Relic abundance
# 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.

## Calculation

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

The temperature scaled abundance of a particle is defined by

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

where $n$ is the number density:

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

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

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

