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

def lookback_time_integrand(self, z):

    if isiterable(z):
        zp1 = (1.0 + np.asarray(z))
    else:
        zp1 = (1.0 + z)
    return (self.inv_efunc(z) / zp1)
