Timeline for Could it be possible that the Universe is expanding in some areas while contracting in other areas?
Current License: CC BY-SA 4.0
23 events
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| Aug 3, 2023 at 8:41 | comment | added | Sten | @blademan9999 You are asking how objects that were once close are now "too far for light to traverse" despite not traveling faster than light? The reason is dark energy's gravitational repulsion. A similar thing happens with black holes, where after an object falls in, we can no longer receive its light, even though it never traveled faster than light. | |
| Aug 3, 2023 at 8:26 | comment | added | blademan9999 | Except galaxies further enough away are inaccessible, as no ligh from us can now reach them. | |
| Aug 3, 2023 at 8:09 | comment | added | Sten | @blademan9999 see my answer at astronomy.stackexchange.com/a/54225/47607 | |
| Aug 3, 2023 at 7:48 | comment | added | blademan9999 | But how do you then get FTL expansion, | |
| Apr 15, 2023 at 18:27 | vote | accept | CommunityBot | ||
| Mar 27, 2023 at 18:51 | comment | added | FlatterMann | @Sten Three of the six phase space dimensions are spatial, so expanding spatial volume seems to imply expanding phase space, which is all that I meant. I do agree with your observation that even with the increase in volume the occupied volume of phase space seems to drop rapidly. The observable universe (can that be a stand-in for "local"?) seems to be emptying out rapidly, with the visible boundary becoming a rapidly cooling surface. One could have an argument whether we should count empty space as physically relevant... in which case "the system" is shrinking. | |
| Mar 27, 2023 at 18:40 | comment | added | Sten | @FlatterMann In cosmology, expansion of space usually refers to 3D spatial volumes, not 6D phase space. It's not really clear to me how to interpret cosmic expansion in terms of phase space volumes (because volumes grow but energies drop) -- but that's anyway a concern about the global, and not local, nature of cosmic expansion. | |
| Mar 27, 2023 at 18:21 | comment | added | Sten | @Edouard Whether space is equivalent to nothingness is probably a matter of semantics (and I'll make no claim on that), but the real point here is that there is no objective sense in which space can (locally) expand, contract, or move. In general relativity, the only local property of space is the (tensor) spacetime curvature. If quantum mechanics adds a vacuum energy, that's really the same thing (magnitude aside) as a cosmological constant and doesn't change the previous point. In particular, the vacuum energy is frame invariant, so it doesn't introduce motion to space. | |
| Mar 27, 2023 at 17:47 | comment | added | Edouard | In response to your question-marked reply to me, I'm obligated to say that it entered the discussion when the OP capitalized "Universe": Before the concept of inflation was formulated by Guth & Vilenkin, "universe" was never (except when it opened a statement or exclamation) capitalized in correct English, as it had meant "everywhere". Also, when I had started reading on this site, I would've agreed with you that space is equivalent to nothingness, but, since then, it's been brought to my attention that space has microscopic effects in the form of "vacuum energy", discussed on Wikipedia. | |
| Mar 27, 2023 at 17:34 | history | edited | Sten | CC BY-SA 4.0 |
make note of dark energy
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| Mar 27, 2023 at 17:14 | comment | added | Sten | @Edouard Sorry, I'm not seeing where a multiverse entered the discussion? Anyway I restructured the answer to make it clearer. | |
| Mar 27, 2023 at 17:13 | history | edited | Sten | CC BY-SA 4.0 |
restructure
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| Mar 27, 2023 at 16:43 | comment | added | Edouard | Sorry to be picky, but I'm an interpreter (not a physicist), and, as far as I've been able to tell, "Universe", in the terminology of physics, currently refers to our local universe in a multiverse (since place names are capitalized in English), whereas "universe" refers to a universe in the older sense in which it was not considered to be a "multiverse" of local universes causally separated from each other: Consequently, the answer doesn't (yet) quite correspond to the question, especially since the multiverse concept is not universally accepted among physicists. | |
| Mar 27, 2023 at 16:42 | comment | added | FlatterMann | I am not sure I can follow the discussion at this point. "space" to me means "phase space". If we are observing that matter and radiation have more degrees of freedom today than in the early universe, then to me "space is expanding". I don't know what symmetry it would take for a proper Noether version of a "spatial volume conservation law", but the universe does not seem to have that symmetry. Am I misunderstanding this? | |
| Mar 27, 2023 at 16:18 | comment | added | Andrew Steane | Yes I agree (related: physics.stackexchange.com/questions/422892/…; physics.stackexchange.com/questions/2110/…) | |
| Mar 27, 2023 at 15:53 | comment | added | Sten | @JohnDoty See above comment (can only notify one person per comment) | |
| Mar 27, 2023 at 15:50 | comment | added | Sten | @AndrewSteane Yeah, I forgot to add the caveat about the total volume of a closed universe growing (when measured on synchronous hypersurfaces). I've added that. The key point is that the question "where is space expanding?" is not meaningful because expanding space doesn't have local meaning. We can only answer "where are the contents of the universe expanding?" | |
| Mar 27, 2023 at 15:46 | history | edited | Sten | CC BY-SA 4.0 |
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| Mar 27, 2023 at 14:22 | comment | added | Andrew Steane | I agree we need to counter misconceptions and indeed there is no "expansion force" which has to be resisted etc. Nonetheless "expanding space" is a physical phenomenon at the global level because it means, for example, that a universe of finite spatial volume (e.g. a 3-sphere) can have a volume which changes with time, and for all we know the universe might be like that. | |
| Mar 27, 2023 at 13:09 | comment | added | John Doty | Calling real physics "not physical" because it isn't part of your mathematical model isn't proper physical reasoning. If the material is expanding, but the space is not, where does the extra space occupied by the material come from? | |
| Mar 27, 2023 at 10:44 | history | edited | Sten | CC BY-SA 4.0 |
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| Mar 27, 2023 at 10:36 | history | edited | Sten | CC BY-SA 4.0 |
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| Mar 27, 2023 at 10:29 | history | answered | Sten | CC BY-SA 4.0 |