Comoving units and code units

Units for Cosmological Datasets

yt has additional capabilities to handle the comoving coordinate system used internally in cosmological simulations. Simulations that use comoving coordinates, all length units have three other counterparts correspoding to comoving units, scaled comoving units, and scaled proper units. In all cases ‘scaled’ units refer to scaling by the reduced Hubble parameter - i.e. the length unit is what it would be in a universe where Hubble’s parameter is 100 km/s/Mpc.

To access these different units, yt has a common naming system. Scaled units are denoted by dividing by the scaled Hubble parameter h (which is in itself a unit). Comoving units are denoted by appending cm to the end of the unit name.

Using the parsec as an example,

pc
Proper parsecs, \rm{pc}.
pccm
Comoving parsecs, \rm{pc}/(1+z).
pccm/h
Comoving parsecs normalized by the scaled hubble constant, \rm{pc}/h/(1+z).
pc/h
Proper parsecs, normalized by the scaled hubble constant, \rm{pc}/h.