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Archive 1 | Archive 2 | Archive 3 |
Nice article. Hope you like the small changes made. They are as follows:
Nice work! FT2 ( Talk | email) 15:49, 16 July 2006 (UTC)
The observable universe is flat. It may be non-Euclidean on unobserved scales, but cosmic inflation prevents us from knowing what kind of curvature the manifold happens to have. -- ScienceApologist 22:43, 16 July 2006 (UTC)
I've got two ways to explain this for lay-readers, to try and make the point simply.
One is this:
This captures the idea that it is not about objects moving, but about the nature of space changing. As a lay-explanation I think its a good starting point to show how very different metric expansion is and make it comprehensible.
The other is this diagram (self drawn):
A fractral-like analogy. An observer at A (the left) and one and B (the right) or indeed any two observers, will see the "ground" between A and B as remaining almost flat, however on a macro scale, as one moves from top to bottom there is a gradually increasing distance for a 1-D walk between any two points.
The crucial emphasis in these two descriptions being (unlike the ant and raisins) to show that it is the metric of space that is expanding. The more common analogies, although well known, actually demonstrate normal expansion to a lay-person -- exactly what is being attempted not to be shown. I feel these are two ways to describe it for Wikipedia, that show the key idea that space itself is changing.
Like em? FT2 ( Talk | email) 13:46, 17 July 2006 (UTC)
Your image could do with some tweaking. I think what you are trying to convey is local flatness which is a feature of general relativity as well as the geometry of our particular space time. What would be better would be to have a picture of a space with simple curvature (not ripples but a single curve) and an ant crawling along. Then enlarge that image to show that the ant perceives space to be flat because the curvature is macroscopic. -- ScienceApologist 13:55, 17 July 2006 (UTC)
It's neither. I'm trying to think of ways to convey the idea that the sense/appearance/reality of "distance" itself is changing, even if the objects themselves separate from the metric expansion, are not. the current analogies both imply "explosion of objects into a bigger unknown, which is exactly what metric expansion isn't. That's the problem I have with it -- that is a fundamental distinction, its not expansion into emptiness, its a change in the distance/metric of existing space. Those are 2 ways I can think of to convey that idea. Can you think of a way that works for you? (and yes, "growing" is a good edit) FT2 ( Talk | email) 15:16, 17 July 2006 (UTC)
I agree that we aren't there yet. Keep on editting. Oh, and if you come across any references (especially print) that you can use to start footnoting the article, please do so. I am going to dig out some Astro 101 texts to try to provide some, but I might not get to it for a few days. -- ScienceApologist 22:32, 17 July 2006 (UTC)
This isn't the place really to learn about basic physics and mathematics. It is really a talkpage about the article. I will answer your questions here on your talkpage, but please, if you have more, don't clutter up this talkpage which is ostensibly supposed to be about making the article better. -- ScienceApologist 12:17, 3 November 2006 (UTC)
Please, keep it shorter and simpler. The intro is an short introduction, for newcomers, we need to resist the temptation to make it a substantial technical "advanced readers" subarticle of its own :)
I'll revert some of it, trying to not lose any of the info, but I think honestly the previous level of detail was better. It's just gone a bit too far back towards "technical creep". Will try not to lose any valuable edits though. A couple of edits I'm thinking don't need to be in the intro include:
Hope this is okay with you. Its just slipping to the point its unhelpful, otherwise. FT2 ( Talk | email) 21:41, 18 July 2006 (UTC)
Not sure that's even so, really. Compare:
Compare them... The 1st says "Its is the phenomenon where space expands over time. That doesnt explain much, and its slightly misleading on 2 scores: its a (speculated theoretical) hypothesis not an (observed) phenomenon, and it emphasises space expanding rather than the metric describing space expanding -- a really crucial distinction to introduce up front (especially given the title). It then goes on to talk about the vaccuum of space, where at this poiint just "space" is almost as accurate and much simpler, and says the tensor is relevatistic which again is too much detail at this point.
The 2nd says, its a part of our understanding, in which space is described by a metric that is changing, which explains how the universe expands. Much simpler. Thoughts? FT2 ( Talk | email) 22:21, 18 July 2006 (UTC)
Comments
-- ScienceApologist 17:57, 19 July 2006 (UTC)
If thats the case, I've drafted an "evidence" section based on my understanding of the above. As ever, can you fix it for any misunderstandings and errors? Thanks. FT2 ( Talk | email) 22:40, 19 July 2006 (UTC)
Quickies for you:
Can you discuss or explain how time behaves below, a bit, and clarify these two issues? Thanks FT2 ( Talk | email) 16:03, 19 July 2006 (UTC)
Clarifications
-- ScienceApologist 17:48, 19 July 2006 (UTC)
This is a great article, however it is only sourced by 2 science magazines. If you can get better referencing, and expansion in general, it can defintely become a Good Article. -- GoOdCoNtEnT 19:21, 10 August 2006 (UTC)
I'm another reference stick-in-the-mud, often asking for them in reviewing articles. There are purposes for asking for in-line references and they have nothing to do with note stacking to make it look impressive.
The first is to give academic credit to someone who discovers a fact, compiles a statistic, creates a hypothesis, comes to a reasoned conclusion, etc. (The negative side is to avoid charges of plagiarism) My rule of thumb is that, if a statement can be found in two or more expert sources, no need for a citation. I also arbitrarily look for at least one cite per section.
The second is to help a reader find the exact source of a notion if they want to learn more. This is what I actually urge my students to use the wiki for.
The third is to help us with verification, especially where there is controversy among editors (this one may not apply to you).
