# 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.