Talk:Observable universe/Archive 4

Clarification about possible radius of the universe
In the section "universe vs observable universe" there is this claim: "There are also lower estimates claiming that the entire universe is in excess of 250 times larger (3,450 billion light-years) (by volume, not by radius) than the observable universe".

I don't get where the numbers come from. If the 250x factor is by volume, then the radius should be approximately 6.3 times (cubic root of 250) the observable one (radius approx 293 billion light-years). If it is 250x the radius, it should be 11625 billions light-years. In both cases I don't get where the 3450 comes from. I'm most probably missing something, since I'm not an expert, but it would be nice if the page was clearer even for non-expert readers like me :). Thanks! — Preceding unsigned comment added by 158.110.144.102 (talk) 12:43, 9 December 2020 (UTC)


 * I don't understand the claim. It was introduced by an editor in April 2020. I reverted the first edits by the editor, but the editor reinstated it, and I left them to avoid getting into an editing war. It makes no sense to me. TowardsTheLight (talk) 20:59, 9 December 2020 (UTC)


 * In accordance with the top cleanup template I have removed the badly sourced content. The first claim (under question here above) was sourced by a personal website. The second claim was sitting there tagged for wp:secondary sources since April 2019. When proper secondary sources can be found, we can restore some of the content. Hope this helps. - DVdm (talk) 23:41, 9 December 2020 (UTC)

Misconceptions about size section
Does the misconceptions about size section serve a useful purpose? To start, I think the premise is flawed. There are a variety of ways to express the size of the observable universe, and several of those are at least arguably reasonable ways of doing so. The referencing is poor in a few of them. Especially the self-published opinion piece by Ned Wright is problematic. Wright is a recognized cosmologist, so WP:SPS doesn't require us to automatically throw it out, but there are plenty of reasons why the light travel distance is a reasonable way to express the scale of the observable universe, and it's also a size that sources commonly use -- including the American Museum of Natural History, whose plaque describing 13.8 billion light years as a size of the observable universe was included as an example of this misconception until I removed it a moment ago. In fact, I think that plaque is every bit as reliable a source as Wright's self-published web page, as the AMNH is definitely a reliable source.

But more generally, I'm just not clear that listing misconceptions is really appropriate for Wikipedia. It's not Wikipedia's job to correct everything that the media has said that's wrong. Is there a good reason to keep it or any of the content there? We could turn it around and state correctly that 13.8 billion light years is the light travel distance to the edge of the observational universe instead of focusing on secondary sources which arguably misrepresent that and other distances. —Alex (Ashill &#124; talk &#124; contribs) 17:56, 18 January 2021 (UTC)
 * I generally agree with you on "misconceptions" sections. I can't comment more specifically on the size issue, because I haven't looked into it in detail. --Trovatore (talk) 20:14, 18 January 2021 (UTC)
 * I've gone ahead and deleted the subsection. I merged the discussion of 13.8 billion light years into the main size section, describing it as the light-travel distance and including the part about it not having much physical meaning. Stating the correct fact instead of stating the misconception and then correcting the misconception is better, I think.
 * I simply removed the extended discussion of the Cornish et al estimate of a lower bound on the total size of the Universe. It isn't really relevant here. —Alex (Ashill &#124; talk &#124; contribs) 23:34, 20 January 2021 (UTC)

"Unbounded"
Commenting on this diff, where restored "finite but unbounded" as the "technical term".

It is true that one very often sees "finite but unbounded" as a possibility for the size of the whole universe ("infinite" being also a possibility). It is possible that this is a "technical term" for cosmologists.

But from a mathematician's point of view, it's a very strange way to put it. If the universe is finite, then it's a bounded metric space, meaning there's a finite number M such that the distance between any two points is less than M. So to call it "unbounded" is really very strange indeed. What seems to be meant is that it's "without boundary" &mdash; that is, that it's a manifold, rather than just a "manifold with boundary".

