'Return the density parameter for photons at redshift ``z``.
Parameters
z : array-like
Input redshifts.
Returns
Ogamma : ndarray, or float if input scalar
The energy density of photons relative to the critical
density at each redshift.'

def Ogamma(self, z):

    if isiterable(z):
        z = np.asarray(z)
    return ((self._Ogamma0 * ((1.0 + z) ** 4)) * (self.inv_efunc(z) ** 2))