The only other observation I have as a complete layman on this subject is that, while nicely written, I'm still having difficulty wrapping my brain around the subject. This would not stop my promoting it, but thought you all would like to know. -- CTSWyneken (talk) 11:23, 22 August 2006 (UTC)
I've put some little questions into <-- SGML comments --> within the article. -- Hoary 07:01, 19 August 2006 (UTC)
I'm sure physicists can happily live without advice from physics-ignorant people (that's me!) with a vague memory of some philosophy of science courses. Still, I have to point out that every crackpot 20th century theory of the psyche was able to come up with observations that confirmed what it proclaimed. A question (if I may hugely oversimplify) was of whether and how it risked empirical refutation. (Notoriously, nothing we can even imagine would show that the Freudian model is wrong, and therefore the model is scientifically worthless.) So I'm not so impressed by a lot of the "Observational evidence", at least as it is described. I am interested in the predictive power, as alluded to in the final paragraph of that section. But this is described very vaguely. What are these predictions? Did earlier theories make the wrong predictions, less accurate predictions, or no predictions at all? -- Hoary 07:13, 19 August 2006 (UTC)
You flatter my question, but on its behalf I thank you all the same. I hope my prodding here and there doesn't irritate you. Something about the contrast between this article and the vapidity of the subject-matter of many indubitably good articles sticks in my craw; I'd like to see this promoted to "Good Article" too. -- Hoary 14:34, 19 August 2006 (UTC)
I put the "Good article" nomination on hold on this article, because I had some concerns with factual correctness. I hope to fix these minor nits personally, with appropriate feedback from the originator & other editors to "keep it simple". I then propose that someone else review the final result. I have some concerns about verifiability of this article - the factual errors crept in in the first place due to writing from memory rather than citing sources. I'm going to commit the same mistake, probably, because all my references are too technical to be useful. But I'll leave the end decision on the fate of the article up to the final reviewer. Meanwhile I want to fix some stuff Pervect 20:49, 22 August 2006 (UTC)
My technical side would really like to add a note explaning that the metric of space-time computes not ordinary distance, but the Lorentz interval.
However, I'm afraid that this might scare readers, and not otherwise add much to the article, since the Lorentz interval reduces to proper distance for spacelike paths, and proper time for timelike paths.
Thus at the moment I'm leaving well enough alone. Pervect 23:08, 22 August 2006 (UTC)
At first I thought it was me, but these are clearly self-contradictions:
So, which is is that changes, the metric of space, or the metric of space-time? Lurk22 01:17, 17 September 2006 (UTC)
Note: This article has a small number of in-line citations for an article of its size and subject content. Currently it would not pass criteria 2b.
Members of the
Wikipedia:WikiProject Good articles are in the process of doing a re-review of current
Good Article listings to ensure compliance with the standards of the
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Agne
05:51, 26 September 2006 (UTC)
Reply to comments left on my talk page
Please read
this. Very briefly, I as a (distinctly!) non-expert suggested that the ideas presented in this article are now mainstream within physics; the interested person can therefore make his or her own choice of mainstream physics book for verifying or reading more about it. The (apparently) expert editors seemed to agree. Meanwhile, the original physics research papers would be hugely too difficult for any but a minuscule percentage of WP readers; the tiny number of people who can read them would not be reading WP articles about the subject. -- Hoary 08:41, 26 September 2006 (UTC)
As a contributor to the above dispute, I concur. Please note my comments there. I would weigh in on the talk page at the GA space, but the argument is already long and any additional reflections I have would probably only add to the noise. But this article highlights the problem with having a rigid citation rule. The subject matter is arcane to uninformed readers, but to scientists, the discussion is quite elementary and fundamental. It is like requiring a citation that the sun rises in the east. I worry that the effect of applying WP:CITE blindly can lead to an amateurish presentation of subjects that will sponsor laughs from an informed reader; in other words, out of place for a serious encyclopedic project. (Don't misunderstand me though: generally, citation is hugely important.) Also, citations are not always good citations. I failed the GA nom for Marcel Proust in part b/c the works cited were woefully inadequate. Having a cite policy that reqires x or y number, however, may lead reviewers to feel that material has passed some kind of verifiability test, even when the cited sources would lead an informed reader of the subject to look askance. Eusebeus 14:41, 26 September 2006 (UTC)
The policy is not X or Y number but for the benefit of WP:V all major claims should be attached to a reliable source that verifies that claim. Simple as that. If a reviewer is blindly impressed with a number then they are simply not a good reviewer with that lacking falling on them and not the criteria. In all honestly, this is not new. It is something that the foundational wikipedia guidelines have asked for all along and in a time when Jimbo and the rest of the Wikipedia community are steering towards more quality over quantity, the GA project is moving accordingly. I don't doubt the truth of this article or that it is mainstream physics. But it has never been about truth but rather verifiability. Agne 17:31, 26 September 2006 (UTC)
I have made a request regarding this issue here. -- ScienceApologist 21:01, 26 September 2006 (UTC)'
This article would be much stronger with more specific reference to the relationship between metric expansion and gravitation - specifically, the metric tensor that describes how matter expands, shrinking space over time to produce the effects of gravity. It needn't and shouldn't be mathematical or obscure, and can actually clarify the article for lay readers.
Like space, matter itself is expanding - in ways that are easier to measure with a scale than a ruler. We can verify that the earth's surface is expanding at an acceleration of g by placing an object of known mass on a scale. To observers also being pushed by the earth's surface, this motion of the surface creates an impression that inertial objects are falling toward the earth. In this sense, Newtonian gravity is a fictitious force like the centrifugal force experienced by a rider on a merry go round. The strongest clue that it's really the earth's surface accelerating outward is, all the inertial objects appear to accelerate downward at the same rate, independent of their mass or lack of it. The equivalence of gravitational and inertial mass is a fundamental observation of general relativity.
The 'real' force responsible for our weight is the outward expansion of the earth's surface. Gravity doesn't pull us; the earth's surface pushes against us where we contact it. Since every atom and object at the earth's surface is expanding at the same rate, a ruler made of atoms won't show it.