Of course this is not a math article, and we aren't here to reform the usage of cosmologists, if "finite but unbounded" really is their technical usage. Still, it wouldn't hurt to explain it a little, given that it's so completely contrary to the way "bounded" is used in mathematics, even relatively low-level mathematics. --Trovatore (talk) 01:26, 30 January 2021 (UTC)


 * Related discussion at WP:RD/Math, eventually expected to land at Reference desk/Archives/Mathematics/2021 January 30. --Trovatore (talk) 20:00, 30 January 2021 (UTC)

It looks like the right way to say it is captured by geodesic manifold, possibly with an exception for paths that end at the singularity of a black hole (but maybe not even that; I get confused on these things). I would love to add a clarifying note on this point to the article, "unbounded" just being so terribly terribly BAD as a word choice. I don't suppose anyone knows where to find a source? --Trovatore (talk) 19:20, 1 February 2021 (UTC)

I added an explanatory note. --Trovatore (talk) 04:01, 2 February 2021 (UTC)
 * This isn't a reliable source, but it's a good explanation of what I agree is the common usage in astronomy/cosmology: Ask an Astronomer: How likely is it that the Universe is closed, rather than flat?. "Unbounded" in common cosmology usage means "has no edge"; the example of the surface of the Earth as an unbounded surface is one that is routinely used in astronomy classes, including mine. But I am an astrophysicist, not a mathemetician! This would be one of many instances in which astronomy usage is inconsistent with other fields; for exhibit A, see metallicity.
 * "Closed" is jargon that is also used to mean finite but unbounded, but I think "finite but unbounded" is more clear than "closed". That said, checking two textbooks I have handy (Sparke & Gallagher, Galaxies in the Universe, an upper-level undergraduate text, and Bennet et al, The Cosmic Perspective, a first-year undergraduate survey text), both used "closed" but I can't find the word "unbounded" in a quick search. I think the note you added is appropriate. —Alex (Ashill &#124; talk &#124; contribs) 18:39, 2 February 2021 (UTC)

Issue with video in image caption
There is problem with the image caption  Cropped frame of the Constrained Local Universe Evolution Simulation (CLUES) (cropped variant 1).jpg  image. There is presumably a video there, but does not show up. I can't figure out whats wrong. Somebody know how to fix it? Andrew Z. Colvin • Talk 00:27, 17 February 2021 (UTC)
 * All I can say is the video works fine for me (tried Safari, Firefox, and Chrome on my Mac). When I click play, a player pops up in the middle of my browser and the movie starts. It runs slowly; takes several seconds for the first noticeable change to appear. —Alex (Ashill &#124; talk &#124; contribs) 06:24, 17 February 2021 (UTC)
 * The video works for me too, it's just that the clickable space appears as a tiny period inside a tiny grey box. It's not actually a thumb of the video like I surmise it is supposed to be. Andrew Z. Colvin • Talk 20:57, 17 February 2021 (UTC)

Confusing lede image
Does anyone think the lede image: https://en.wikipedia.org/wiki/Observable_universe#/media/File:Observable_Universe_with_Measurements_01.png has axes that are confusingly labeled "1 Billion light years"? This presumably corresponds to the tiny tickmarks, but to the uninitated, I think this will not be clear. Attic Salt (talk) 18:45, 27 January 2021 (UTC)
 * I agree; it's pretty terrible. When you zoom in, you can see that there are scale bars that show clearly that the labels correspond to the tickmarks, but that scale bar is illegible at the scale of display in the article. At the least, the small scale bars should be bright green like the text. Or they could just be dropped because the big scale (93 billion light years/28 billion parsecs) at the top is clear. I don't have the ability to easily modify the image. —Alex (Ashill &#124; talk &#124; contribs) 18:42, 2 February 2021 (UTC)
 * Easy to insult someones art when you don't make it, huh? Andrew Z. Colvin • Talk 23:59, 16 February 2021 (UTC)
 * It wasn't meant as an insult, though it did come across that way. I apologize. I see you improved it, and I think the small change makes it much better; thanks! —Alex (Ashill &#124; talk &#124; contribs) 00:30, 17 February 2021 (UTC)
 * No problem. It's kind of a weird thing to represent visually so I figured that the smaller tickmarks can help readers understand the scale a little better than simply having one scale bar. Iv'e considered improving the overall design further as I recently changed the galaxies into filaments instead of the cloud-like render that existed before. Any recommendations would be great! Andrew Z. Colvin • Talk 21:01, 17 February 2021 (UTC)

