In
cosmology
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe, the cosmos. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', with the meaning of "a speaking of the wo ...
, the relic abundance of a given
elementary particle
In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. The Standard Model presently recognizes seventeen distinct particles—twelve fermions and five bosons. As a c ...
is a measure of the present quantity of that particle remaining from the
Big Bang
The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...
.
Uses
Relic abundance is modelled for
WIMP
WiMP is a music streaming service available on mobile devices, tablets, network players and computers. WiMP, standing for "Wireless Music Player," was a music streaming service that emphasized high-quality audio. WiMP offered music and podcast ...
s (weakly interacting massive particles) in the study of
dark matter
In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
.
[Kim Griest]
"Relic Abundance in More Detail"
The Net Advance of Physics: The Nature of Dark Matter, Section 6C, MIT
Calculation
Assuming that an elementary particle was formerly in
thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in t ...
, its relic abundance may be calculated using a
Boltzmann equation
The Boltzmann equation or Boltzmann transport equation (BTE) describes the statistical behaviour of a thermodynamic system not in a state of equilibrium; it was devised by Ludwig Boltzmann in 1872.Encyclopaedia of Physics (2nd Edition), R. G ...
.
The temperature scaled abundance of a particle is defined
by
:
where
is the number density:
:
that is, number of particles per physical volume (not the
comoving volume).
The relic abundance of a particle is shown by
indicates the asymptotic value of abundance of a species of a particle which it will reach after its "freeze-out".
References
Physical cosmology
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