Explaining this can give readers a clearer idea of both metric expansion and gravitation, and why metric expansion may not be obvious from our point of view. Moreover, metric expansion of space helps explain how the earth can be expanding this way without colliding with say, the sun: space itself is expanding too. Thus, the two should be discussed together. -- Dlevity 12:26, 19 November 2006 (UTC)
I've got a model for an expanding universe that allows for all the various effects to be incorporated. Universal expansion, a time of acceleration from a point and a time of acceleration back into a point.
I'm autocading the design right now.
Is there a particular format I should use for formatting the article? -- Mad Morlock 19:30, 11 December 2006 (UTC)
Old text:
This is not quite correct. We cannot arbitrarily chose different metrics. There is a physical distance between any two nearby points that is well defined. The metric calculates the distance between nearby points. Thus there is no freedom to chose the metric coefficients once we have chosen a coordinate system, only one set of metric coeffients will give the correct answer for distance.
In the example given, the distance between New York and Lodon is not strictly defined until one defines the exact curve connecting New York and London. The usual convention is to chose a 'great circle' connecting them, but one might chose a different curve.
But this choice of connecting curve does not matter to the metric coefficients, which determine the distance between nearby points. When two points are close, the curvature does not matter, and we can consider the points to be in an essentially Euclidean space.
When we go to the relativistic metric, we replace the concept of distance by the concept of Lorentz interval. This is something that should perhaps be added into the article, but I'm not sure if/how it can be done without confusing the reader. Once we have chosen the coordinate system, and once we have chosen whether the metric should represent pure distances (as on the surface of the Earth) or the Lorentz interval, we have no freedom in choosing the metric coefficients. Pervect 01:24, 12 December 2006 (UTC)
Hi, I love this article, because it's explained expansion to me, in a way that makes sense to me. If you take a bunch of points, and multiply their coordinates by 3.0, it doesn't matter where you set the origin, it'll all appear the same: like everything is going away from everything else, depending on the distance. I get it: theres more space between stuff. I love this article.
I have one small request: Can you somehow make it clearer that the Inflationary period at the time of the big bang is something very different than the Metric Expansion of space? That, it's a very brief time, during which the metric expansion of space happened very quickly, but that otherwise, the increase of the metric expansion of space is constant? That was something that I didn't get from the article, but learned later, from a usenet conversation. (I may be misled.) I kept getting the metric expansion of space confused with the inflationary period, and I think this page could more clearly distinguish them.
Thank you, Lion Kimbro.
LionKimbro 05:03, 21 December 2006 (UTC)
It should be mentioned that the expansion of the universe is often used as evidence for a Creator in Christian and Islamic Apologetics.
Christian sources:
Islamic sources:
-- Jorfer 23:48, 26 January 2007 (UTC)
I tried to generate an animation to illustrate this concept, but I ran into difficulties.
My idea was to draw a couple of circles (representing bodies in space) on a square grid, where the number of grid squares represents the measured distance between them. Then I was planning on shrinking the size of the grid squares over time. This would result in an increase in the number of squares (i.e. measured distance) between the two bodies, without them visually moving apart.
This appears at first glance to be a great illustration, but it results in the measured diameter of each body increasing over time at the same rate that the separation increases.
On the assumption that the metric expansion of space does not entail the expansion of all bodies within space, the obvious solution is to decrease the size of the circles so that their diameter is constant relative to the shrinking squares. However, if you visualise what such an animation would look like, you can see that it would look an awful lot like the 'camera' was zooming out from an image of two discs that were moving apart from one another. Is this assumption wrong? Are bodies supposed to expand, or is it only the space between them that does so? If the former, then the raisin bread analogy is flawed because the raisins don't expand. Hubble's Law makes no distinction between light from stars in our galaxy and light from other galaxies, right? So what are the "raisins"?
Can anyone shed any light on this? SheffieldSteel 21:32, 1 March 2007 (UTC)
Problems visualising this may alway be. No model is prefect. I like the balloon model, to demonstrate, I will put dots on it. As the balloon is inflated-expands the dots get farther apart. The dots on the balloon are galaxies. The key is the curve of the balloon is not just space but time. We call this time-space curvature. In the expansion of universe, not just space is expanding, but time as well. The galaxies are on the surface of the 4D universe as it expands. The expansion started with the Big Bang 13.7 billion years ago, when all space, time and matter came into existence. As the universe expands the time-space curvature is less. In the early universe time-space curvature was greater and the galaxies closer to each other. Telecine Guy 08:32, 12 March 2007 (UTC)
I was hoping to find some mention of Olbers's Paradox in this article. — Loadmaster 22:32, 23 July 2007 (UTC)
What is the rate of this metric expansion? Does it increase the distances in our solar system over time? If so, doesn't it affect the length of a year? Does mass remain constant with this metric expansion of space? Landroo 05:23, 27 July 2007 (UTC)
In the Overview, the article explains in a somewhat roundabout and indirect, but understandable way how a metric of distance can be constructed and applied to a coordinate space. This is good.
However, the part that says "The metric of space appears from current observations to be Euclidean, on a large scale. The same cannot be said for the metric of space-time, however. The non-Euclidean nature of space-time manifests itself by the fact that the distance between points with constant coordinates grows with time, rather than remaining constant." is, I think, confusing. It's not clear what "distance" is growing with time. Is it "just some obscure mathematical construct" (bear with me, I'm trying to see this from a complete layman's perspective, which isn't difficult because I almost am one), or is it the "real distance" between the two points? How is this distance, the distance that grows with time, measured? In other words, what is the metric of the universe (the one that is expanding)?
