bugGNU Astronomy Utilities - Bugs: bug #65195, CosmicCalculator prevents non-flat...

 
 

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bug #65195: CosmicCalculator prevents non-flat FLRW calculations; angular diam dist only on flat

Submitter:  Boud Roukema <boud>
Submitted:  Tue 23 Jan 2024 06:26:13 PM UTC
   
 
Category:  CosmicCalculator Severity:  3 - Normal
Item Group:  Output not reasonable Status:  Fixed
Privacy:  Public Assigned to:  boud
Open/Closed:  Closed

Sun 04 Feb 2024 05:06:46 PM UTC, comment #3: 

Great! Thanks a lot Boud :-).

Mohammad Akhlaghi <makhlaghi>
Group administrator
Sun 04 Feb 2024 04:27:47 PM UTC, comment #2: 

comment #1:

> About the correct function to measure the angular diameter distances in a non-flat model, can you define a new "task"? In the task, it would also be great if you could point to the relevant part of cosmdist, so some time in the future we can adopt it in Gnuastro too?


Done :) https://savannah.gnu.org/bugs/index.php?65254

Boud Roukema <boud>
Group Member
Fri 02 Feb 2024 05:27:42 PM UTC, comment #1: 

Thanks a lot Boud for catching this important point! Your commit (with cosmetic modifications as you will see) is now in the 'master' branch as Commit 5a6cf1d15.

About the correct function to measure the angular diameter distances in a non-flat model, can you define a new "task"? In the task, it would also be great if you could point to the relevant part of cosmdist, so some time in the future we can adopt it in Gnuastro too?

Thanks again for reporting and fixing this bug :-).

Mohammad Akhlaghi <makhlaghi>
Group administrator
Tue 23 Jan 2024 06:26:13 PM UTC, original submission:  

Commit 4aca51a7 prevents non-flat FLRW calculations (for curvature density parameter values greater than 1e-8 in absolute value). Although LCDM is flat by definition, there's not much point excluding non-flat models for general cosmological analysis (e.g. [1]).

I prepared commit https://codeberg.org/boud/gnuastro/commit/0b01bb6c3f8373f394badd10beac1dd64dcea0b5 for merging into gnuastro, but the problem is that the current code does not calculate the angular diameter distance at all: it just divides the radial comoving distance by (1+z), which is valid in the flat case, but not the general case.

Here are some examples of commit 0b01bb6c, with cross-checks against cosmdist 0.3.12 [2]:

radial comoving distance: (agreement)

$ for OmLam0 in 0.1 0.8 0.9; do echo 9 | ./cosmdist -r 0 -m 0.9 -l ${OmLam0} ; done
6105.05
6851.35
6986.83

$  for OmLam0 in 0.0 0.1 0.8 0.9; do ./bin/cosmiccal/astcosmiccal --properdistance --oradiation=0.0 --omatter=0.9 --olambda=${OmLam0} --H0=70 --redshift=9; done |& grep "^[0-9]"
6105.053148
6851.349603
6986.825942


angular diameter distance: (disagreement in the hyperbolic and spherical cases)

$ for OmLam0 in 0.0 0.1 0.8 0.9; do echo 9 | ./cosmdist --angular-diameter -r 0 -m 0.9 -l ${OmLam0} ; done
621.97
610.51
498.13
475.85
$ for OmLam0 in 0.0 0.1 0.8 0.9; do ./bin/cosmiccal/astcosmiccal --angulardimdist --oradiation=0.0 --omatter=0.9 --olambda=${OmLam0} --H0=70 --redshift=9; done |& grep "^[0-9]"
601.956856
610.505315
685.134960
698.682594


In the non-flat cases, cosmdist is correct and astcosmiccal is clearly wrong: for fixed H0 and Omega_{m0}, the hyperbolic (spherical) case gives a greater (smaller) one-radian-comoving-arclength (= the areal radius, from "area" + "al"), and correspondingly (dividing by (1+z)), the angular diameter distance must be greater or smaller; astcosmiccal only gets this right in the flat case.

What to do about this depends on how people wish to use astcosmiccal in practice. For the moment, cosmdist is available for general FLRW cosmological calculations (not completely general, there are checks that could still be added), without restricting to the flat case or wrongly calculating angular diameter distances [2].

[1] Yang, Weiqiang; Giarè, William; Pan, Supriya; Di Valentino, Eleonora; Melchiorri, Alessandro; Silk, Joseph, PRD, 107, 063509, https://arxiv.org/abs/2210.09865

[2] https://codeberg.org/boud/cosmdist

Boud Roukema <boud>
Group Member

 

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    Date Changed by Updated Field Previous Value => Replaced by
    2024-02-02 makhlaghi StatusNone Fixed
        Assigned toNone boud
        Open/ClosedOpen Closed
        SummaryCosmicCalculator prevents non-flat FLRW calculations; angular diam dist is generically wrong CosmicCalculator prevents non-flat FLRW calculations; angular diam dist only on flat

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