'The lookback distance is the light travel time distance to a given
redshift. It is simply c * lookback_time.  It may be used to calculate
the proper distance between two redshifts, e.g. for the mean free path
to ionizing radiation.
Parameters
z : array-like
Input redshifts.  Must be 1D or scalar
Returns
d : `~astropy.units.Quantity`
Lookback distance in Mpc'

def lookback_distance(self, z):

    return (self.lookback_time(z) * const.c).to(u.Mpc)