Later, the article goes on to say "In spaces that expand, the metric changes with time in a way that causes distances to appear larger at later times" -- this seems to imply that they aren't "really" larger, they just "appear" to be larger. I realize that this is very difficult to explain at this basic level, but since this seems to be what the article is attempting to do (which I think is a good idea), maybe you could try harder. The reader has to let go of the everyday concept of distance to avoid confusion; see below.
A simplified description of the metric should, I think, appear earlier in the article.
Another issue giving rise to confusion is the common-sense argument that as everything we observe is in space, right, then a hypothetical tape measure we could measure distances with would also be in space; hence, if space were to expand, then the tape measure would expand with it, and we'd still be reading the same distance. The statement "the metric itself changed exponentially, causing space to change from smaller than an atom to around 100 million light years across" seems, approached from a common-sense perspective, to imply that there is some unchanging "super-space" "outside our space", and that during the early life of our universe, our space, measured on a tape measure in this "super-space", was smaller than the size of an atom across, but then suddenly expanded, again measured on the tape measure "outside", to a size of 100 million light years. The "size of the atom" would then, apparently, be the size an atom is now, measured on this absolute tape measure; since, if space is expanding, the space inside an atom is also expanding, making the atoms bigger, surely? But, since our non-absolute tape measures are also likewise expanding, we're none the wiser, right?
This apparent implication should, I think, be addressed to remove confusion. Maybe explain that distances can be shown to have increased because light is taking longer to traverse them, and also why this doesn't mean that light slowed down, or that time speeded up (which, superficially, would both appear to be plausible explanations of light taking longer to cross a fixed distance).
I think the "Measuring distances" section should be expanded to serve as an introductory-level description of the problem and solution of measuring "long" distances. As it is, anyone familiar with the subject matter will probably understand it, but a layman, even armed with a fair understanding of mathematics, will probably not. The construction of the comoving distance metric seems arbitrary, and its physical meaning is unclear. I'm not sure how it could be explained in an intuitive way. Also, it's unclear whether it is in fact the, or only the, comoving distance that is increasing as space expands, or whether the other possible distance measures alluded to are also affected, and what the physical meaning of this is.
The figure caption only adds to the confusion as it says "The Hubble constant can change in the past ..." -- "you what?", my internal semantic parser thinks. :) How can a constant change? And how can it change in the past, yet in the present tense? Don't you mean "it can/may have changed" or "it could/might have changed"? Also, how can it change dependent on the observed value of density parameters? Surely, it doesn't matter whether we observe them or not?
The Copernican Principle as cited seems to contradict (again, based on common sense) what the article says earlier about the early universe having the size of an atom - surely, that atom-sized structure had a well-defined center, and well-defined boundaries? Where did these go, if they aren't around anymore? And if space is expanding, it is "expanding away" from the original center of that atom-sized structure, right? That is, there "must be" a well-defined center of the universe -- in fact, if the amount of matter is finite, it is even possible to define it as, say, the center of mass of all matter, right? And if expansion is symmetric, then this point is really the center of the universe, right? Wow, awesome! Can we put up a sign? :)
About "Ant on a balloon": the analogy is good, but there is an unaddressed issue related to the "everything is in space" argument above. The problem is that the ant is not itself in the "plane" of the balloon surface, but on it and is thus not itself directly affected by the balloon growing (maybe it has to shuffle its legs around a bit to keep them close to each other :). If the ant were in the fabric of the balloon (as we, observers of the universe, are in the fabric of said universe), then it would expand just as the balloon expands, and, depending on how it measures the distance between two points (e.g. by counting the number of steps required to reach B from A), it wouldn't necessarily detect a change in distance.
The raisin bread model does talk about the problem of the expanding ruler (or tape measure) a bit, but I think it's too little, too late. The concept of being "bound", and how and why it affects expansion, should be introduced earlier. Also, since gravity has an infinite range (although its effects approach zero as distance from the mass approaches infinity), all objects in the universe are "bound", aren't they? The distance between any two objects is finite, hence they exert a finite gravitational force on each other, right? OK, it's not called a force in relativity, but still, they both curve spacetime to a finite degree around the other object, making them "bound", right?
Modern science is a series of assaults on common sense: the Earth is not flat even though it appears to be; the Sun doesn't revolve around it even though it appears to; etc. I think it's important to explain why the assertions that apparently violate common sense are still true, and how common sense is mistaken/misled.
-- 195.56.53.118 23:47, 2 August 2007 (UTC)
I agree with 195.56.53.118. Galaxies are moving apart from each other so co-moving co-ordinates are chosen as a convenience. Other co-ordinate systems could be used to cover the manifold, eg. lots of different systems in describing black holes. Just because the co-ordinates are moving doesn't mean space is. What does it mean that space is expanding? It doesn't mean that objects can't move closer together. It doesn't make atoms get any larger. I can't see that it means anything, for example, how could you tell the difference between stationary objects in expanding space and stationary space with objects moving apart? Please tell me where I'm wrong and be as technical as you like. I see Ned Wright's cosmology tutorial seems to agree with me. Neodymion 04:53, 3 September 2007 (UTC)
According to the source I linked: It seems that we can't see the ones that haven't had time for their light to reach us yet. As to their velocity being limited by c, it is, but in the same way as SR, where adding velocities by Lorentz boosting always gives an answer less than or equal to c. Thanks for answering my question, though, at least I don't feel like I'm completely missing something now. Neodymion 12:50, 3 September 2007 (UTC)
This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | Archive 2 | Archive 3 |
Nice article. Hope you like the small changes made. They are as follows:
Nice work! FT2 ( Talk | email) 15:49, 16 July 2006 (UTC)
The observable universe is flat. It may be non-Euclidean on unobserved scales, but cosmic inflation prevents us from knowing what kind of curvature the manifold happens to have. -- ScienceApologist 22:43, 16 July 2006 (UTC)
I've got two ways to explain this for lay-readers, to try and make the point simply.
