Relic abundance

In cosmology, the relic abundance of a given elementary particle is a measure of the present quantity of that particle remaining from the Big Bang.

Uses

Relic abundance is modelled for WIMPs (weakly interacting massive particles) in the study of dark matter.^[1]

Calculation

Assuming that an elementary particle was formerly in thermal equilibrium, its relic abundance may be calculated using a Boltzmann equation.^[2]

The temperature scaled abundance of a particle is defined^[3] by

${\displaystyle Y\equiv {\frac {n}{T^{3}}}}$

where ${\displaystyle n}$ is the number density:

${\displaystyle n\equiv {\frac {N}{V}}}$

that is, number of particles per physical volume (not the comoving volume).

The relic abundance of a particle is shown by ${\displaystyle Y_{\infty }}$ indicates the asymptotic value of abundance of a species of a particle which it will reach after its "freeze-out".^[4]