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

[Pengfei Chen]

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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