One is this:
This captures the idea that it is not about objects moving, but about the nature of space changing. As a lay-explanation I think its a good starting point to show how very different metric expansion is and make it comprehensible.
The other is this diagram (self drawn):
A fractral-like analogy. An observer at A (the left) and one and B (the right) or indeed any two observers, will see the "ground" between A and B as remaining almost flat, however on a macro scale, as one moves from top to bottom there is a gradually increasing distance for a 1-D walk between any two points.
The crucial emphasis in these two descriptions being (unlike the ant and raisins) to show that it is the metric of space that is expanding. The more common analogies, although well known, actually demonstrate normal expansion to a lay-person -- exactly what is being attempted not to be shown. I feel these are two ways to describe it for Wikipedia, that show the key idea that space itself is changing.
Like em? FT2 ( Talk | email) 13:46, 17 July 2006 (UTC)
Your image could do with some tweaking. I think what you are trying to convey is local flatness which is a feature of general relativity as well as the geometry of our particular space time. What would be better would be to have a picture of a space with simple curvature (not ripples but a single curve) and an ant crawling along. Then enlarge that image to show that the ant perceives space to be flat because the curvature is macroscopic. -- ScienceApologist 13:55, 17 July 2006 (UTC)
It's neither. I'm trying to think of ways to convey the idea that the sense/appearance/reality of "distance" itself is changing, even if the objects themselves separate from the metric expansion, are not. the current analogies both imply "explosion of objects into a bigger unknown, which is exactly what metric expansion isn't. That's the problem I have with it -- that is a fundamental distinction, its not expansion into emptiness, its a change in the distance/metric of existing space. Those are 2 ways I can think of to convey that idea. Can you think of a way that works for you? (and yes, "growing" is a good edit) FT2 ( Talk | email) 15:16, 17 July 2006 (UTC)
I agree that we aren't there yet. Keep on editting. Oh, and if you come across any references (especially print) that you can use to start footnoting the article, please do so. I am going to dig out some Astro 101 texts to try to provide some, but I might not get to it for a few days. -- ScienceApologist 22:32, 17 July 2006 (UTC)
This isn't the place really to learn about basic physics and mathematics. It is really a talkpage about the article. I will answer your questions here on your talkpage, but please, if you have more, don't clutter up this talkpage which is ostensibly supposed to be about making the article better. -- ScienceApologist 12:17, 3 November 2006 (UTC)
Please, keep it shorter and simpler. The intro is an short introduction, for newcomers, we need to resist the temptation to make it a substantial technical "advanced readers" subarticle of its own :)
I'll revert some of it, trying to not lose any of the info, but I think honestly the previous level of detail was better. It's just gone a bit too far back towards "technical creep". Will try not to lose any valuable edits though. A couple of edits I'm thinking don't need to be in the intro include:
Hope this is okay with you. Its just slipping to the point its unhelpful, otherwise. FT2 ( Talk | email) 21:41, 18 July 2006 (UTC)
Not sure that's even so, really. Compare:
Compare them... The 1st says "Its is the phenomenon where space expands over time. That doesnt explain much, and its slightly misleading on 2 scores: its a (speculated theoretical) hypothesis not an (observed) phenomenon, and it emphasises space expanding rather than the metric describing space expanding -- a really crucial distinction to introduce up front (especially given the title). It then goes on to talk about the vaccuum of space, where at this poiint just "space" is almost as accurate and much simpler, and says the tensor is relevatistic which again is too much detail at this point.
The 2nd says, its a part of our understanding, in which space is described by a metric that is changing, which explains how the universe expands. Much simpler. Thoughts? FT2 ( Talk | email) 22:21, 18 July 2006 (UTC)
Comments
-- ScienceApologist 17:57, 19 July 2006 (UTC)
If thats the case, I've drafted an "evidence" section based on my understanding of the above. As ever, can you fix it for any misunderstandings and errors? Thanks. FT2 ( Talk | email) 22:40, 19 July 2006 (UTC)
Quickies for you:
Can you discuss or explain how time behaves below, a bit, and clarify these two issues? Thanks FT2 ( Talk | email) 16:03, 19 July 2006 (UTC)
Clarifications
-- ScienceApologist 17:48, 19 July 2006 (UTC)
This is a great article, however it is only sourced by 2 science magazines. If you can get better referencing, and expansion in general, it can defintely become a Good Article. -- GoOdCoNtEnT 19:21, 10 August 2006 (UTC)
I'm another reference stick-in-the-mud, often asking for them in reviewing articles. There are purposes for asking for in-line references and they have nothing to do with note stacking to make it look impressive.
The first is to give academic credit to someone who discovers a fact, compiles a statistic, creates a hypothesis, comes to a reasoned conclusion, etc. (The negative side is to avoid charges of plagiarism) My rule of thumb is that, if a statement can be found in two or more expert sources, no need for a citation. I also arbitrarily look for at least one cite per section.
The second is to help a reader find the exact source of a notion if they want to learn more. This is what I actually urge my students to use the wiki for.
The third is to help us with verification, especially where there is controversy among editors (this one may not apply to you).
