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Expansion of the universe article. This is not a forum for general discussion of the article's subject. |
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Expansion of the universe was one of the Natural sciences good articles, but it has been removed from the list. There are suggestions below for improving the article to meet the good article criteria. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake. | |||||||||||||
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A summary of this article appears in Big Bang. |
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Metric expansion of space has been answered. Set the |answered= or |ans= parameter to no to reactivate your request. |
In reference 24 (inline "ref" tag following text "radiation from the Big Bang was demonstrably warmer at earlier times." the URL http://www.eso.org/outreach/press-rel/pr-2000/pr-27-00.html should be updated to https://www.eso.org/public/news/eso0043/ thanks to link-rot. Sigh. What is thet "U" in "URL" any "URI" supposed to mean? 50.0.193.12 ( talk) 06:00, 12 August 2017 (UTC)
The last sentence of the following article segment reads thusly:
The spatial and temporal universality of physical laws was until very recently taken as a fundamental philosophical assumption that is now tested to the observational limits of time and space.
Aside from this sentence appearing rather convoluted to me (admittedly not a physicist), it suspiciously relies on the phrases "...was until very recently..." and "...is now [being]..." without providing a source (never mind the fact that, even with a source, I wouldn't know how to interpret the intended meaning of said sentence). As such, I call weasel words on it.
It doesn't seem to add much to the rest of the section, either. And correct me if I'm wrong, but isn't the "universality of physical laws" some modern physics teaches us? If so, the sentence is incorrect as well as convoluted (assuming I'm even reading it right).
If the article wasn't locked, I would make a propositional edit and just remove that one sentence. Thoughts? All the best, 155.4.130.19 ( talk) 19:54, 16 August 2017 (UTC)
I suggest that the following two sentences from the article be joined: Current version: These problems arise from the observation that to look like it does today. The Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang. Proposed version: These problems arise from the observation that to look like it does today, the Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang. — Preceding unsigned comment added by Chris.Jankowski ( talk • contribs) 17:29, 18 October 2017 (UTC)
I suggest a grammatical correction to this sentence: Is: No field has yet discovered what is responsible for the cosmic inflation. Should be: No field has yet been discovered that is responsible for the cosmic inflation. Chris.Jankowski ( talk) 17:33, 18 October 2017 (UTC) Please do this. The current version is just wrong. Hutoth ( talk) 11:18, 25 February 2018 (UTC)
I suggest that the following two sentences from the article be joined:
Current version: These problems arise from the observation that to look like it does today. The Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang.
Proposed version: These problems arise from the observation that to look like it does today, the Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang.
Chris.Jankowski ( talk) 17:40, 18 October 2017 (UTC)
How about the universe is actually spinning... Just like a galaxy that has a super-massive black hole in the middle...how about just after the big bang an astronomical black hole formed and the whole universe spins around it.
Kind regards
Sorin — Preceding unsigned comment added by Sorin262000 ( talk • contribs) 17:31, 8 January 2018 (UTC)
The result of the move request was: move. The editor who uses the pseudonym " JamesBWatson" ( talk) 10:04, 14 April 2018 (UTC)
Metric expansion of space → Expansion of the universe – the current name is too technical and should be replaced with the more common "expansion of the universe" Vpab15 ( talk) 23:18, 6 April 2018 (UTC)
I have a PhD in molecular biology and I think this article, like so many STEM but espicially math articles, the intro sucks, to use technical lingo, cause it is not written for the general user go on, block me or delete me or whatever, I don't care I am so tired of articles writtenw ith complex jargon lke "gravitatinally bound"
This just dropped into my inbox: [2] nagual design 14:33, 12 July 2018 (UTC)
In the following sentence (I quote)
In 2001, Dr. Wendy Freedman determined space to expand at 72 kilometers per second per megaparsec - roughly 3.3 million light years - meaning that as we move away from Earth every 3.3 million light years is moving 72 kilometers a second faster.
the part I've italicized is apparently intended to clarify ('meaning that'), but it's confusing. WHAT is moving faster ... 3.3 million light-years? Too terse by far.
