title: Relic abundance
text: # 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".
  
  