The only other observation I have as a complete layman on this subject is that, while nicely written, I'm still having difficulty wrapping my brain around the subject. This would not stop my promoting it, but thought you all would like to know. -- CTSWyneken (talk) 11:23, 22 August 2006 (UTC)
I've put some little questions into <-- SGML comments --> within the article. -- Hoary 07:01, 19 August 2006 (UTC)
I'm sure physicists can happily live without advice from physics-ignorant people (that's me!) with a vague memory of some philosophy of science courses. Still, I have to point out that every crackpot 20th century theory of the psyche was able to come up with observations that confirmed what it proclaimed. A question (if I may hugely oversimplify) was of whether and how it risked empirical refutation. (Notoriously, nothing we can even imagine would show that the Freudian model is wrong, and therefore the model is scientifically worthless.) So I'm not so impressed by a lot of the "Observational evidence", at least as it is described. I am interested in the predictive power, as alluded to in the final paragraph of that section. But this is described very vaguely. What are these predictions? Did earlier theories make the wrong predictions, less accurate predictions, or no predictions at all? -- Hoary 07:13, 19 August 2006 (UTC)
You flatter my question, but on its behalf I thank you all the same. I hope my prodding here and there doesn't irritate you. Something about the contrast between this article and the vapidity of the subject-matter of many indubitably good articles sticks in my craw; I'd like to see this promoted to "Good Article" too. -- Hoary 14:34, 19 August 2006 (UTC)
I put the "Good article" nomination on hold on this article, because I had some concerns with factual correctness. I hope to fix these minor nits personally, with appropriate feedback from the originator & other editors to "keep it simple". I then propose that someone else review the final result. I have some concerns about verifiability of this article - the factual errors crept in in the first place due to writing from memory rather than citing sources. I'm going to commit the same mistake, probably, because all my references are too technical to be useful. But I'll leave the end decision on the fate of the article up to the final reviewer. Meanwhile I want to fix some stuff Pervect 20:49, 22 August 2006 (UTC)
My technical side would really like to add a note explaning that the metric of space-time computes not ordinary distance, but the Lorentz interval.
However, I'm afraid that this might scare readers, and not otherwise add much to the article, since the Lorentz interval reduces to proper distance for spacelike paths, and proper time for timelike paths.
Thus at the moment I'm leaving well enough alone. Pervect 23:08, 22 August 2006 (UTC)
At first I thought it was me, but these are clearly self-contradictions:
So, which is is that changes, the metric of space, or the metric of space-time? Lurk22 01:17, 17 September 2006 (UTC)
Note: This article has a small number of in-line citations for an article of its size and subject content. Currently it would not pass criteria 2b.
Members of the
Wikipedia:WikiProject Good articles are in the process of doing a re-review of current
Good Article listings to ensure compliance with the standards of the
Good Article Criteria. (Discussion of the changes and re-review can be found
here). A significant change to the GA criteria is the mandatory use of some sort of in-line citation (In accordance to
WP:CITE) to be used in order for an article to pass the
verification and reference criteria. It is recommended that the article's editors take a look at the inclusion of in-line citations as well as how the article stacks up against the rest of the Good Article criteria. GA reviewers will give you at least a week's time from the date of this notice to work on the in-line citations before doing a full re-review and deciding if the article still merits being considered a Good Article or would need to be de-listed. If you have any questions, please don't hesitate to contact us on the Good Article project
talk page or you may contact me personally. On behalf of the Good Articles Project, I want to thank you for all the time and effort that you have put into working on this article and improving the overall quality of the Wikipedia project.
Agne
05:51, 26 September 2006 (UTC)
Reply to comments left on my talk page
Please read
this. Very briefly, I as a (distinctly!) non-expert suggested that the ideas presented in this article are now mainstream within physics; the interested person can therefore make his or her own choice of mainstream physics book for verifying or reading more about it. The (apparently) expert editors seemed to agree. Meanwhile, the original physics research papers would be hugely too difficult for any but a minuscule percentage of WP readers; the tiny number of people who can read them would not be reading WP articles about the subject. -- Hoary 08:41, 26 September 2006 (UTC)
As a contributor to the above dispute, I concur. Please note my comments there. I would weigh in on the talk page at the GA space, but the argument is already long and any additional reflections I have would probably only add to the noise. But this article highlights the problem with having a rigid citation rule. The subject matter is arcane to uninformed readers, but to scientists, the discussion is quite elementary and fundamental. It is like requiring a citation that the sun rises in the east. I worry that the effect of applying WP:CITE blindly can lead to an amateurish presentation of subjects that will sponsor laughs from an informed reader; in other words, out of place for a serious encyclopedic project. (Don't misunderstand me though: generally, citation is hugely important.) Also, citations are not always good citations. I failed the GA nom for Marcel Proust in part b/c the works cited were woefully inadequate. Having a cite policy that reqires x or y number, however, may lead reviewers to feel that material has passed some kind of verifiability test, even when the cited sources would lead an informed reader of the subject to look askance. Eusebeus 14:41, 26 September 2006 (UTC)
The policy is not X or Y number but for the benefit of WP:V all major claims should be attached to a reliable source that verifies that claim. Simple as that. If a reviewer is blindly impressed with a number then they are simply not a good reviewer with that lacking falling on them and not the criteria. In all honestly, this is not new. It is something that the foundational wikipedia guidelines have asked for all along and in a time when Jimbo and the rest of the Wikipedia community are steering towards more quality over quantity, the GA project is moving accordingly. I don't doubt the truth of this article or that it is mainstream physics. But it has never been about truth but rather verifiability. Agne 17:31, 26 September 2006 (UTC)
I have made a request regarding this issue here. -- ScienceApologist 21:01, 26 September 2006 (UTC)'
This article would be much stronger with more specific reference to the relationship between metric expansion and gravitation - specifically, the metric tensor that describes how matter expands, shrinking space over time to produce the effects of gravity. It needn't and shouldn't be mathematical or obscure, and can actually clarify the article for lay readers.
Like space, matter itself is expanding - in ways that are easier to measure with a scale than a ruler. We can verify that the earth's surface is expanding at an acceleration of g by placing an object of known mass on a scale. To observers also being pushed by the earth's surface, this motion of the surface creates an impression that inertial objects are falling toward the earth. In this sense, Newtonian gravity is a fictitious force like the centrifugal force experienced by a rider on a merry go round. The strongest clue that it's really the earth's surface accelerating outward is, all the inertial objects appear to accelerate downward at the same rate, independent of their mass or lack of it. The equivalence of gravitational and inertial mass is a fundamental observation of general relativity.