Inconsistent page ?
The page gives mass of observable universe 1.5*10^53, and it gives diameter of observable universe 8.8*10^26, these are fine enough, but then it gives density of universe 9.9×10^-27 kg/m^3. The thing is if one divide 1.5*10^53/SphereVolumeObservableUniverse, one gets 1.5*10^53/(4/3*pi*(4.4*10^26)^3)=4.4 *10^-28 kg/m^3

In other words the values given are not consistent. There exist a series of different estimates on for example the mass of the universe, therefore one should take the whole set from a single source, not mass from one and density from another one. This causes confusion for people not well studied on the topic. One cannot have mass 1.5*10^53 and radius 4.4*10^26 m at the same time one have density 9.9×10^-27 kg/m^3, this is inconsistent.

There are many inconsistencies on wikipedia pages, but we that try to fix them are often overrun by long term editors that are more interested in promoting their personal views than getting objective and good information up on wikipedia. ChrisCalif (talk) 17:20, 23 April 2021 (UTC)
 * Check your figures. You've done the arithmetic wrong. --Trovatore (talk) 17:43, 23 April 2021 (UTC)

what is wrong? the volume of a sphere is 4/3*pi*R^3. Please take the given mass of the observable universe given on the wikipedia page and divide by the volume one get from the radius of the observable universe given on the same page and see what density you get. Is it really 9.9×10^-27 kg/m^3 ??

of course I can have calculated wrong, but pls check! Tell me if you agree there is consistency or inconsistency in the numbers given omg the wikipedia page. I suspect inconsistency as they likely are collected from different sources using different estimates for H etc. This will cause confusion for non specialists on the topic, because should not mass divided by volume give mass density, sure it should! ChrisCalif (talk) 17:48, 23 April 2021 (UTC)

Wikipedia gives mass 1.5*10^53, 8.8×10^26 m (4.4×10^26 m radius), volume 4×10^80 m^3, and density 9.9×10−27 kg/m3. So no matter if you use the volume given, or the spherical volume from the radius given u will not get the density of 9.9×10−27 kg/m3, will you? Pls correct me if I am wrong.ChrisCalif (talk) 18:00, 23 April 2021 (UTC)
 * Oh wait; somehow I didn't see the 10^-27 in your second figure. Sorry about that. UPDATE: Ah, the reason I didn't see it is that it wasn't there; you added it in this diff, after my comment.  You might have mentioned that. --Trovatore (talk) 18:10, 23 April 2021 (UTC)
 * Anyway the $$\frac{4}{3}\pi R^3$$ formula is for a sphere in Euclidean 3-space, whereas here we're talking about general relativity. I don't know if that would explain the discrepancy or not (maybe not, since space is supposedly asymptotically flat to the best we can tell) but the question could be subtler than you think. Are we taking a slice of constant comoving time? It could be fairly sensitive to exactly what definitions are being used. --Trovatore (talk) 18:01, 23 April 2021 (UTC)

So either add explanation of why it not match up on wikipedia, or take for example all from one source! Several of my sources gives numbers matching perfectly up. The page has given a volume and a mass, mass density is mass divided by volume, if not using sphere due to whatever explanation one still get a volume. No the page is sloppy and have got approximate numbers from different sources and just put ut up on the page without any check of quality. ChrisCalif (talk) 18:07, 23 April 2021 (UTC)

