# [AstroPy] Cosmology Recession Velocity

Thøger Emil Rivera-Thorsen trive at astro.su.se
Tue Feb 23 19:35:36 EST 2016

```Sorry, that did not come out very clear.

The two different values you find are the recession velocity at the time
of emission and the recession velocity now. Since the Universe has
expanded by a lot since then, a current value of 62 c seems reasonable.

I have a notebook demonstrating some key Astropy features for students
at our department, including this very calculation, here:
http://nbviewer.jupyter.org/urls/ttt.astro.su.se/~trive/teaching/tools-package/Astropy-intro.ipynb
(Scroll to the bottom)

On 02/24/2016 01:14 AM, Thøger Emil Rivera-Thorsen wrote:
> 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) 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.
>>
>> Michael
>>
>>
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>
>
>
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