# [AstroPy] astropy.cosmology

Rick White rlw at stsci.edu
Wed Dec 11 15:17:47 EST 2013

```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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