Talk:Distance measure

Page creation rationale
I made this page because information about various "distance measures in cosmology" seems to be scattered about Wikipedia. Standard textbooks often devote a chapter to the idea of distance measures and then subsections to each of the various kinds, so why not Wikipedia too?

Also, in looking around the articles, the articles on some distanc measures define other distance measures or talk about the relation between them, etc. There were also suggestions on some of the pages that a meta-distance measure page be created.

This page might be a useful introduction to what distance measures are used for, and a good place to describe how they are related to each other. For details we can just refer to the individual pages on each measure. Wesino 09:58, 22 November 2006 (UTC)

Merge from comoving distance
I noticed that the whole comoving distance article was tagged with a "merge" into Distance measures (cosmology). In my opinion this might be a bit much -- certainly comoving distance is important enough to have its own page? I think that the section regarding comparison with other distance measures on the comoving distance page, though, should go into this page. Any other opinions? Wesino 00:36, 6 December 2006 (UTC)

merged comparison section to distance measures (cosmology)
Well, no other comments since December so I'll do the move. Wesino |  t  | 09:41, 9 May 2007 (UTC)

removed article merge tag to distance measures (cosmology)
The only opinions on here for six months (mine) say no to the merge of the whole article. I just merged the comparison to distance measures (cosmology) so now I'll remove the article merge suggestion tag. Wesino |  t  | 09:48, 9 May 2007 (UTC)

Unintroduced variables
Might be because of merging and stuff, but I notice at least two variables that are used without a real introduction or explanation. First, there's chi, which seems to first come up in "If and only if the curvature is zero, then proper motion distance and comoving distance are identical, i.e. dpm = χ," which seems to mean it represents comoving distance, but it would be nice if that were explicitly defined first. Then, there's dp, first used in "A practical formula for numerically integrating dp to a redshift z...." Even less explanation of what that one is. Could someone who knows clear this up? -- J. Randall Owens | (talk) 01:16, 30 September 2007 (UTC)

Confusing Notation
Please clarify: "Angular diameter distance or proper motion distance. Angular Diameter Distance is a good indication (especially in a flat universe) of how near an astronomical object was to us when it emitted the light that we now see." -- ie. They are equal.

"d_a = d_pm / (1 + z)" -- ie.They are not equal.

Further, the lines: "   * called the angular size distance by Peebles 1993, but should not be confused with angular diameter distance [1])" and "    * sometimes dpm is called the angular diameter distance" are contradictory.
 * Yes the need for the factor (1+z) here and elsewhere is nowhere explained. JFB80 (talk) 22:08, 21 February 2014 (UTC)

proper-motion distance
Above one can read:

"d_a = d_pm / (1 + z)" -- ie.They are not equal.

This is correct. I have changed the text correspondingly. That is, the proper-motion distance is in general nost the same as the comoving distance, but rather the same as the transverse comoving distance. The proper-motion distance is the same as the comoving distance in a flat universe because there the transverse comoving distance and the proper-motion distance are the same. 87.122.202.6 (talk) 09:42, 11 November 2019 (UTC)

Complaint about reversion
You undid my edit, with the comment "E is not the Hubble parameter, and the ordering of the elements of E(z) is intentional (decreasing exponent)".
 * 1) So why did you revert my addition of a wikilink to Dimensionless Hubble constant?
 * 2) Why did you revert my addition of $$-\Omega_r$$ to the equation giving $$\Omega_k$$? Do you think it was correct before?
 * 3) It's true that I don't understand why the expression for $$E(z)$$ would be a dimensionless Hubble parameter, but that's what the editor said who put it there in the first place, and our version still says that it equals $$\frac {H(z)} {H_0}.$$ So is the version you reverted to wrong?
 * 4) I think the order in which we define the Ωs should be the same as the order they appear in the formula, and since $$\Omega_k$$ is given in terms of the others, it has to be last.

Meanwhile I am restoring the first two of these points.Eric Kvaalen (talk) 09:29, 5 January 2021 (UTC)


 * Hi, I'm the editor who put the equation there. I'm not sure what the confusion is. E(z)=H(z)/H_0 is a common definition in cosmology for convenience, see e.g. 1. Would it help to call it the "normalized Hubble parameter"? Even though 2 yields less results, for what it's worth. Quantum Doughnut (talk) 07:44, 31 March 2022 (UTC)

Merger proposal
I propose to merge Comoving and proper distances into Distance measures (cosmology). The former are two particular cases of the latter. &mdash; A. di M.  21:05, 12 January 2021 (UTC)
 * Oppose merge; the current structure is in WP:SUMMARY for, with a brief summary at Distance measures (cosmology) linking to the main article at Comoving and proper distances which provides greater detail. This structure seems to work well for readers, and is consistent with policy. Klbrain (talk) 15:17, 14 August 2021 (UTC)
 * Oppose merge; for those of us whose brains are not the size required for cosmology, it is helpful to have the longer, more explanatory article at Comoving and proper distances so we don't have to tackle the entire complexity of yet more distances in one go. Complex subjects are best dealt with in bite-sized bits. The distance-measures article is then a useful overview of all the different distances. Elemimele (talk) 07:20, 4 April 2022 (UTC)
 * Closing, given the objections and no support. Klbrain (talk) 07:42, 23 April 2022 (UTC)