cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount (lexicographer), Thomas Blount's ''Glossographia'', and in 1731 taken up in ...

, 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. Particles currently thought to be elementary include electrons, the fundamental fermions ( quarks, leptons, an ...

is a measure of the present quantity of that particle remaining from the

Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...

Uses

Relic abundance is modelled for

WIMP Wimp, WIMP, or Wimps may refer to: Science and technology * Weakly interacting massive particle, a hypothetical particle of dark matter * WIMP (computing), the "window, icon, menu, pointer" paradigm * WIMP (software bundle), the web stack of Wind ...

s (weakly interacting massive particles) in the study of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

.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 ...

, 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, devised by Ludwig Boltzmann in 1872.Encyclopaedia of Physics (2nd Edition), R. G. Lerne ...

. The temperature scaled abundance of a particle is defined by :

Y \equiv \frac

where

n

is the number density: :

n \equiv \frac

that is, number of particles per physical volume (not the comoving volume). The relic abundance of a particle is shown by

Y_\infty

indicates the asymptotic value of abundance of a species of a particle which it will reach after its "freeze-out".

References

Physical cosmology {{particle-stub