The 'real' force responsible for our weight is the outward expansion of the earth's surface. Gravity doesn't pull us; the earth's surface pushes against us where we contact it. Since every atom and object at the earth's surface is expanding at the same rate, a ruler made of atoms won't show it.
Explaining this can give readers a clearer idea of both metric expansion and gravitation, and why metric expansion may not be obvious from our point of view. Moreover, metric expansion of space helps explain how the earth can be expanding this way without colliding with say, the sun: space itself is expanding too. Thus, the two should be discussed together. -- Dlevity 12:26, 19 November 2006 (UTC)
I've got a model for an expanding universe that allows for all the various effects to be incorporated. Universal expansion, a time of acceleration from a point and a time of acceleration back into a point.
I'm autocading the design right now.
Is there a particular format I should use for formatting the article? -- Mad Morlock 19:30, 11 December 2006 (UTC)
Old text:
This is not quite correct. We cannot arbitrarily chose different metrics. There is a physical distance between any two nearby points that is well defined. The metric calculates the distance between nearby points. Thus there is no freedom to chose the metric coefficients once we have chosen a coordinate system, only one set of metric coeffients will give the correct answer for distance.
In the example given, the distance between New York and Lodon is not strictly defined until one defines the exact curve connecting New York and London. The usual convention is to chose a 'great circle' connecting them, but one might chose a different curve.
But this choice of connecting curve does not matter to the metric coefficients, which determine the distance between nearby points. When two points are close, the curvature does not matter, and we can consider the points to be in an essentially Euclidean space.
When we go to the relativistic metric, we replace the concept of distance by the concept of Lorentz interval. This is something that should perhaps be added into the article, but I'm not sure if/how it can be done without confusing the reader. Once we have chosen the coordinate system, and once we have chosen whether the metric should represent pure distances (as on the surface of the Earth) or the Lorentz interval, we have no freedom in choosing the metric coefficients. Pervect 01:24, 12 December 2006 (UTC)
Hi, I love this article, because it's explained expansion to me, in a way that makes sense to me. If you take a bunch of points, and multiply their coordinates by 3.0, it doesn't matter where you set the origin, it'll all appear the same: like everything is going away from everything else, depending on the distance. I get it: theres more space between stuff. I love this article.
I have one small request: Can you somehow make it clearer that the Inflationary period at the time of the big bang is something very different than the Metric Expansion of space? That, it's a very brief time, during which the metric expansion of space happened very quickly, but that otherwise, the increase of the metric expansion of space is constant? That was something that I didn't get from the article, but learned later, from a usenet conversation. (I may be misled.) I kept getting the metric expansion of space confused with the inflationary period, and I think this page could more clearly distinguish them.
Thank you, Lion Kimbro.
LionKimbro 05:03, 21 December 2006 (UTC)
It should be mentioned that the expansion of the universe is often used as evidence for a Creator in Christian and Islamic Apologetics.
Christian sources:
Islamic sources:
-- Jorfer 23:48, 26 January 2007 (UTC)
I tried to generate an animation to illustrate this concept, but I ran into difficulties.
My idea was to draw a couple of circles (representing bodies in space) on a square grid, where the number of grid squares represents the measured distance between them. Then I was planning on shrinking the size of the grid squares over time. This would result in an increase in the number of squares (i.e. measured distance) between the two bodies, without them visually moving apart.
This appears at first glance to be a great illustration, but it results in the measured diameter of each body increasing over time at the same rate that the separation increases.
On the assumption that the metric expansion of space does not entail the expansion of all bodies within space, the obvious solution is to decrease the size of the circles so that their diameter is constant relative to the shrinking squares. However, if you visualise what such an animation would look like, you can see that it would look an awful lot like the 'camera' was zooming out from an image of two discs that were moving apart from one another. Is this assumption wrong? Are bodies supposed to expand, or is it only the space between them that does so? If the former, then the raisin bread analogy is flawed because the raisins don't expand. Hubble's Law makes no distinction between light from stars in our galaxy and light from other galaxies, right? So what are the "raisins"?
Can anyone shed any light on this? SheffieldSteel 21:32, 1 March 2007 (UTC)
Problems visualising this may alway be. No model is prefect. I like the balloon model, to demonstrate, I will put dots on it. As the balloon is inflated-expands the dots get farther apart. The dots on the balloon are galaxies. The key is the curve of the balloon is not just space but time. We call this time-space curvature. In the expansion of universe, not just space is expanding, but time as well. The galaxies are on the surface of the 4D universe as it expands. The expansion started with the Big Bang 13.7 billion years ago, when all space, time and matter came into existence. As the universe expands the time-space curvature is less. In the early universe time-space curvature was greater and the galaxies closer to each other. Telecine Guy 08:32, 12 March 2007 (UTC)
I was hoping to find some mention of Olbers's Paradox in this article. — Loadmaster 22:32, 23 July 2007 (UTC)
What is the rate of this metric expansion? Does it increase the distances in our solar system over time? If so, doesn't it affect the length of a year? Does mass remain constant with this metric expansion of space? Landroo 05:23, 27 July 2007 (UTC)
In the Overview, the article explains in a somewhat roundabout and indirect, but understandable way how a metric of distance can be constructed and applied to a coordinate space. This is good.
