Talk:Expansion of the universe

Semi-protected edit request on 12 August 2017

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)

Done jd22292 (Jalen D. Folf) (talk) 19:08, 12 August 2017 (UTC)

Apparent weasel words?

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)

The above proposed edit really ought to be considered. Anyone? 217.119.171.154 (talk) 10:13, 21 August 2017 (UTC)

I've removed the sentence: it really didn't add anything to the article, and felt like word salad to me. - Parejkoj (talk) 18:40, 22 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)

idk whether you really thought the hypothetical scenario where he did provide a source. Awella (talk) 15:33, 10 July 2022 (UTC)

Suggested grammatical error correction

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)

You can change sentences to make them grammatical without asking here first. If you are adding extra information, it must be reputably sourced. Danielklein (talk) 07:08, 29 May 2018 (UTC)

Logical flow improvement

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 not actually expanding

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)

As the article currently says, "All that is certain is that the manifold of space in which we live simply has the property that the distances between objects are getting larger as time goes on. ...
Even if the overall spatial extent is infinite and thus the universe cannot get any "larger", we still say that space is expanding because, locally, the characteristic distance between objects is increasing. As an infinite space grows, it remains infinite." So in fact, it may be that space is not expanding, but that every "object" (particle, energy waveform, etc) is actually shrinking in size. Eventually (even after heat death) the "obecjts" will be infinitesimally small, or "space" between them will be unimaginably huger. Same thing, in a relative universe. 2.31.166.197 (talk) 20:24, 27 February 2018 (UTC)

A universe that spins around a huge black hole would not be homogeneous or isotropic. No evidence for that has been observed despite many surveys. --Geek3 (talk) 10:44, 16 April 2018 (UTC)

Wikipedia is not a place for "how about?". The only information that should be here is what can be verified. Have there been any reputable articles published about this idea, etc. If you want to know whether this could work or not, you can ask a question on https://physics.stackexchange.com/ Danielklein (talk) 07:05, 29 May 2018 (UTC)

In Newtonian physics there is space and there is time; in GR there is only a single structure of space-time. So, one can't even begin to talk literally about "The Universe" expanding; the phrase "The expanding Universe" is a metaphor! ([[David edwards, 8/26/18, http://alpha.math.uga.edu/~davide/) — Preceding unsigned comment added by David edwards (talk • contribs) 09:05, 26 August 2018 (UTC)

Requested move 6 April 2018

The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review. No further edits should be made to this section.

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)

• Support; WP:COMMONTERM. TJRC (talk) 15:24, 7 April 2018 (UTC)
• Support; yes - agreed - WP:COMMONTERM. Drbogdan (talk) 15:53, 7 April 2018 (UTC)
• Support; clearly the common name. Rreagan007 (talk) 18:42, 7 April 2018 (UTC)
• Support; agreed. From a mathematical point of view, "Metric expansion of space" suggests something more general. CapitalSasha ~ talk 10:29, 12 April 2018 (UTC)

---

The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.

intro sucks cause it is written for phd not general user

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"

Hubble constant in the news

This just dropped into my inbox: [2] nagualdesign 14:33, 12 July 2018 (UTC)

Say what?

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)

@Twang: On your first point, the part of the sentence you italicised was nonsense. I have rephrased it in the hope of making it clearer, but if someone else can make it clearer still that will be welcome. As for your second point, I'm afraid I don't know what you mean. Can you try to clarify your question? The editor who uses the pseudonym "JamesBWatson" (talk) 16:38, 18 November 2018 (UTC)

EDITS: Measurement of expansion and change of rate of expansion

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 https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe

Improve~enwiki (talk) 18:45, 5 November 2019 (UTC)

Mass as the Source

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)

Thank you for the article

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)

Graphical representation of the expansion of the universe

A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left. This diagram can be confusing because the expansion of space looks like it is happening into an empty "nothingness". However, this is a choice made for convenience of visualization: it is not a part of the physical models which describe the expansion.

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;

• a) its apparent (but misleading) insignificance compared to inflationary expansion,
• b) the fact it is not uniformly expanding in the positive and negative direction of the y axis (it is nearly flat at the high end of the y axis), and;
• c) the fact the constant expansion only results in the observable universe increasing approximately 2x in size since the end of inflation (it really should be something like x10^(15+9+2=26)=x100000000000000000000000000, and so its proper visualisation requires a minimum expansion of 10x.