SIDE NOTE: There's another question I didn't see addressed in the article: what limits/controls/determines the 'metric expansion rate'? (Not the value of the what, but the what? IOW: are there theoretical considerations that keep the rate from being much slower? or infinitely faster? (NB please don't mail me on this.) Twang ( talk) 05:17, 16 November 2018 (UTC)
Figure in the "Measurement of expansion and change of rate of expansion" section does not depict the form of redshift relevant to cosmology. In fact, trying to disentangle the doppler redshift from cosmological/gravitational redshift was the biggest source of uncertainty in the early days of the cosmic distance ladder. This is why the very bright Type Ia Supernovae are used to measure the expansion rate of the universe at very large distances. That way the doppler contribution to the redshift is much smaller than the cosmological contribution.
Suggestion: This figure should be removed for now. Not sure if there already exists one for cosmological redshift already in the wikimedia.
I have also updated paragraph 2 of this section to reflect recent results, correct a few misconceptions, and provide a bit more context for the cosmic microwave background measurement and its interesting conflict with distance ladder measurements.
Taylorjhoyt ( talk) 22:05, 10 December 2018 (UTC)
1. This website says "expansion has been mainly due to inertia." The truth is, the universe didn't explode (like bits of dynamite flying across a grid), rather, the grid (called spacetime) is expanding. The inertia is not causing the expansion.
2. This website also says that "the matter in the very early universe was flying apart for unknown reasons (most likely as a result of cosmic inflation) and has simply continued to do so, though at an ever decreasing rate due to the attractive effect of gravity." While old books on the subject might have the wrong answer, the correct (modern) information is that the rate is not decreasing, it is increasing according to another Wikipedia article "The accelerating expansion of the universe is the observation that the expansion of the universe is such that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time." This other website is /info/en/?search=Accelerating_expansion_of_the_universe
Improve~enwiki ( talk) 18:45, 5 November 2019 (UTC)
Is there nothing in the research that indicates mass is the source of Universal Expansion?
Because.... The Earth's radius expands at one surface gravity per second. (Galileo's Tower of Pisa experiment).
The compression wave coming off of Earth puts the Moon in an equilibrium where inner and outer halves of the Moon encounter equal amounts of space in each orbit. Therefore, space must be more compressed closer to Earth.
Another thing to wonder.... If the expansion of space can't be measured with a ruler, does that mean light speed is expanding with space also? ie. Does the distance component of light speed scale up? — Preceding
unsigned comment added by
45.72.152.47 (
talk) 21:19, 28 December 2018 (UTC)
I assert that the expansiveness of space is proportional to the density of mass within the space. — Preceding unsigned comment added by 45.72.224.241 ( talk) 00:18, 16 September 2019 (UTC)
I'd just like to say thanks to all those that worked on this. It has to be one of the most accessible explanations of a complex topic I have read anywhere. Period. The concept that expansion is the result of the metric changing explains - in 10 seconds - something I've been wondering about for the past 30 years!
I recognise that the section on measuring distances may not be strictly encyclopedic, but perhaps it is all the better for that. It's well written and very clear. An encyclopedia needs to be understandable as well as accurate. That's not always the case on wikipedia.
46.227.49.108 ( talk) 07:34, 29 April 2019 (UTC)
File:CMB Timeline300 no WMAP.jpg is a really misleading graphic and it has been inadvertently copied all over the internet (as of 2019). It should only ever be used in the context of explaining theoretical inflation (and I wouldn't recommend using this graphic in that context either - unless one was attempting to make a specific point about the intensity of inflationary expansion). The theoretical inflationary epoch is believed to have expanded the observable universe only up unto approximately 10-100cm^3, nothing like what is shown in the graphic (ie size of current observable universe/2). Furthermore, it is difficult to observe the approximate constant expansion of the universe in the graphic (what the graphic is typically used to demonstrate) due to;
Note to demonstrate the concept of constant expansion (Hubble's constant), I recommend linking to a 3D simulation such as; "Simulation of the Big Bang and expanding universe" ( https://www.youtube.com/watch?v=WGUBt-vNFC8).
I have created a modified version of the graphic below;
"Because the surface of the Earth is two-dimensional..."
