[AstroPy] Cosmology Recession Velocity

[Michael Seifert]

Hi,

today I tried to determine cosmological recession velocities based on the paper of Davis, Lineweaver, 2001 (http://arxiv.org/pdf/astro-ph/0011070v2.pdf) with astropy.cosmology.

But I'm not sure what I'm doing wrong (or if I just misunderstood their reasoning). But they said the recession velocity is just "v_rec = da(z)/dt * comoving distance(z)" and "da/dt = H(z)*a(z)" so the recession velocity should be with "cosmo = astropy.cosmology.WMAP9" and redshift "z=1000":

v_rec = cosmo.H(z) * cosmo.scale_factor(z) * cosmo.comoving_distance(z)

but this gives me 62 times the speed of light where it should only be between 2 and 4 (based on the paper). But just using

v_rec = cosmo.H0 * cosmo.comoving_distance(z)

gives me 3.25 * c which seems to fit their calculations. Is my formula or reasoning wrong or is there something strange in the comoving distance calculation of WMAP9?

I hope this question is not too basic but I'm not very deep into cosmology and it just seemed strange to me.

Many thanks in advance,
Michael
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