Be aware the mass in many texts are given as extreme approximate, they dont even care if off by factor 2, as they are interested in if 10^53, or 10^52 etc. If use accurate calculations of all parameters from same (Hubble data set) it should indeed match up with logic. ChrisCalif (talk) 18:09, 23 April 2021 (UTC)
 * Value 9.9×10^-27 kg/m^3 refers to the total energy density, which includes the dark energy, dark matter and ordinary matter. Basically this is the critical density. On the other hand, the mass of ordinary matter is 1.5*10^53 kg (~4.1% of the total). Ruslik_ Zero 18:50, 23 April 2021 (UTC)
 * Ah, nice. That seems to be reasonably clear in the infobox, which I think is where ChrisCalif was seeing those numbers.  In any case the first number already has an explanatory footnote, which could be expanded to point out the distinction being made more explicitly. --Trovatore (talk) 18:58, 23 April 2021 (UTC)

Okay so 1.5*10^53 is the mass of ordinary matter according to Ruslik. And the density is due to total matter-energy (ordinary + dark matter + dark energy), but then the mass is 9.9×10−27 kg/m3*4/3*pi*(4.4*10^26)^26=1.12*10^54, 1.5*10^53 kg divided by this is 13% which do not fit (~4.1% of the total), in particular here the inflated radius (as is given in the box) is indeed the relevant radius (Not for example the Hubble radius). Please advice ChrisCalif (talk) 20:35, 23 April 2021 (UTC)
 * 9.9×10−27 kg/m3*4/3*pi*(4.4*10^26)^26=3.53*10^54. Ruslik_ Zero 08:20, 24 April 2021 (UTC)

The critical mass density is indeed around 9.9×10−27 kg if using H=72.7 (decent H value, inside consensus), but someone should check carefully if one then should operate with the volume and mass given without further explanation, personally I find the page confusing as it is now, it can be improved I think here, to be more clear for example that this is the critical mass-energy density, and not then the assumed density of the universe as it is assumed to be expanding. If it is the critical mass, then Λ is zero, so then how can dark energy be part of this density? Is that not additional to the critical density as we then have a non zero Λ that is linked to dark energy and expansion. 8.8*10^26 is the diameter of the inflated universe, after expansion at current time. ChrisCalif (talk) 20:48, 23 April 2021 (UTC)

DE has approx 70% density of the critical density. The critical density should be used with a radius(volume) before expansion. The dark energy should be used for expanded volume, then indeed one get the correct ratio of ordinary matter. I think Ruslik is wrong when saying dark energy part of the density, as it is the critical density which is before one add dark energy, which is why critical density is linked to Λ zero. Will probably take quite some work to clarity the page more, but something to think about, just a thought.ChrisCalif (talk) 21:17, 23 April 2021 (UTC)
 * Thinking in terms of the volume or radius of the Universe is just not how measurements work; it's the density that we relatively-directly measure, and it's the density that the references provide. Extrapolating to a volume is fine (it's basic arithmetic), but it's not the starting point. The 70% (really 68.3%) number is what comes directly from the references.
 * is correct in saying that dark energy is part of the total energy density of the Universe. I think expressing densities in kg/m^3 is a bit odd, since it's an energy density not a matter density, but it's fine (matter and energy are equivalent, after all). I also think it's a bit confusing to list the density of total matter/energy but the total mass of ordinary matter; the way it's presented suggests to a reader that one should be able to multiply the density times the volume and get the mass/energy, which is not true with the way it's presented. We should either report densities of both (my preference) or total mass/energy of both (or both density and total of each) so they can be directly compared.
 * Slightly rewording what Ruslilk0 said: The critical density is the critical density. The total energy density of the Universe is radiation (close to zero in the present-day so often ignored in lists of the contents of the Universe, including in this article) + matter + dark matter + dark energy. We measure the sum of the three (or four) terms, and that sum is almost exactly the critical density. If you don't include dark energy, the sum is only about 30% of the critical density. —Alex (Ashill &#124; talk &#124; contribs) 22:36, 23 April 2021 (UTC)

The critical density is for a flat universe (Λ is zero in Friedmann) when Euclidean space holds, so the volume is then a classical sphere. So it is just that things are a bit diffusely labeled on this page, so room for improvements. ChrisCalif (talk) 09:30, 24 April 2021 (UTC)

Faster than light
Were it says,

"it refers to the physical limit created by the speed of light itself. Because no signals can travel faster than light, any object farther away from us than light could travel in the age of the universe"