What does this mean? The surface of the Earth is not two-dimensional in any geometric sense, it clearly has three spatial dimensions. It seems very confusing to start an explanatory example by flatly stating something that is obviously not true.
The opening paragraph states, "Technically, neither space nor objects in space move." That's not true. By any measurement or observation, galaxy clusters are moving away from each other. Place a long enough yardstick from Earth to a far off galaxy cluster, and the measurement will show that the distance to that cluster is increasing. Tie a rope between one cluster and another, and that rope will break because the objects are moving away from each other. Objects in space are flying apart from each other, and the space/time metric is changing because of it, not the other way around.
If you turn time backward to "the beginning", one of these pictures implies a spatial singularity while the other does not. One involves space changing as a function of time while the other involves time changing as a function of time. These seem like significant conceptual differences, yet these pictures are metrologically indistinguishable. Is there any reason to accept one over the other? 2001:480:91:FF00:0:0:0:16 ( talk) 16:34, 16 July 2021 (UTC)
Many people say "but we have no proof that other universes exist". Great. But one word cannot magically convey all possible interpretations; and when it supposedly does, it becomes vague. These people say "see the context", but that's not a proper way to write. We have to be specific. In everyday language we can confuse moon with Moon and universe with Universe, but encyclopedically it's improper. — Preceding unsigned comment added by 2A02:2149:8737:2900:64D0:A994:6EF:F61E ( talk) 14:30, 1 October 2021 (UTC)
That's what I'm looking for. Maybe there's a separate article for that. I will check. Even so, the "expansion of the universe" begs the question, Will the expansion eventually peter out and the universe collapse? 2600:8801:BE31:D300:A8E7:F7D4:F07D:1027 ( talk) 14:12, 17 April 2022 (UTC) James.
To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer.
Link for the nearest galaxies lead to local cluster. Local clusters are obviously bounded by gravity.
But
What do you mean the nearest galaxies?
a single local cluster bounded to the observer? Or Every local clusters of the observable universe?
P.S "And that all but" dude? really!
103.217.156.220 ( talk) 11:19, 17 October 2022 (UTC)
This article was the subject of a Wiki Education Foundation-supported course assignment, between 7 September 2022 and 24 December 2022. Further details are available on the course page. Student editor(s): Jeffrey Sucuzhanay ( article contribs).
— Assignment last updated by Janyahmercedes ( talk) 02:52, 26 January 2023 (UTC)
If 100 kids scatter and run away from each other in the playground of a kindergarten, can we say the school is expanding? 14.0.225.81 ( talk) 06:25, 2 March 2023 (UTC)
This article is propagating a lot of misconceptions. Fundamentally, expansion of space is not different from galaxies moving apart. It is only a common coordinate choice.
I'll try to make edits when I get a chance, but I'm adding this topic for discussion. Some relevant literature:
- [3] https://arxiv.org/abs/0809.4573
- [4] https://arxiv.org/abs/0808.1081
- [5] https://arxiv.org/abs/1605.08634
Aseyhe ( talk) 11:02, 14 July 2023 (UTC)
I'm planning to remove most of the section on metrics. The description of comoving coordinates is useful, but the rest is extraneous at best and wrong or misleading at worst. Metric expansion is not a fundamental feature of cosmic expansion and is not important to understanding it.
Aseyhe ( talk) 03:57, 7 August 2023 (UTC)
While restructuring the article last year I basically moved a bunch of miscellany into the "Conceptual considerations and misconceptions" section. Probably the whole section can be removed, but I haven't had the time to sift through what's worth keeping.
Aseyhe ( talk) 02:40, 25 March 2024 (UTC)
- The introduction lacks any proof or evidence, that the universe isn't expanding into something, and that space is expanding. Where is the proof ?
- Later, the article states that there are 4 dimensions not 3, and concludes that spacetime is not flat. Maybe, but space remains flat inside of space-time. All this rhetoric of space-time is not proven either.
