[AstroPy] astropy.cosmology

Neil Crighton neilcrighton at gmail.com
Wed Dec 11 18:37:38 EST 2013

The cosmology.inverse function idea sounds good, I vote to do that. We can
give a warning for cases that are non-monotonic for the standard cosmology,
but I think we should trust users to understand what they're doing.

That's weird there's no age function, I thought all the methods and
functions were eventually mirrored. I'm also not sure why the comoving
volume isn't described in the documentation. Probably this is my fault :)

We can also add the comoving volume element, dV_comoving.  Are there any
other quantities it would be useful to have?

I can open a PR making these changes in the next day or so, unless someone
beats me to it.


On 12 December 2013 07:17, Rick White <rlw at stsci.edu> wrote:

> A problem with inverse angular diameter distance is that it is not a
> single-valued function. Luminosity & comoving distance both increase
> monotonically with redshift, so the inverse is unambiguous.  Angular
> diameter distance increases, then has a maximum and decreases as redshift
> increases.
> So there would be some justification for providing inverses only for
> luminosity & comoving distance...
> On Dec 11, 2013, at 3:07 PM, Benjamin Weiner wrote:
> [Erik wrote]
>> Suppose you have a galaxy where you think you know what its absolute
>> magnitude should be (say its a standard candle) and have an apparent
>> magnitude, and you want to use it to know what redshift it's at.  Then
>> you need this, because the distance modulus is tied to the luminosity
>> distance (this example came to mind because I've actually needed this
>> for actual science in the past).  My point here is mainly that just
>> because we don't want all of them, doesn't mean some other users don't
>> have a need.
> Hi,
> Not to belabor this point, but it is very common to
> want an inverse function for redshift or comoving distance
> when you are trying to deal with selection limits.  For example
> the 1/Vmax luminosity function calculation asks, given an
> object of known luminosity, and a selection limit, to what
> Dmax could I see it?  So you know a distance modulus
> and want to know comoving_volume(z(distmod)).
> Wanting the inverse of angular distance is less common
> but I could imagine using it, for example, answering to what
> redshift can HST resolve a 1 kpc diameter?
> As an aside, I didn't think astropy.cosmology included a
> function for the comoving volume element dV_c or the
> integrated comoving volume from 0 to z, which are both
> useful.  However, looking in the source, comoving_volume
> is there, but it and several other functions were not in the
> documentation.
> Ben
> --
> Benjamin Weiner
> Associate Astronomer, Steward Observatory
> bjw at as.arizona.edu
> http://mingus.as.arizona.edu/~bjw/
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