Talk:Expansion of the universe/Archive 1

message: Nice article. Hope you like the small changes made. They are as follows:
# Group the "ant" and "raisin" models together under one top-level section, "models used to explain the expansion"
# Move the technical paragraph (that it is a feature of certain solutions of XYZ... time-varying universe..." etc) to an "overview" section, because this really is too "heavy" fro newcomers to be hit with on line #1. Also merge in with it, the "expansion in our universe" -- together they make a really good overview section of the field
# Add simpler replacement para's for the intro. Might not be technically 100% literally perfect at PhD level, but at lay-level they give a good idea without discouraging further reading. If any significant misinformation has been introduced please do fix it -- I can't warrant this myself, though I * think * its ok.
# I'm still unsure about my edit to para 3 of the intro. Some review of the DIFF (before and after) would help. You can probably see what I'm trying to do, but I'm not sure if it's worked well yet.
Nice work! FT2 (Talk | email) 15:49, 16 July 2006 (UTC)
thread_title:  Changes 

message: Quickies for you:
# On the one hand we describe the inflationary epoch as 10^-32 sec. on the other hand you say "inflation wasn't brief when it happened, it occurred over a time that was multiples of the age of the universe". Can you clarify this for me?
# On a similar theme you say "It is absolutely required that space and time are not absolutes". I thought time also was variable. In some places time is treated as absolute, in others like this it sounds like a metric too. Again, can you clarify how time behaves over time, so to speak(!)
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)
replies: text: Yes, as a former mariner I'm intrigued by this because I think time actually is immutable, that it is immaterial and intangible and cannot possibly be affected by material forces although the quality of what occurs during it can vary.  The book "Longitude" is interesting in this regard. As a died in the wool agnostic, I am open to evidence to the contrary, however. Thank you. Tom Cod 15:25, 1 November 2006 (UTC)
thread_title: Two further quick questions on Time

message: 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)
replies: text: My view on sourcing is very different yet partly compatible. I'll repeat what I wrote elsewhere: in my very limited understanding of cosmology this article is an attempt to summarize what's pretty much agreed among interested physicists and it's therefore unnecessary to say that A comes from X and B comes from Y. If I'm right here, then I suggest that the article should get either (i) some discursive footnotes, describing roughly what in the article comes from where (in addition of course to any needed footnotes saying which more controversial assertion comes from precisely where), or (ii) descriptions in the list of references, wherein the contribution of the source to the article is briefly explained. -- Hoary 07:01, 19 August 2006 (UTC)
replies: text: What a great criticism. Indeed, the article is both organic and redundant, but it is needed. There are different "levels" of articles sometimes needed (see for example introduction to special relativity and special relativity). This article is actually a more basic explanation of Big Bang. It is needed so that people are able to understand the basics of expansion before they can explore the big bang in its totality. --ScienceApologist 08:42, 19 August 2006 (UTC)
replies: text: When you wrote that, I wasn't quite sure what you meant, and I'm still not sure. I notice that you've renominated the article in the meantime, but before doing this you haven't taken up either of my recommendations (or explicitly turned them down). Again, I think it would be a good idea if you either (i) added a few discursive footnotes (explaining not merely that the immediately preceding assertion came from such-and-such a place, but rather that the following content of kind A comes from W and X, and that of kind B comes from Y and Z, etc.), or (ii) added comments after each item in the lists of external links and printed works saying how the item has contributed. As it is, some of what's in the list is very surprising, notably Eddington's work, written half a century or so before what I learn (or misread) was a kind of crystallization of the notion of the metric expansion of space. -- Hoary 15:10, 20 August 2006 (UTC)
replies: text: I'm not the best at doing this. I'll leave it to other editors to do this task. The Eddington reference is surprising because it is such a good explanation of the expanding universe. Realize that although it has taken decades to confirm the expanding universe, people assumed the universe was expanding (based on reasonable assumptions and the evidence at hand) since the 1930s. --ScienceApologist 20:07, 20 August 2006 (UTC)

text:  I've taken a look at the article. In my opinion, the only thing that separates it from Good Article status is a lack of inline references. As I said on the dispute page:
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)
thread_title: Good Article

message: I've put some little questions into <-- SGML comments --> within the article. -- Hoary 07:01, 19 August 2006 (UTC)
thread_title: Little questions