2A02:A03F:615C:9C00:F955:71F2:3D0C:DAB1 ( talk) 12:24, 15 March 2024 (UTC)
What reason is there to believe objects in rapidly expanding space are incapable of gravitational collapse? At this point, the article has just informed us that expansion only applies to the scale factor of the spacetime manifold, not to the objects within it. So the objects of the inflationary epoch are of relatively constant size; it's the space between them that's expanding. But then this section quickly moves the goalposts by saying gravitational collapse does not "apply to rapidly expanding space," conflating the expansion of objects and the expansion of space. Unless I'm misunderstanding (well within the realm of possibilities), there's some kind of equivocation going on here.
The statement, "commonly used calculations for gravitational collapse are usually based upon objects of relatively constant size," is also doubtful by itself. Calculations for gravitational collapse aren't "based on" physical objects, they're simple mathematical models that approximately describe nature. There are all kinds of regimes cited on the gravitational collapse article, including some that are thought to be responsible for primordial black holes, which may have formed during the inflationary epoch.
I highlighted the weasel word "usually," since it's problematic here. The statement seems to be intended to create an impression that we can't apply models of gravitational collapse to the inflationary epoch because gravitational collapse models are "based upon objects of relatively constant size." But if that impression is true, then the word "usually" is unnecessary. Conversely, if it's true that gravitational collapse models are only usually based upon objects of constant size, then indeed we can apply at least some gravitational collapse models to the inflationary epoch, leading us right back to the original question, "why no collapse?" The article should answer that question with reference to the most accurate models, not the least accurate.
I also highlighted the weasel word "relatively," since it's also problematic in this context. Relative to what? I don't mean to say this word should never be used, but in this case it seems to be used in the colloquial sense that's synonymous with "somewhat" or "moderately," indicating a position between two extremes. But since there's no limit to how rapidly something might expand, it's unclear what "between the most extreme rate and the least extreme rate" could mean in absolute terms, so how do we know that the inflationary epoch falls outside that definition? Aminomancer ( talk) 07:41, 21 April 2024 (UTC)
This is the
talk page for discussing improvements to the
Expansion of the universe article. This is not a forum for general discussion of the article's subject. |
Article policies
|
Find sources: Google ( books · news · scholar · free images · WP refs) · FENS · JSTOR · TWL |
Archives: 1, 2, 3 |
Expansion of the universe was one of the Natural sciences good articles, but it has been removed from the list. There are suggestions below for improving the article to meet the good article criteria. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake. | |||||||||||||
| |||||||||||||
Current status: Delisted good article |
This
level-5 vital article is rated C-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | |||||||||||||||||||||||||||
|
A summary of this article appears in Big Bang. |
This article links to one or more target anchors that no longer exist.
Please help fix the broken anchors. You can remove this template after fixing the problems. |
Reporting errors |
This
edit request to
Metric expansion of space has been answered. Set the |answered= or |ans= parameter to no to reactivate your request. |
In reference 24 (inline "ref" tag following text "radiation from the Big Bang was demonstrably warmer at earlier times." the URL http://www.eso.org/outreach/press-rel/pr-2000/pr-27-00.html should be updated to https://www.eso.org/public/news/eso0043/ thanks to link-rot. Sigh. What is thet "U" in "URL" any "URI" supposed to mean? 50.0.193.12 ( talk) 06:00, 12 August 2017 (UTC)
The last sentence of the following article segment reads thusly:
The spatial and temporal universality of physical laws was until very recently taken as a fundamental philosophical assumption that is now tested to the observational limits of time and space.
Aside from this sentence appearing rather convoluted to me (admittedly not a physicist), it suspiciously relies on the phrases "...was until very recently..." and "...is now [being]..." without providing a source (never mind the fact that, even with a source, I wouldn't know how to interpret the intended meaning of said sentence). As such, I call weasel words on it.
It doesn't seem to add much to the rest of the section, either. And correct me if I'm wrong, but isn't the "universality of physical laws" some modern physics teaches us? If so, the sentence is incorrect as well as convoluted (assuming I'm even reading it right).
