Talk:Comoving distance

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To-do list for Comoving distance: edit · history · watch · refresh

tasks for expert:

  • completely reorganize material into new articles on Notions of distance in cosmology (many kinds; see Peebles! note some articles already exist) versus comoving coordinates
  • introductions of all articles should be comprehensible to the general reader! Introduce technical complications gracefully.

tasks for anyone:

  • put references in uniform format suggested here.

Contents

[edit] Universe or universe"

Shouldn't "the Universe" be capitalized when it refers to our Universe? David W. Hogg 00:59, 24 September 2005 (UTC)

[edit] Speak Simple English!!!

Can someone translate this to Simple English?

Yes, PLEASE! This article is even more unfathomable than most scientific ones in Wikipedia. And that's saying something.
"General relativity is also a local theory, but it is used to constrain the local properties of a Riemannian manifold, which itself is global. In the context of general relativity, the assumption of Weyl's postulate is that a favoured reference frame in space-time can be decided."
Gawd almighty.
Don't know if it's any better now, but I tried :-). I also tried to correct a boatload of technical errors. BTW, the above example was just pure techno-babble (in my opinion). Pervect 04:23, 21 August 2006 (UTC)

[edit] Expert tag philosophy

This article could still use attention by an expert, but, couldn't any article?

My perception of the "expert tag" is that it should be applied to articles with technical issues, such as articles written by enthusiastic amateurs that need revision for factual accuracy.

This article previously had that exact problem. I think that I have now eliminated most of the factual problems with it, though there is certainly a lot that could be done to expand and refine the article. Most of the formulas are presented without motivation or derivation, for instance.

Pervect 21:00, 21 August 2006 (UTC)

I improved the diction. It will be a little unclear as to what changes I made since I accidently (for a moment) pasted some extraneous text and submitted it. But I have removed that by now, and the article is now much clearer.Kmarinas86 04:42, 30 August 2006 (UTC)

Oh wait, it shouldn't be that hard to compare them. I've reminded myself of the "compare selected versions" feature.Kmarinas86 05:03, 30 August 2006 (UTC)

[edit] Notions of Distance in Cosmology page

I think the suggestion that there should be some kind of notions of distance in cosmology page is a good one. The situation with the distance measures pages is kind of confused and disorganized, I think. There is now a page on Distance measures (cosmology) and a redirection from notions of distance in cosmology.

Hopefully this means that each of the pages on individual distance measures can now focus on the distance measure themselves (for example, should the comoving distance page really be defining the other measures and giving formulae for the proper motion distance?) and the Distance measures (cosmology) page can describe the overall idea and the relations between measures.

Specifically, I think a bunch of the material in the "other distances used in cosmology" might be better suited to the new page, since it discusses things beyond the scope of the title of this article. Does this sound good? Wesino 10:16, 22 November 2006 (UTC)

I noticed that the whole 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 comparison with other distance measures, though, should go on the distance measures page. Any other opinions? Wesino 00:32, 6 December 2006 (UTC)

[edit] Equation

\chi = \int_{t_e}^{t} { c \; \mbox{d} t' \over a(t')}

Should there not be a "with respect to t" "dt" at the end of that? Like this: \chi = \int_{t_e}^{t} {c \; \mbox{d} t' \over a(t')} \; dt ?

Stuart Morrow 19:03, 26 February 2007 (UTC)

I think it's with respect to \; dt'Wolfmankurd 21:59, 8 May 2007 (UTC)

[edit] More on Distance Equation

What do the te and t refer to? The integration is performed along a path of constant cosmological time, so they can't refer to cosmological time (unless all distances are identically zero, which makes for a useless definition). —The preceding unsigned comment was added by 155.148.10.80 (talk) 19:56, 25 April 2007 (UTC).

[edit] Confusion over terms

The use of technical terminology in this article is confused. Comoving distance and Proper distance are not the same thing in my books. In fact the article states:

Most textbooks and research papers define the comoving distance between comoving observers to be a fixed unchanging quantity independent of time, while calling the dynamic, changing distance between them 'proper distance'.

which is true, in which case we really shouldn't include proper distance as an alternative name in bold at the top of the article.

The following quote from the article is also wrong:

Despite being an integral over time, this does give the distance that would be measured by a hypothetical tape measure at fixed time t.

Surely this is the definition of proper distance but not of comoving distance, which is the distance travelled by a photon between two times, i.e. along a null geodesic. In fact, that whole paragraph (with the exception of the equation) is defining proper distance. —Preceding unsigned comment added by Cosmo0 (talk • contribs) 15:15, 20 September 2007 (UTC)

Granted that at a fixed time, comoving and proper distances are simply related by the expansion factor at that time, but I still think it's confusing. Cosmo0 15:27, 20 September 2007 (UTC)