<p>The gravitational repulsion from the dark energy is stronger than the gravitational attraction of the present-day average matter density. Thus, if you were to magically halt the cosmic expansion, it would resume again.</p>
<p>If you were to delete all of the dark energy and halt the cosmic expansion, then the universe would collapse in about the free-fall time, <span class="math-container">$\sqrt{3\pi/(32G\rho)}\simeq 40~\text{billion years}$</span>, where <span class="math-container">$\rho\simeq 40~\text{M}_\odot\text{kpc}^{-3}$</span> is the present-day average density of the matter.</p>
<p>The end result would not be a &quot;supergalaxy&quot;. Galaxy clusters would grow quickly, but they would grow in mass more rapidly than they grow in size, such that eventually they would become huge black holes. In the final collapse of the universe, these black holes would all merge together.</p>

CC BY-SA 4.0
2024-10-09T16:20:30.970Z
What would occur if the gravitational repulsion of dark energy was magically made equal to the (current) average magical density? Would that allow something closer to the scenario I am imagining, where the only change is the universe is no longer expanding nor contracting, merely remaining unchanging for eternity and allowing it to evolve without the long-term expansion or contraction placing a hard limit on large-scale evolution.

Unless you mean to say that supermassive galaxies would eventually become so dense that they would begin collapsing into their supermassive black holes entirely.
Cryoraptor
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2024-10-09T16:33:34.860Z
*Average matter density, apologies
Cryoraptor
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2024-10-09T17:11:31.230Z
@Cryoraptor That leaves you with [Einstein's static universe](https://en.wikipedia.org/wiki/Einstein%27s_static_universe). It's unstable, though, and regions that are overdense or underdense in the matter would evolve very differently. Small overdense "islands" could probably host galaxies in the long term. Larger overdense regions (like superclusters) might make black holes. Underdense regions, like voids, would exponentially expand and eventually dominate the total volume (although the universe would be so inhomogeneous that space, and hence volume, is no longer straightforward to define).
Sten
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2024-10-10T03:14:35.213Z
I think you won't get the Einstein static universe, because when $\dot a=0$ you have $K=4πG(ρ+p)$ (where $K=k/a^2$) independent of $Λ$, and we observe $|K|\ll 4πGρ$, and $K$ can't change during the magic stop unless it's so magical that it rearranges the galaxies, so you need some sort of exotic matter with $ρ+p<0$ to prevent collapse.
benrg
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2024-10-10T06:02:30.023Z
@benrg good point. I think the resolution is that you have to be careful about when you magically stop the expansion. Due to the different shapes of worldline-orthogonal spatial surfaces in expanding and not-expanding universes, you can stop expansion at the "expanding" same time or the "not expanding" same time, but not both. And I think you need a preferred observer whose motion does not change. My intuition is that in the first case the preferred observer ends up in the densest part of the universe, while in the second case they end up in the sparsest, so there's probably a middle ground...
Sten
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2024-10-10T06:02:36.157Z
where you retain something like homogeneity in the density. But no matter how you do it, there is time for the universe's mass to be redistributed between the "expanding universe" spatial surface and a later "not-expanding universe" spatial surface since due to their different shapes, you can't make them arbitrary close.
Sten
CC BY-SA 4.0
2024-10-10T20:56:08.220Z
@Sten Do you have any references for the hypothetical scenario you describe with Einstein's static universe that go into more detail or elaborate further? I'm really curious about that, it's not immediately obvious to me how that would work and I'd like to dig more into it.
Idran
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2024-10-09T15:53:14.233Z
2024-10-10T01:32:55.600Z
2024-10-10T01:32:55.600Z
Sten
<p>Assuming that after abruptly ending expansion, the universe remained completely static until the end of time, how long would it take for every single galaxy in the observable universe to merge into one gigantic ultramassive galaxy?</p>
<p>Discounting any galaxies or galatic remnants, stars, gas etc. that are ejected during mergers, assuming it was possible for any object to reach an escape velocity that would make it permanently unbound from anything else in the universe in this scenario.</p>
<p>As an aside, how massive would this final universal galaxy be, and how large? And would this process take longer than the time required for every galaxy in the universe to dissolve via stellar encounters and devouring by the supermassive black holes? Although presumably the super- and hypermassive black hole remnants could continue on merging with each other long after galaxies themselves have dissolved.</p>
<p>I have purposely used the observable universe for this scenario, and not the whole universe, as we don't know how large the entire universe is. Obviously, if the universe is incredibly large, or even infinite, galaxies would dissolve and black holes decay via Hawking radiation before such a universal galaxy could form.</p>

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2024-10-09T13:44:00.513Z
Policy on hypothetical questions: https://astronomy.meta.stackexchange.com/questions/786/what-exactly-count-as-a-hypothetical-question
Nilay Ghosh
<galaxy><universe><hypothetical>
Assuming the expansion of the universe somehow abruptly stopped tomorrow, how long would it take for all galaxies in the observable universe to merge?
Cryoraptor
# Assuming the expansion of the universe somehow abruptly stopped tomorrow, how long would it take for all galaxies in the observable universe to merge?