If the article wasn't locked, I would make a propositional edit and just remove that one sentence. Thoughts? All the best, 155.4.130.19 ( talk) 19:54, 16 August 2017 (UTC)
I suggest that the following two sentences from the article be joined: Current version: These problems arise from the observation that to look like it does today. The Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang. Proposed version: These problems arise from the observation that to look like it does today, the Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang. — Preceding unsigned comment added by Chris.Jankowski ( talk • contribs) 17:29, 18 October 2017 (UTC)
I suggest a grammatical correction to this sentence: Is: No field has yet discovered what is responsible for the cosmic inflation. Should be: No field has yet been discovered that is responsible for the cosmic inflation. Chris.Jankowski ( talk) 17:33, 18 October 2017 (UTC) Please do this. The current version is just wrong. Hutoth ( talk) 11:18, 25 February 2018 (UTC)
I suggest that the following two sentences from the article be joined:
Current version: These problems arise from the observation that to look like it does today. The Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang.
Proposed version: These problems arise from the observation that to look like it does today, the Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang.
Chris.Jankowski ( talk) 17:40, 18 October 2017 (UTC)
How about the universe is actually spinning... Just like a galaxy that has a super-massive black hole in the middle...how about just after the big bang an astronomical black hole formed and the whole universe spins around it.
Kind regards
Sorin — Preceding unsigned comment added by Sorin262000 ( talk • contribs) 17:31, 8 January 2018 (UTC)
The result of the move request was: move. The editor who uses the pseudonym " JamesBWatson" ( talk) 10:04, 14 April 2018 (UTC)
Metric expansion of space → Expansion of the universe – the current name is too technical and should be replaced with the more common "expansion of the universe" Vpab15 ( talk) 23:18, 6 April 2018 (UTC)
I have a PhD in molecular biology and I think this article, like so many STEM but espicially math articles, the intro sucks, to use technical lingo, cause it is not written for the general user go on, block me or delete me or whatever, I don't care I am so tired of articles writtenw ith complex jargon lke "gravitatinally bound"
This just dropped into my inbox: [2] nagual design 14:33, 12 July 2018 (UTC)
In the following sentence (I quote)
In 2001, Dr. Wendy Freedman determined space to expand at 72 kilometers per second per megaparsec - roughly 3.3 million light years - meaning that as we move away from Earth every 3.3 million light years is moving 72 kilometers a second faster.
the part I've italicized is apparently intended to clarify ('meaning that'), but it's confusing. WHAT is moving faster ... 3.3 million light-years? Too terse by far.
SIDE NOTE: There's another question I didn't see addressed in the article: what limits/controls/determines the 'metric expansion rate'? (Not the value of the what, but the what? IOW: are there theoretical considerations that keep the rate from being much slower? or infinitely faster? (NB please don't mail me on this.) Twang ( talk) 05:17, 16 November 2018 (UTC)
Figure in the "Measurement of expansion and change of rate of expansion" section does not depict the form of redshift relevant to cosmology. In fact, trying to disentangle the doppler redshift from cosmological/gravitational redshift was the biggest source of uncertainty in the early days of the cosmic distance ladder. This is why the very bright Type Ia Supernovae are used to measure the expansion rate of the universe at very large distances. That way the doppler contribution to the redshift is much smaller than the cosmological contribution.
Suggestion: This figure should be removed for now. Not sure if there already exists one for cosmological redshift already in the wikimedia.
I have also updated paragraph 2 of this section to reflect recent results, correct a few misconceptions, and provide a bit more context for the cosmic microwave background measurement and its interesting conflict with distance ladder measurements.
Taylorjhoyt ( talk) 22:05, 10 December 2018 (UTC)
1. This website says "expansion has been mainly due to inertia." The truth is, the universe didn't explode (like bits of dynamite flying across a grid), rather, the grid (called spacetime) is expanding. The inertia is not causing the expansion.
2. This website also says that "the matter in the very early universe was flying apart for unknown reasons (most likely as a result of cosmic inflation) and has simply continued to do so, though at an ever decreasing rate due to the attractive effect of gravity." While old books on the subject might have the wrong answer, the correct (modern) information is that the rate is not decreasing, it is increasing according to another Wikipedia article "The accelerating expansion of the universe is the observation that the expansion of the universe is such that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time." This other website is /info/en/?search=Accelerating_expansion_of_the_universe
Improve~enwiki ( talk) 18:45, 5 November 2019 (UTC)
Is there nothing in the research that indicates mass is the source of Universal Expansion?
