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Shouldn't "the Universe" be capitalized when it refers to our Universe? David W. Hogg 00:59, 24 September 2005 (UTC)
Can someone translate this to Simple English?
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)
The discussion/formula on distances is not entirely correct... and so probably confusing to some non-expert readers.
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)
Should there not be a "with respect to t" "dt" at the end of that? Like this: ?
Stuart Morrow 19:03, 26 February 2007 (UTC)
User:Blanclar Right! —Preceding undated comment added 16:46, 13 September 2020 (UTC)
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).
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)
Cosmo0 is right about confusion over "comoving distance" vs. "proper distance". The sentence in the article,
"The comoving distance from an observer to a distant object (e.g. galaxy) can be computed by the following formula:"
should read,
"The proper distance from an observer to a distant object (e.g. galaxy) can be computed by the following formula:"
Also, the following sentence presently included in the article just about had me convinced that the whole topic was a fallacy:
“Despite being an integral over time, this does give the distance that would be measured by a hypothetical tape measure at fixed time t.”
What kind of tape measure is this; is it analogous to an ideal physical tape with infinite elastic modulus? What is meant by, “fixed time t”? Does this mean that the expansion of space is put on pause for an instant while the measure is being taken? Or does it mean that the tape only exists at the one instant when the distance is measured? Neither scenario is conducive to understanding. To make common sense of comoving coordinates, without resorting to general relativity, you need to postulate a tape that exists longer than an instant, and you must not pause the expansion even for an instant.
Suppose you have comoving galaxies, A & B, whose centers (in proper distance) are 1026 meter (ten billion light years) apart at time = t. Shortly before t, you place your tape measure in line between the galaxies with the center of the tape measure midway between them and comoving with them. This being an ideal tape measure, its length is fixed in its own inertial frame; the ends of the tape measure are therefore not comoving with the center of the tape and not comoving with the galaxies. While the ends of the tape remain a constant distance apart in the inertial frame of the tape, the proper distance between the galaxies is increasing at about 2.5 x 108 m/s (.833 c). (Actually, the relative motion in the inertial frame of the tape would be less than .833c because the time dilation would slow the expansion of space in the inertial coordinates of the tape. In the comoving frame of the galaxies, the relative motion would be less than .833 c because of the relativistic contraction of the tape; observers moving with the galaxies and with the ends of the tape would perceive the same relative velocity. The math for calculating that relative velocity is beyond my ability.) This gives relativistic motion between the galaxies and the ends of the tape, and it is not merely “apparent motion” because both the tape and the galaxy exist at the same place and time. If the tape measure has intermediate distance marks, near its ends they will be about half their correct distance apart according to observers moving with the galaxies; intermediate marks near the middle of the tape will be the correct distance apart in both comoving and inertial frames. Clocks moving with the ends of the tape will run about half as fast as the observer’s clock moving with the galaxy. If you hit the “pause” button to stop the expansion just for an instant, you would have each end of the tape in two places at the same time, and it would experience infinite acceleration. You would also be nullifying the gravity field associated with the expansion (acceleration being equivalent to gravity, according to Einstein).
Here’s my solution: Let’s use the same galaxies, A & B, as above. Instead of a tape, let’s postulate a measuring chain between the two galaxies. The length of each link is 1022 meter (1 million light years). Shortly before t, the centers of the links at opposite ends of the chain are fastened to the centers of the galaxies, and each of the remaining links is comoving with the galaxies, leaving an equal amount of slack between successive links. As t approaches, the slack disappears at a rate of about 25 km/s (.0000833 c). The chain and its links may be imaginary, but the relative motion between links is real. The relativistic contraction and time dilation of each link as seen from the adjacent link is also real, but due to the relatively short length of the links, the relativistic effects are insignificant (less than ten parts per billion). The last bit of slack disappears at t, giving the proper distance between the galaxies as the sum of the lengths of the chain links.
I propose the following to replace the present wording:
“Despite being an integral over time, this does give the distance that would be measured at time = t by a hypothetical measuring chain of equally spaced comoving links at the instant when the expansion of space eliminates the last bit of slack between links. For each new t, a new measuring chain must be postulated, because the old hypothetical chain has snapped.
For shorter links, relativistic contraction between adjacent links is proportional to the square of the length of each link. If each link is 1022 meter (1 million light years) long, relative motion between successive links is about .00008 c, and relativistic contraction of successive links relative to one another is then less than ten parts per billion. That is why relative motion between comoving galaxies is referred to as only apparent, though in inertial coordinate systems, it is real.”