However, the part that says "The metric of space appears from current observations to be Euclidean, on a large scale. The same cannot be said for the metric of space-time, however. The non-Euclidean nature of space-time manifests itself by the fact that the distance between points with constant coordinates grows with time, rather than remaining constant." is, I think, confusing. It's not clear what "distance" is growing with time. Is it "just some obscure mathematical construct" (bear with me, I'm trying to see this from a complete layman's perspective, which isn't difficult because I almost am one), or is it the "real distance" between the two points? How is this distance, the distance that grows with time, measured? In other words, what is the metric of the universe (the one that is expanding)?
Later, the article goes on to say "In spaces that expand, the metric changes with time in a way that causes distances to appear larger at later times" -- this seems to imply that they aren't "really" larger, they just "appear" to be larger. I realize that this is very difficult to explain at this basic level, but since this seems to be what the article is attempting to do (which I think is a good idea), maybe you could try harder. The reader has to let go of the everyday concept of distance to avoid confusion; see below.
A simplified description of the metric should, I think, appear earlier in the article.
Another issue giving rise to confusion is the common-sense argument that as everything we observe is in space, right, then a hypothetical tape measure we could measure distances with would also be in space; hence, if space were to expand, then the tape measure would expand with it, and we'd still be reading the same distance. The statement "the metric itself changed exponentially, causing space to change from smaller than an atom to around 100 million light years across" seems, approached from a common-sense perspective, to imply that there is some unchanging "super-space" "outside our space", and that during the early life of our universe, our space, measured on a tape measure in this "super-space", was smaller than the size of an atom across, but then suddenly expanded, again measured on the tape measure "outside", to a size of 100 million light years. The "size of the atom" would then, apparently, be the size an atom is now, measured on this absolute tape measure; since, if space is expanding, the space inside an atom is also expanding, making the atoms bigger, surely? But, since our non-absolute tape measures are also likewise expanding, we're none the wiser, right?
This apparent implication should, I think, be addressed to remove confusion. Maybe explain that distances can be shown to have increased because light is taking longer to traverse them, and also why this doesn't mean that light slowed down, or that time speeded up (which, superficially, would both appear to be plausible explanations of light taking longer to cross a fixed distance).
I think the "Measuring distances" section should be expanded to serve as an introductory-level description of the problem and solution of measuring "long" distances. As it is, anyone familiar with the subject matter will probably understand it, but a layman, even armed with a fair understanding of mathematics, will probably not. The construction of the comoving distance metric seems arbitrary, and its physical meaning is unclear. I'm not sure how it could be explained in an intuitive way. Also, it's unclear whether it is in fact the, or only the, comoving distance that is increasing as space expands, or whether the other possible distance measures alluded to are also affected, and what the physical meaning of this is.
The figure caption only adds to the confusion as it says "The Hubble constant can change in the past ..." -- "you what?", my internal semantic parser thinks. :) How can a constant change? And how can it change in the past, yet in the present tense? Don't you mean "it can/may have changed" or "it could/might have changed"? Also, how can it change dependent on the observed value of density parameters? Surely, it doesn't matter whether we observe them or not?
The Copernican Principle as cited seems to contradict (again, based on common sense) what the article says earlier about the early universe having the size of an atom - surely, that atom-sized structure had a well-defined center, and well-defined boundaries? Where did these go, if they aren't around anymore? And if space is expanding, it is "expanding away" from the original center of that atom-sized structure, right? That is, there "must be" a well-defined center of the universe -- in fact, if the amount of matter is finite, it is even possible to define it as, say, the center of mass of all matter, right? And if expansion is symmetric, then this point is really the center of the universe, right? Wow, awesome! Can we put up a sign? :)
About "Ant on a balloon": the analogy is good, but there is an unaddressed issue related to the "everything is in space" argument above. The problem is that the ant is not itself in the "plane" of the balloon surface, but on it and is thus not itself directly affected by the balloon growing (maybe it has to shuffle its legs around a bit to keep them close to each other :). If the ant were in the fabric of the balloon (as we, observers of the universe, are in the fabric of said universe), then it would expand just as the balloon expands, and, depending on how it measures the distance between two points (e.g. by counting the number of steps required to reach B from A), it wouldn't necessarily detect a change in distance.
The raisin bread model does talk about the problem of the expanding ruler (or tape measure) a bit, but I think it's too little, too late. The concept of being "bound", and how and why it affects expansion, should be introduced earlier. Also, since gravity has an infinite range (although its effects approach zero as distance from the mass approaches infinity), all objects in the universe are "bound", aren't they? The distance between any two objects is finite, hence they exert a finite gravitational force on each other, right? OK, it's not called a force in relativity, but still, they both curve spacetime to a finite degree around the other object, making them "bound", right?
Modern science is a series of assaults on common sense: the Earth is not flat even though it appears to be; the Sun doesn't revolve around it even though it appears to; etc. I think it's important to explain why the assertions that apparently violate common sense are still true, and how common sense is mistaken/misled.
-- 195.56.53.118 23:47, 2 August 2007 (UTC)
I agree with 195.56.53.118. Galaxies are moving apart from each other so co-moving co-ordinates are chosen as a convenience. Other co-ordinate systems could be used to cover the manifold, eg. lots of different systems in describing black holes. Just because the co-ordinates are moving doesn't mean space is. What does it mean that space is expanding? It doesn't mean that objects can't move closer together. It doesn't make atoms get any larger. I can't see that it means anything, for example, how could you tell the difference between stationary objects in expanding space and stationary space with objects moving apart? Please tell me where I'm wrong and be as technical as you like. I see Ned Wright's cosmology tutorial seems to agree with me. Neodymion 04:53, 3 September 2007 (UTC)
According to the source I linked: It seems that we can't see the ones that haven't had time for their light to reach us yet. As to their velocity being limited by c, it is, but in the same way as SR, where adding velocities by Lorentz boosting always gives an answer less than or equal to c. Thanks for answering my question, though, at least I don't feel like I'm completely missing something now. Neodymion 12:50, 3 September 2007 (UTC)