Because.... The Earth's radius expands at one surface gravity per second. (Galileo's Tower of Pisa experiment).
The compression wave coming off of Earth puts the Moon in an equilibrium where inner and outer halves of the Moon encounter equal amounts of space in each orbit. Therefore, space must be more compressed closer to Earth.
Another thing to wonder.... If the expansion of space can't be measured with a ruler, does that mean light speed is expanding with space also? ie. Does the distance component of light speed scale up? — Preceding
unsigned comment added by
45.72.152.47 (
talk) 21:19, 28 December 2018 (UTC)
I assert that the expansiveness of space is proportional to the density of mass within the space. — Preceding unsigned comment added by 45.72.224.241 ( talk) 00:18, 16 September 2019 (UTC)
I'd just like to say thanks to all those that worked on this. It has to be one of the most accessible explanations of a complex topic I have read anywhere. Period. The concept that expansion is the result of the metric changing explains - in 10 seconds - something I've been wondering about for the past 30 years!
I recognise that the section on measuring distances may not be strictly encyclopedic, but perhaps it is all the better for that. It's well written and very clear. An encyclopedia needs to be understandable as well as accurate. That's not always the case on wikipedia.
46.227.49.108 ( talk) 07:34, 29 April 2019 (UTC)
File:CMB Timeline300 no WMAP.jpg is a really misleading graphic and it has been inadvertently copied all over the internet (as of 2019). It should only ever be used in the context of explaining theoretical inflation (and I wouldn't recommend using this graphic in that context either - unless one was attempting to make a specific point about the intensity of inflationary expansion). The theoretical inflationary epoch is believed to have expanded the observable universe only up unto approximately 10-100cm^3, nothing like what is shown in the graphic (ie size of current observable universe/2). Furthermore, it is difficult to observe the approximate constant expansion of the universe in the graphic (what the graphic is typically used to demonstrate) due to;
Note to demonstrate the concept of constant expansion (Hubble's constant), I recommend linking to a 3D simulation such as; "Simulation of the Big Bang and expanding universe" ( https://www.youtube.com/watch?v=WGUBt-vNFC8).
I have created a modified version of the graphic below;
"Because the surface of the Earth is two-dimensional..."
What does this mean? The surface of the Earth is not two-dimensional in any geometric sense, it clearly has three spatial dimensions. It seems very confusing to start an explanatory example by flatly stating something that is obviously not true.
The opening paragraph states, "Technically, neither space nor objects in space move." That's not true. By any measurement or observation, galaxy clusters are moving away from each other. Place a long enough yardstick from Earth to a far off galaxy cluster, and the measurement will show that the distance to that cluster is increasing. Tie a rope between one cluster and another, and that rope will break because the objects are moving away from each other. Objects in space are flying apart from each other, and the space/time metric is changing because of it, not the other way around.
If you turn time backward to "the beginning", one of these pictures implies a spatial singularity while the other does not. One involves space changing as a function of time while the other involves time changing as a function of time. These seem like significant conceptual differences, yet these pictures are metrologically indistinguishable. Is there any reason to accept one over the other? 2001:480:91:FF00:0:0:0:16 ( talk) 16:34, 16 July 2021 (UTC)
Many people say "but we have no proof that other universes exist". Great. But one word cannot magically convey all possible interpretations; and when it supposedly does, it becomes vague. These people say "see the context", but that's not a proper way to write. We have to be specific. In everyday language we can confuse moon with Moon and universe with Universe, but encyclopedically it's improper. — Preceding unsigned comment added by 2A02:2149:8737:2900:64D0:A994:6EF:F61E ( talk) 14:30, 1 October 2021 (UTC)
That's what I'm looking for. Maybe there's a separate article for that. I will check. Even so, the "expansion of the universe" begs the question, Will the expansion eventually peter out and the universe collapse? 2600:8801:BE31:D300:A8E7:F7D4:F07D:1027 ( talk) 14:12, 17 April 2022 (UTC) James.
To any observer in the universe, it appears that all of space is expanding, and that all but the nearest galaxies (which are bound by gravity) recede at speeds that are proportional to their distance from the observer.
Link for the nearest galaxies lead to local cluster. Local clusters are obviously bounded by gravity.
