# Cosmology¶

One quick note about cosmology: the convention in *ARES* is to eliminate all factors of “little h,” i.e., the Hubble parameter in units of \(100 \ \mathrm{km} \ \mathrm{s}^{-1} \ \mathrm{Mpc}^{-3}\). The most noticeable place where this happens is in the `ares.physics.HaloMassFunction`

class. For example, whereas the *hmf* code yields the halo mass function with implicit h’s, *ARES* “undoes” these factors, meaning, e.g., that the halo mass function stored in `ares.physics.HaloMassFunction.tab_dndm`

is simply in units of \(\mathrm{Mpc}^{-3}\), not \(h^4 \mathrm{Mpc}^{-3}\), so the user need not multiply tab_dndm by \(h^4\) to obtain the “true” mass function. The same goes for halo masses themselves (no need to divide by \(h\)) and the cumulative mass function (no need to multiply by \(h^3\) or \(h^2\) for \(n(>m)\) and \(m(>m)\), respectively).

For a nice discussion of little h check out this paper by Darren Croton.