energy conditions. The form of
these equations depends on the nature of the matter in the
Universe. It is assumed here that the usual description of
matter on a cosmological scale as a "perfect fluid" is adequate
at most times. Then (24, 25) the stress tensor of each component
of matter or energy present takes the form where µ is the
(relativistic) energy density of the fluid and p its pressure.
To complete the description of matter, we must give suitable
equations of state for the matter variables; in the hot early
Universe, the "standard" theory assumes (except during phases of
pair annihilation) that these are describing a "Fermi gas" with
temperature T (107, 109). THE CONSERVATION EQUATIONS The stress
tensor Tab of matter that does not interact with other matter or
radiation obeys the equations Tdb