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;

A graphical representation of the expansion of the universe from the Big Bang. To visualize a constant expansion of space, see "Simulation of the Big Bang and expanding universe" ([1])

Great circle example in the section regarding overview of metrics

"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.

In mathematics, if its unit of measurement is area it two dimensional. If its unit of measurement is volume it three dimensional. If its unit of measurement is length its one dimensional. Doesn't matter how curvy a string is, it's still considered one dimensional. Cloudswrest (talk) 23:26, 17 January 2020 (UTC)

But it is confusing to the reader, because we are talking about spatial dimensions if we call a line "one dimensional". A curved line is only "one dimensional" in a purely mathematical sense. Geometrically, it occupies at least two spatial dimensions. The same is true of the surface of a sphere, the fact that we can define a polar coordinate system that only requires two coordinates to uniquely define a location on the sphere's surface, most assuredly does not make the surface of that sphere "two-dimensional" in a spatial sense. We only need two coordinates to specify a point, because we have already specified that we are working with a sphere! That is the only reason the polar coordinate system can uniquely identify a point on the surface. The third dimension is still there; it is implied in the premise that we are defining coordinates for a sphere. Each point on the sphere's surface, when treated independently of the happy fact that it happens to be on the surface of a sphere, requires three coordinates to uniquely identify its location in space. 67.4.71.16 (talk) 21:43, 30 January 2023 (UTC)

The surface of a sphere is in fact a 2d surface. How do you suggest that be worded to be clearer? - Parejkoj (talk) 18:18, 17 January 2020 (UTC)

The surface of a sphere is unequivocally not a 2d surface. The only reason we can use only two coordinates to uniquely specify any arbitrary point on that surface, is because we have specified as a given that the polar coordinate system is being applied to a sphere, which is an inherently 3d object. The surface of a sphere clearly occupies three spatial dimensions. The entire statement in the article: "Because the surface of the Earth is two-dimensional, points on the surface of the Earth can be specified by two coordinates – for example, the latitude and longitude." ... is not only misleading, it is incorrect. 67.4.71.16 (talk) 22:16, 30 January 2023 (UTC)

By (topological) convention, a sphere's surface is a 2-dimensional manifold, and in GR-physics (and many rivals for the theoretical center stage), manifolds seem to answer to more real phenomena than our conventional Euclidean and coordinate paradigms. We have to select the best tool for the context. JohndanR (talk) 19:03, 14 January 2024 (UTC)

Objects in Space are Moving

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.

Definitely true. Now corrected. jps (talk) 22:51, 31 December 2020 (UTC)

Expansion of space or contraction of time?

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)

Please see WP:NOTFORUM. However, you could try posting this question at Wikipedia:Reference desk/Science. nagualdesign 16:54, 16 July 2021 (UTC)

Universe vs universe

• Universe: our universe; the universe we (the thinking beings) live in
• universe: any mathematically possible universe, including the supposed infinite inaccessible ones but also then non-string theory based spacetimes (because string theory gives a limited amount of universes; but there is no proof that spacetimes of different foundations are impossible.

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)

Nothing about the "big crunch" theory?

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.

See Big Crunch. --Amble (talk) 05:47, 18 April 2022 (UTC)

Ambiguous, please clarify.

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)

Wiki Education assignment: PHY 381 History of Modern Physics

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)

Wrong theory of expanding universe

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)

Misconceptions about expanding space

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)

Proof ?

- 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)

In the introduction, I've attempted to make the wording more precise. The observed cosmic expansion has nothing to do with the matter of whether the universe might expand into something. (Also, the introduction already does not say that space is expanding.) Aseyhe (talk) 02:49, 25 March 2024 (UTC)

Density of universe during expansion

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)

You are right, this subsection does not make sense. The black hole question is a common one but tricky to answer both succinctly and accurately.

Before trying to correct it, though, I wonder: is this even appropriate for the Wikipedia article? (Actually, I wonder this about the whole of the last section.) Aseyhe (talk) 04:17, 22 April 2024 (UTC)