'Integrand of the lookback time.
Parameters
z : float
Input redshift.
Returns
I : float
The integrand for the lookback time
References
Eqn 30 from Hogg 1999.'

def _lookback_time_integrand_scalar(self, z):

    args = self._inv_efunc_scalar_args
    return (self._inv_efunc_scalar(z, *args) / (1.0 + z))