This is wrong since the universe has been expanding in the meanwhile. Actually we observe galaxies so far (z>1.5) that when it's light was emitted the galaxy was receding faster than light. Juan lacruz (talk) 16:37, 19 May 2021 (UTC)
 * Fixed, although feel free to do it yourself next time. Unusually, this is also the wording used in cosmological horizon. I suspect the reason the original quote is misleading is because it quotes the age of the universe at 13.7 billion years, which is not directly related to the cosmological horizon. Banedon (talk) 00:55, 20 May 2021 (UTC)

Topology and boundaries and all that
When I reverted 's edit (this one) I didn't have time to discuss my objections in detail. I thought maybe Gatewaycat would open a discussion on talk and we could talk about it.

Since he/she didn't, I'll do it myself, because I do think there's some legitimate stuff to discuss here, though I'm not sure whether or not we can boil it down to something useful to say in the article. For convenience, Gatewaycat added the following text:
 * The terms "finite" and "unbounded" in cosmology correspond to the terms "bounded" and "boundaryless" in mathematics, respectively. See Boundary (topology).

Here are the issues I see: I think, in the end, the closest standard mathematical expression of the cosmologist's idea of "unbounded" is that the universe is a geodesic manifold &mdash; that is, you can go an arbitrary distance in any direction. See by the way this old refdesk discussion.
 * Equating a cosmologist's "finite" with a mathematician's "bounded" might work, if we specify that the latter is bounded in the sense of a metric space, but the former might also mean finite volume, which is not exactly the same thing.
 * "Boundary" has too many meanings in topology for it to be clear which one is intended here.
 * One possibility would be the meaning from point-set topology, but that only makes sense for a set embedded in some background topological space. It's not clear what the whole physical universe is supposed to be embedded in.
 * Another way of reading it would be that the universe should be a manifold, rather than just a manifold with boundary. But that doesn't capture the cosmological idea, I think.  For example, if the universe looked like the set of all (x, y, z) with x &ge; 0, then it would not be a manifold, but only a manifold with boundary, so that works .. but if it looks like all (x, y, z) with x &gt; 0, then all of a sudden it would be a full-fledged manifold (without boundary).  From the point of view of cosmology, you can't really distinguish these cases, so this analysis doesn't seem to be getting at the root point.

So then the question is, is it a good idea to try to incorporate any of this into the article? It seems difficult to make precise in a way that will make sense to very many readers, and it might be tricky to source. --Trovatore (talk) 04:06, 23 September 2021 (UTC)
 * I definitely think having a better explanation of this language here would be useful, but we definitely would need sources, and I'm don't recall many cosmology books being that precise in their use of mathematical language in this regard. - Parejkoj (talk) 17:35, 23 September 2021 (UTC)
 * Yeah, I don't know where to find sources either. Complicating the issue, as well, is that to capture the cosmologist's "unbounded", we are (I assume) only interested in the universe "in the large" &mdash; we don't care about black-hole singularities, we don't care about quantum foam, we don't care about the notion of space being possibly quantized at extremely small distances.  Formalizing exactly what we don't care about is likely to be challenging.  But it would be very cool if someone could find something, as I think lots of people would potentially be interested in how this maps to mathematics. --Trovatore (talk) 20:43, 23 September 2021 (UTC)
 * I think that all that's needed, is a thorough re-read of Wald's General relativy and a distillation of something out of it. I made my way through it once. I don't feel like doing it again, alas. - DVdm (talk) 22:16, 23 September 2021 (UTC)
 * Hmm, I don't have it. I have Misner, Thorne, and Wheeler, Gravitation, but I don't recall such a discussion there.  I could also take a look through Barrett O'Neill's Semi-Riemannian Geometry. --Trovatore (talk) 22:37, 23 September 2021 (UTC)

Calculation
Im trying to understand the basis for the calculation that the universe is roughly 93 Gly across. My calculation is about a factor of 3 off at 28 Gly.

