4

I am wondering if could it be possible that the Universe is expanding in some areas while contracting in other areas.

I have wondered if perhaps as one area of the Universe is squeezed inward by some force, a force such as dark energy, all the matter that is within that squeezed space is flowing outward in all directions and causing the areas outside of the squeezed space to expand via the influx of new matter. This would be similar to when you hold a small water balloon in your hand and then squeeze it.

Moreover, perhaps there may be multiple areas throughout the Universe that are being squeezed and multiple areas that are expanding at any given moment. Perhaps this could be an alternate explanation as to why galaxies are moving in different directions throughout the Universe. The overall volume of the Universe would always same the same, especially considering that energy/matter cannot be created or destroyed.

Could it be possible that the Universe is expanding in some areas while contracting in other areas?

user57467
  • 462
  • 2
    https://en.wikipedia.org/wiki/Great_Attractor – mmesser314 Mar 27 '23 at 02:58
  • I'm not the expert on this matter, but this explanation really has the power - it makes a multiverse a closed system. So no need for a "dark energy"- some universes expands, just because co-universes contracts. Also because multi-verse becomes a closed one,- now energy conservation law becomes fixed,- input of "dark energy" into our universe is compensated by output of same energy in other co-universes, so total amount of energy in multiverse keeps the same, now. It should feel like a right way to solve many "conundrums" in astrophysics. – Agnius Vasiliauskas Mar 27 '23 at 10:55
  • 1
    see: https://physics.stackexchange.com/questions/422892/is-space-expansion-causing-increasing-distances-between-galaxies/674077#674077 – Andrew Steane Mar 27 '23 at 16:19

2 Answers2

5

The universe is indeed expanding in some places and contracting in others. That's why we have galaxies: regions of the universe that were denser than average eventually stopped expanding, turned around, and collapsed. At larger scales, our Local Group is contracting but has not collapsed yet. Conversely, cosmic voids are regions where the density is lower than average, and the expansion there is faster than the global expansion rate.

You will notice that I am speaking of the expansion of the material within the universe, and not the expansion of space itself. That's because the expansion of space is not even a local physical phenomenon in the first place! It's a common misconception that space is some sort of medium that expands and carries things with it. Expanding space has no such effect -- it is really just a convention that simplifies the mathematics in cosmological contexts. It's a coordinate choice.

See this answer for further reading on "expanding space" not being a local physical phenomenon. There are also other Stack Exchange discussions of this point, e.g. "Is the universe actually expanding?" and "Can the Hubble constant be measured locally?" Incorrect answers to these questions are unfortunately common and frequently upvoted, a testament to how tenacious this misconception is, but I've linked correct answers.

It's worth noting, though, that expanding space can have concrete meaning globally, in the sense that the total volume of a closed universe -- measured on the synchronous surfaces of comoving observers -- grows.

Also, there is a local expansion force induced by dark energy. This is sometimes conflated with cosmic expansion, but it's really just a consequence of the local dark energy content (or cosmological constant). If you intend to ask whether this force spatially varies, to the best of our knowledge it does not, but this would be better suited for a separate answer (perhaps even a separate question).

Sten
  • 4,851
  • 1
  • 13
  • 30
  • 1
    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? – John Doty Mar 27 '23 at 13:09
  • 3
    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. – Andrew Steane Mar 27 '23 at 14:22
  • 1
    @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?" – Sten Mar 27 '23 at 15:50
  • @JohnDoty See above comment (can only notify one person per comment) – Sten Mar 27 '23 at 15:53
  • 2
    Yes I agree (related: https://physics.stackexchange.com/questions/422892/is-space-expansion-causing-increasing-distances-between-galaxies/674077#674077; https://physics.stackexchange.com/questions/2110/why-does-space-expansion-not-expand-matter/441211#441211) – Andrew Steane Mar 27 '23 at 16:18
  • 1
    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? – FlatterMann Mar 27 '23 at 16:42
  • 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. – Edouard Mar 27 '23 at 16:43
  • 1
    @Edouard Sorry, I'm not seeing where a multiverse entered the discussion? Anyway I restructured the answer to make it clearer. – Sten Mar 27 '23 at 17:14
  • 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. – Edouard Mar 27 '23 at 17:47
  • 2
    @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. – Sten Mar 27 '23 at 18:21
  • 1
    @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. – Sten Mar 27 '23 at 18:40
  • @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. – FlatterMann Mar 27 '23 at 18:51
  • But how do you then get FTL expansion, – blademan9999 Aug 03 '23 at 07:48
  • @blademan9999 see my answer at https://astronomy.stackexchange.com/a/54225/47607 – Sten Aug 03 '23 at 08:09
  • Except galaxies further enough away are inaccessible, as no ligh from us can now reach them. – blademan9999 Aug 03 '23 at 08:26
  • @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. – Sten Aug 03 '23 at 08:41
3

If expansion is growing separation of comoving and force free worldlines and contraction is shrinking separation between them then yes, in the close field where matter collapses you can describe this as space falling into the center of mass if you use the Raindrop analogy as you see in the Gullstrand Painlevé equivalent form of the FLRW metric in Proper Distance coordinates where the radial space flow velocity $\rm v=g_{tr}=-c\sqrt{r_s/r}$ is replaced by $\rm v=g_{tr}=+H r$, so in that sense in the close field where the sign is negative, space can be described as contracting while in the far field where the sign becomes positive as expanding. For a single mass in an otherwise dark energy dominated universe the distance where the sign changes would be at $\rm r=\sqrt[3]{G M/H^2}$.

Yukterez
  • 11,553
  • But in reality angular momentum expels most of the mass even from these regions and what's left in black holes succumbs to black hole evaporation. In other words, gravity can never stabilize regions of the universe forever. It just delays the inevitable decay into the (empty) ground state. – FlatterMann Mar 27 '23 at 04:07
  • 3
    Sure, but angular momentum doesn't invalidate the River model, for the infalling space analogy that only contributes as frame dragging but even if the test particles are in orbit at constant radius, the space itself is still regarded as infalling (or expanding when Hr becomes large enough) in that analogy. – Yukterez Mar 27 '23 at 04:12