[yt-users] potential unit error in cosmology.py

[Britton Smith]

Hi Pengfei,

I'm not able to reproduce what you're seeing, but before I go into that, I
can explain a few things about how the cosmology calculator works.  In yt,
we are only able to define a comoving unit system in relation to a proper
unit system at a specific redshift.  We are unable to define a comoving
unit system that is the base unit system.  This is a problem for the
cosmology calculator, which only calculates comoving quantities.
Therefore, in this case, we are forced to set the comoving and proper unit
systems to be the same.  You can see this for yourself by running the
following script:

from yt.utilities.cosmology import Cosmology
co = Cosmology()
print (co.comoving_radial_distance(1.92, 2).to("Mpc/h"))
print (co.comoving_radial_distance(1.92, 2).to("Mpccm/h"))

Both will be the same answer.

Now, back to the issue you're seeing.  I ran the following script:
http://paste.yt-project.org/show/6741/
to calculate the redshifts dumps needed for a 80 Mpc/h box to go from z = 2
to z = 1.92 and got the following result:
CosmologyOutputRedshift[0] = 2.000
CosmologyOutputRedshift[1] = 1.926

This is pretty close to what you expected.  Additionally, if I run the
following script,
http://paste.yt-project.org/show/6743/
with the above redshifts added to the parameter file, I get that dz_max
(the change in redshift when moving 80 Mpccm/h at z = 0)
of 0.0746521607364, which also seems to agree with what you are finding.

Can you share with me the simulation parameter file and script that you're
using to make your light rays?  I can take a look at that and see if I
still get what I expect.

Britton

On Mon, Aug 8, 2016 at 6:13 AM, Pengfei Chen <madcpf at gmail.com> wrote:

> Hi all,
>
> I was trying to generate light rays using yt 3.4-dev, but found that the
> light rays have wrong redshift intervals. For example, my simulation box is
> 80Mpccm/h per side, and I expect a light ray generated in a data dump at
> z=2 with length =1 simulation unit (i.e. 80Mpccm/h) would have a redshift
> interval from z=2 to z=1.92. However, the light ray generated by yt gives
> spans from z=2 to z=1.97.
>
> As I look into this, I found that the function comoving_radial_distance
> in cosmology.py might return the wrong unit. I think the return value
> should be in comoving units, instead of physical units. To see this
> directly, I made the following change in cosmology_splice.py:
>
> (root) ~/yt-conda/src/yt-hg/yt/analysis_modules/cosmological_observation
> $diff cosmology_splice.py cosmology_splice.py0
> 373d372
> <             print target_distance, distance2, z2
>
> And it shows:
>
> 80.0 Mpccm/h 4.65320708035e+26 cm 1.97387969592 dimensionless
> 80.0 Mpccm/h 1.19455177073e+26 cm 1.97343368065 dimensionless
> 80.0 Mpccm/h 1.21507519065e+26 cm 1.97342592996 dimensionless
>
> Then I made the following change in cosmology.py:
>
> (root) ~/yt-conda/src/yt-hg/yt/utilities $diff cosmology.py cosmology.py0
> 111,112c111,112
> <         return self.quan((self.hubble_distance() *
> <                 trapzint(self.inverse_expansion_factor, z_i,
> z_f)).value, 'cmcm')
> ---
> >         return (self.hubble_distance() *
> >                 trapzint(self.inverse_expansion_factor, z_i,
> z_f)).in_base(self.unit_system)
>
>
> Then I get:
> 80.0 Mpccm/h 4.65320708035e+26 cmcm 1.92163908776 dimensionless
> 80.0 Mpccm/h 3.62771515661e+26 cmcm 1.92124702027 dimensionless
>
> With the change of unit from cm to cmcm, the light rays have the right
> span.
>
> Even though this solves my problem, I am not sure if similar problem still
> exists. For example, instead of making change in comoving_radial_distance,
> we might need to change hubble_distance into comoving units. Hopefully
> someone familiar with yt unit system could check this.
>
> Thanks,
> Pengfei
>
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>
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