message: 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)
replies: text: I have been dealing with science for years, and this is the best question I have had asked of me about a subject. Scientists often take for granted the fact that models need to predict novel ideas before being accepted as reasonable. The metric expansion of space is a story about how science really works: hundreds of ideas promoted by hundreds of people and then one is selected of the hundreds to explain the observations made by the data-collectors in return. What happened with the expansion is space is something akin to a game of 20-questions. People asked "What kind of universe would fit this data?" an then "What kind of universe would fit that data?" until what was left was a metric expansion of space. The theory grew organically beause ideas about the rationality of science had been in the works long enough to protect scientists' extrapolations about the universe long enough for one of them to be confirmed. That's the present state of science -- and it's one that the metric expansion of space couldn't exit without. --ScienceApologist 08:39, 19 August 2006 (UTC)
thread_title: Confirmation vs lack of refutation and predictive power

message: 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)
thread_title:  On hold status for "Good article" nomination 

message: 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)
thread_title:  Lorentz interval? 

message: At first I thought it was me, but these are clearly self-contradictions:
*From the lead section: "space itself is described by a metric which changes over time"
*From the Overview:  "The metric of space appears from current observations to be Euclidean. The same cannot be said for the metric of space-time, however."
So, which is is that changes, the metric of space, or the metric of space-time? Lurk22 01:17, 17 September 2006 (UTC)
replies: text: The title of the article is "Metric expansion of space" not "Metric expansion of spacetime". The metric of space changes. The metric of spacetime also changes inasmuch as the spatial part of it is expanding. The time part is, however, unaffected by the expansion. As the old saying goes "time marches on". --ScienceApologist 01:31, 17 September 2006 (UTC)

text: I think this is a valid criticism, but I'm not sure what the best way of fixing it is.  By getting more into the physics of the Lorent interval, it would be possible to explain this point further, to explain how 4d space-time is not flat, while the 3d hypersurfaces of simultaneity are flat.  But I'm not sure it is a good idea to get too deep into this issue. I have a feeling even talking about the 3d hypersufaces of simultaneity would be too complex for the level this article is at.  Pervect 19:33, 17 September 2006 (UTC)
replies: text: It indeed would not add anything of value. We can just rely on our weak separation of the metric into spatial and temporal parts. Even though the spacetime interval can mix these two according to the Lorentz interval, it's absolutely true that in all inertial reference frames, it is only the spatial part of the metric that will expand. I'll try to modify the lead so it is clear that while the spacetime metric is the thing that is changing over time, it is only the spatial part that is affected. This may remove the "apparent" contradiction. --ScienceApologist 22:26, 17 September 2006 (UTC)
thread_title:  This article is terrible 

message: 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)
thread_title:  Visualizations of a Hypersphere 

message: Old text:
:We then choose a metric which will be used to calculate the distance between nearby points in terms of the difference in the coordinates between them. In principle we can choose any one of a multitude of metrics, but in practice we choose a metric that seems to fit our purposes best. For example, local mapmaking often ignores curvature, whereas airline flights and distances are based upon a Great Circle. Since the Earth is not flat, the "distance" between two points (such as London and New York may differs depending on how it is to be measured (ie, the metric which is to be utilized). In measuring small distances on Earth, there is in theory always an effect due to curvature, but in practice the the Earth's curvature is so small as to be almost unnoticable for short distances.
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)
replies: text: I've had a go at rewriting this section to give an example (which I think's needed) but not misinform. I'm on shakey ground here, could you (or someone) review it and clean up any misinformation I've inadvertantly added? I also added in your reference to the lorenz interval somehow. Thanks. FT2 (Talk | email) 15:26, 12 December 2006 (UTC)
thread_title:  Recent edits 

message: 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)
replies: text: Does the last paragraph in the lead not do it for you? --ScienceApologist 05:09, 21 December 2006 (UTC)
thread_title:  Metric Expansion of Space != Inflationary Period 

message: 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)
thread_title:  Problems visualising this 

message: I was hoping to find some mention of Olbers's Paradox in this article. — Loadmaster 22:32, 23 July 2007 (UTC)
thread_title:  [[Olbers's Paradox]] 

message: 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)
replies: text: The metric expanion of space only occurs measurably over cosmological scales. There is no solar system influence. Nondistinguished 18:32, 3 September 2007 (UTC)
thread_title:  Local Expansion?