This does not wok for large redshifts.
2) The second method is using a formula that has been extracted from a Taylor
expansion approximation for scale factor.
dp0 = (z*c/H0) * (1-(z*(1+q0)/2))
3) The third method is using astropy's '.lookback_distance()' method.
"""
from astropy.cosmology import Planck13 as cosmo
from astropy.constants import G, c
import numpy as np
import matplotlib.pyplot as plt
Or0 = cosmo.Ogamma(0) + cosmo.Onu(0)
Om0 = cosmo.Om(0)
Ode0 = cosmo.Ode(0)
# deceleration parameter (equation 6.11)
q0 = Or0 + 0.5 * Om0 - Ode0
H0 = cosmo.H(0).to('1 / s')
z = np.linspace(0, 2, 100)
# current proper distance in terms of z (equation 6.12)
dp0_612 = c * z / H0
dp0_612 = dp0_612.to('Mpc')
# current proper distance in terms of z (equation 6.19)