-- Onerock ( talk) 06:35, 2 August 2008 (UTC)
In the integral defining the comoving distance, what does c denote? It appears in the equation without any explanation. Is it the speed of light? - 99.242.17.45 ( talk) —Preceding undated comment added 03:06, 24 January 2010 (UTC).
Yes, c denotes the speed of light. It's now corrected. Daniel Olivaw ( talk) 14:52, 8 June 2010 (UTC)
I've added a layman-accessible summary to the lede. Among other things, we get questions about what "comoving distance" is every couple of months at quasar, with complaints that this article isn't helpful for describing that. The lede should fix that.
I realize that the description is approximate. This is intentional, for the sake of simplicity, and noted in the lede.
Per the other threads on this talk page, it's possible that my edit (and the quasar article) should use the term "proper distance" instead of "comoving distance". I don't have sufficient expertise to settle that question in the above thread; I suggest bringing it to the attention of the people at WT:AST to get it resolved (and to propagate corrections to other articles if changes are needed). -- Christopher Thomas ( talk) 21:03, 17 July 2010 (UTC)
I've often heard it said that "FTL [Faster Than Light] allows time travel". Nonetheless, it seems to me that virtually all matter is, to some approximation, at rest relative to a " comoving frame" (if I understand the idea properly; note that this link doesn't go anywhere useful). By which I mean that if you jump the spacelike interval (comoving distance) to any number of comoving frames at the same time from the creation of the universe, you never move backwards toward the creation of the universe, and thus you never move backward in time relative to any of those frames. It also follows that if only one's velocity relative to the comoving frame would carry over in such a jump, then you come out "more or less at rest" wherever you end up, little things like the whirl of galaxies aside. (Though since revolving matter drags spacetime along with it a little, would the comoving frame pick up some of its velocity...?) Can someone point to a source that investigates such speculations?
Also, has anyone used particle accelerators specifically to observe the properties of matter propelled so as to be at rest relative to the bulk of material objects in the universe in relation to their local comoving frames? Wnt ( talk) 14:16, 29 July 2010 (UTC)
1) I think that sin/h^-1 is better expressed as arcsin/h.
2) arcsin/h() does not accept metric argument values; arcsin(1 meter) is meaningless.
I assume that the intended expressions would rather be R×arcsinh(r/R) and R×arcsin(r/R), respectively. Hilmer B ( talk) 21:02, 6 November 2016 (UTC)
Still - to a common reader it must be utterly confusing with a distance without a metric dimension. Four lines further down says. "the proper distance d(t) at an arbitrary time t is simply given by d(t) = a(t) χ where a(t) is the scale factor" (click that link!). And as far as any reader understands it: 0<a(t)<1, with no metric dimension whatsoever involved. The equations per se are OK, but here they are utterly out of context! Hilmer B ( talk) 22:41, 4 October 2017 (UTC)
The result of the move request was: moved DrStrauss talk 13:45, 9 October 2017 (UTC)
Comoving distance →
Comoving and proper distances – The article is clearly about both of the distances - and even starts by comparing the two. See no reason why the preference should be given to the comoving distance
OlJa 10:33, 2 October 2017 (UTC)
[1] [2] [3] [4] — Preceding unsigned comment added by 2A02:2149:8769:F400:A82C:4E97:EBEE:1488 ( talk) 14:12, 27 February 2018 (UTC)
Although a lot of scientists cackle the same, there's no proof of the Universe actually expanding. The stuff within the Universe may move away from each other and create the illusion that it's expanding, but it's simply not true. The Universe consists out of three entities, time, space and filler (matter/energy). The one that came last moving around in infinite space, does not expand or contract the space, but is simply motion within that infinite space. Also, timespace does not exist, these are separate, time and space, of which time came first, then space. The filler came last. Without time, there can be no change, and thus there can't be change from a time without space to a time with space, so time came before space. (Anything can only have one state at any given time, including Δt=0 and t=0) Since filler can't exist or move or move away from the initial point of release without space, the filler came after space. So, time first, space second, filler last. And if the filler moves, it does not expand space, it just moves within it. Simple, yes?
(The fact that many state the same, does not make true. Once people all said that the Earth was flat, and everyone 'knew' it. Lack of knowledge leads to misunderstanding, and yet, each misunderstanding was never true.)