[AstroPy] Cosmology Recession Velocity

Thøger Emil Rivera-Thorsen trive at astro.su.se
Tue Feb 23 19:14:12 EST 2016


62 c at z=1000 does not sound completely off for the *current value*. I 
suppose the values between 2 and 4 c you get gggfrom Davis & Lineweaver 
is velocities at the time of emission? These two differ *strongly*.



On 02/24/2016 12:25 AM, Michael Seifert wrote:
> 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 
> <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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