# [AstroPy] astropy.cosmology

Juande Santander Vela juandesant at gmail.com
Fri Dec 6 16:15:01 EST 2013

I could do that for a couple of them (Enrique's input might be useful here as use cases), but they should return Quantities, so I should familiarize myself with how to do that.

--
Juande Santander Vela

> On 6 Dec 2013, at 17:19, Erik Tollerud <erik.tollerud at gmail.com> wrote:
>
> Right now I think Juande's solution is indeed the best way right now.
> I think the reason for only having the "forward" version of these is
> that it's not clear exactly which of all the cosmology functions
> should have inverses.
>
> But we could add inverse versions of everything, basically implemented
> the way Juande is suggesting here.  Then they could later be optimized
> when someone wants to make them faster. If anyone wants to issue a PR
> doing that, I think it might be useful...
>
>> On Thu, Dec 5, 2013 at 12:06 PM, Juande Santander Vela <jdsant at iaa.es> wrote:
>> Hi, Enrique,
>>
>> I don’t find the age function in astropy.cosmology, but the loopback_time, but I think the answer stands.
>>
>> If you want to obtain the inverse function, assuming loopback_time is continuous, and it should, you can use scipy.optimize methods to find out the redshift. For instance, the redshift corresponding to a 0.2Gyears lookback time, you can do:
>>
>> from  scipy.optimize import newton
>> import astropy.cosmology as cosmo
>>
>> newton(lambda x: cosmo.lookback_time(x).value-0.2, 0)
>>
>> or, in general:
>>
>> from  scipy.optimize import newton
>> import astropy.cosmology as cosmo
>>
>> def redshift_from_lookback_time(gyears):
>>    return newton(lambda x: cosmo.lookback_time(x).value-gyears, 0)
>>
>> Hope that helps.
>>
>>> El 05/12/2013, a las 11:01, ES.Enrique Perez <eperez at iaa.es> escribió:
>>>
>>> Geetings.
>>>
>>> I may have overlooked the answer in the docs, but really haven't been able to find it:
>>>
>>> astropy.cosmology age(some_redshift) provides the age corresponding to a given redshfit,
>>> but what is the reverse function that provides the redshift corresponding to a given age?
>>>
>>> Thanks,
>>> Enrique
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>>
>> --
>> Juande Santander Vela
>> VIA-SKA Project Manager
>> Instituto de Astrofísica de Andalucía (IAA-CSIC)
>> Glorieta de la Astronomía s/n, E-18008, Granada, Spain
>>
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>>
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>
>
>
> --
> Erik