Calculation is speed of light in m/s divided by hubble constant in km/s/Mpc, times two for the diameter, convert to Gpc and then convert pc to ly by multiplying by 3.26. I get 28ish. — Preceding unsigned comment added by 134.204.1.226 (talk) 20:23, 26 April 2022 (UTC)

Update "most distant observed object" to include Hubble gravitationally lensed star Earendel
Article needs an update to "most distantly observed object" section Danshawen (talk) 01:30, 28 April 2022 (UTC)Danshawen
 * I changed the section slightly, but did not include Earendel, because it's not the most distant observed object. It's the most distant observed star, but this is the observable universe article, not List of the most distant astronomical objects. Banedon (talk) 01:50, 28 April 2022 (UTC)

Recent reverts
hey ! I reverted addition of several sentences backed by Siegel's blog as unencyclopedic. Can you please tell me how is this valuable for the article? It looks like a poorly sourced trivia: An interesting occurrence: the future visibility limit is exactly equal to the reachable limit (of 18 billion light-years) added to the current visibility limit (of 46 billion light-years). This no coincidence; the light that will ultimately reach us is right at that reachable limit today, after journeying 46 billion light-years since the Big Bang. Someday far in the future, it will arrive at our eyes. Artem.G (talk) 07:13, 2 April 2023 (UTC)
 * I don't understand what you mean. The size of the observable universe changes over time. The quote you give explains how the size of the observable universe as well as the size of the reachable universe changes in the future. This article is on the observable universe. Banedon (talk) 14:23, 2 April 2023 (UTC)
 * An interesting occurrence, Someday far in the future, it will arrive at our eyes. doesn't sound like an encyclopedic article, but more like attention-grabbing journalistic piece. Aren't there any better sources from real papers, not from a blog? (even if this blog is by an expert, it's not peer reviewed and thus not entirely reliable) And it should be rewritten anyway, even if you think the sources are acceptable. I'm mostly away for this and the next week, but will ping some users who can have an opinion on this:, , . Artem.G (talk) 06:46, 3 April 2023 (UTC)
 * That's the sort of thing which should be cited to an introductory cosmology book, not a random blog post. (If the only place that a topic is discussed is in a random blog post, then we have to wonder whether it deserves including here.) Also, it needs to be written, not copied. XOR&#39;easter (talk) 11:12, 3 April 2023 (UTC)
 * This is the kind of thing that won't show up in the research literature, because the math is well-known and the results depend on parameters. Like, let's say you have an equation x + y = 10. You can't say what x and y are because the system is unconstrained, but once you experimentally measure one, the theory tells you the value for the other. That's what we have here. Depending on the amount of dark energy & matter in the universe, the reachable limit will vary, same as the visibility limit of the future. You could find papers on how much dark energy & matter there is (c.f. Planck), but not what the reachable/visible limit will be. Banedon (talk) 02:05, 4 April 2023 (UTC)
 * As far as I understand, the expansion of the universe means that the farther reaches of the universe will never be able to reach our eyes.  Serendi pod ous  09:29, 3 April 2023 (UTC)
 * Well if we can already see it, then light from those regions is already able to reach our eyes. It's just that as the universe expands, the size of the observable universe changes; what we can currently see might not be visible in the future. Banedon (talk) 01:56, 4 April 2023 (UTC)
 * I think this is trivial calculation for those are the expert in the field. I could not find introductory book written about it yet, but here is a simple Astronomy paper explain the math the relation ship between the size"reachable/affected universe" with future size of observable universe. Sorry for copy paste of the article, I'm not very articulate in English language. I think one can write down as equation a+b=c. The value of a and c already well source in this article, the value of b (reachable universe) is always overshadow by observable universe article in search engine result. Thus my effort to highlighting it in wikipedia page J.X (talk) 12:52, 4 April 2023 (UTC)
 * Astute readers will notice that the radius of the affectable universe (16.5 billion light years) and that of the observable universe (46.4 billion light years) sum to the greatest distance light can travel (62.9 billion light years). This is not a coincidence, but a fundamental fact of our universe. J.X (talk) 12:56, 4 April 2023 (UTC)