But
What do you mean the nearest galaxies?
a single local cluster bounded to the observer? Or Every local clusters of the observable universe?
P.S "And that all but" dude? really!
103.217.156.220 ( talk) 11:19, 17 October 2022 (UTC)
This article was the subject of a Wiki Education Foundation-supported course assignment, between 7 September 2022 and 24 December 2022. Further details are available on the course page. Student editor(s): Jeffrey Sucuzhanay ( article contribs).
— Assignment last updated by Janyahmercedes ( talk) 02:52, 26 January 2023 (UTC)
If 100 kids scatter and run away from each other in the playground of a kindergarten, can we say the school is expanding? 14.0.225.81 ( talk) 06:25, 2 March 2023 (UTC)
This article is propagating a lot of misconceptions. Fundamentally, expansion of space is not different from galaxies moving apart. It is only a common coordinate choice.
I'll try to make edits when I get a chance, but I'm adding this topic for discussion. Some relevant literature:
- [3] https://arxiv.org/abs/0809.4573
- [4] https://arxiv.org/abs/0808.1081
- [5] https://arxiv.org/abs/1605.08634
Aseyhe ( talk) 11:02, 14 July 2023 (UTC)
I'm planning to remove most of the section on metrics. The description of comoving coordinates is useful, but the rest is extraneous at best and wrong or misleading at worst. Metric expansion is not a fundamental feature of cosmic expansion and is not important to understanding it.
Aseyhe ( talk) 03:57, 7 August 2023 (UTC)
While restructuring the article last year I basically moved a bunch of miscellany into the "Conceptual considerations and misconceptions" section. Probably the whole section can be removed, but I haven't had the time to sift through what's worth keeping.
Aseyhe ( talk) 02:40, 25 March 2024 (UTC)
- The introduction lacks any proof or evidence, that the universe isn't expanding into something, and that space is expanding. Where is the proof ?
- Later, the article states that there are 4 dimensions not 3, and concludes that spacetime is not flat. Maybe, but space remains flat inside of space-time. All this rhetoric of space-time is not proven either.
2A02:A03F:615C:9C00:F955:71F2:3D0C:DAB1 ( talk) 12:24, 15 March 2024 (UTC)
What reason is there to believe objects in rapidly expanding space are incapable of gravitational collapse? At this point, the article has just informed us that expansion only applies to the scale factor of the spacetime manifold, not to the objects within it. So the objects of the inflationary epoch are of relatively constant size; it's the space between them that's expanding. But then this section quickly moves the goalposts by saying gravitational collapse does not "apply to rapidly expanding space," conflating the expansion of objects and the expansion of space. Unless I'm misunderstanding (well within the realm of possibilities), there's some kind of equivocation going on here.
The statement, "commonly used calculations for gravitational collapse are usually based upon objects of relatively constant size," is also doubtful by itself. Calculations for gravitational collapse aren't "based on" physical objects, they're simple mathematical models that approximately describe nature. There are all kinds of regimes cited on the gravitational collapse article, including some that are thought to be responsible for primordial black holes, which may have formed during the inflationary epoch.
I highlighted the weasel word "usually," since it's problematic here. The statement seems to be intended to create an impression that we can't apply models of gravitational collapse to the inflationary epoch because gravitational collapse models are "based upon objects of relatively constant size." But if that impression is true, then the word "usually" is unnecessary. Conversely, if it's true that gravitational collapse models are only usually based upon objects of constant size, then indeed we can apply at least some gravitational collapse models to the inflationary epoch, leading us right back to the original question, "why no collapse?" The article should answer that question with reference to the most accurate models, not the least accurate.
I also highlighted the weasel word "relatively," since it's also problematic in this context. Relative to what? I don't mean to say this word should never be used, but in this case it seems to be used in the colloquial sense that's synonymous with "somewhat" or "moderately," indicating a position between two extremes. But since there's no limit to how rapidly something might expand, it's unclear what "between the most extreme rate and the least extreme rate" could mean in absolute terms, so how do we know that the inflationary epoch falls outside that definition? Aminomancer ( talk) 07:41, 21 April 2024 (UTC)