(Asked by: username_0 on 2024-10-09)

Assuming that after abruptly ending expansion, the universe remained completely static until the end of time, how long would it take for every single galaxy in the observable universe to merge into one gigantic ultramassive galaxy?

Discounting any galaxies or galatic remnants, stars, gas etc. that are ejected during mergers, assuming it was possible for any object to reach an escape velocity that would make it permanently unbound from anything else in the universe in this scenario.

As an aside, how massive would this final universal galaxy be, and how large? And would this process take longer than the time required for every galaxy in the universe to dissolve via stellar encounters and devouring by the supermassive black holes? Although presumably the super- and hypermassive black hole remnants could continue on merging with each other long after galaxies themselves have dissolved.

I have purposely used the observable universe for this scenario, and not the whole universe, as we don't know how large the entire universe is. Obviously, if the universe is incredibly large, or even infinite, galaxies would dissolve and black holes decay via Hawking radiation before such a universal galaxy could form.

**Tags:** galaxy, universe, hypothetical

> **Comments:**
> **username_1**: Policy on hypothetical questions: https://astronomy.meta.stackexchange.com/questions/786/what-exactly-count-as-a-hypothetical-question  

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## 1 answers

### Answer by username_2 on 2024-10-09. Score: 12

The gravitational repulsion from the dark energy is stronger than the gravitational attraction of the present-day average matter density. Thus, if you were to magically halt the cosmic expansion, it would resume again.

If you were to delete all of the dark energy and halt the cosmic expansion, then the universe would collapse in about the free-fall time, $\sqrt{3\pi/(32G\rho)}\simeq 40~\text{billion years}$, where $\rho\simeq 40~\text{M}\_\odot\text{kpc}^{-3}$ is the present-day average density of the matter.

The end result would not be a "supergalaxy". Galaxy clusters would grow quickly, but they would grow in mass more rapidly than they grow in size, such that eventually they would become huge black holes. In the final collapse of the universe, these black holes would all merge together.

> **Comments:**
> **username_0**: What would occur if the gravitational repulsion of dark energy was magically made equal to the (current) average magical density? Would that allow something closer to the scenario I am imagining, where the only change is the universe is no longer expanding nor contracting, merely remaining unchanging for eternity and allowing it to evolve without the long-term expansion or contraction placing a hard limit on large-scale evolution.  Unless you mean to say that supermassive galaxies would eventually become so dense that they would begin collapsing into their supermassive black holes entirely.  
> **username_0**: *Average matter density, apologies  
> **username_2**: @username_0 That leaves you with [Einstein's static universe](https://en.wikipedia.org/wiki/Einstein%27s_static_universe). It's unstable, though, and regions that are overdense or underdense in the matter would evolve very differently. Small overdense "islands" could probably host galaxies in the long term. Larger overdense regions (like superclusters) might make black holes. Underdense regions, like voids, would exponentially expand and eventually dominate the total volume (although the universe would be so inhomogeneous that space, and hence volume, is no longer straightforward to define).  
> **username_3**: I think you won't get the Einstein static universe, because when $\dot a=0$ you have $K=4πG(ρ+p)$ (where $K=k/a^2$) independent of $Λ$, and we observe $|K|\ll 4πGρ$, and $K$ can't change during the magic stop unless it's so magical that it rearranges the galaxies, so you need some sort of exotic matter with $ρ+p<0$ to prevent collapse.  
> **username_2**: @username_3 good point. I think the resolution is that you have to be careful about when you magically stop the expansion. Due to the different shapes of worldline-orthogonal spatial surfaces in expanding and not-expanding universes, you can stop expansion at the "expanding" same time or the "not expanding" same time, but not both. And I think you need a preferred observer whose motion does not change. My intuition is that in the first case the preferred observer ends up in the densest part of the universe, while in the second case they end up in the sparsest, so there's probably a middle ground...  
> **username_2**: where you retain something like homogeneity in the density. But no matter how you do it, there is time for the universe's mass to be redistributed between the "expanding universe" spatial surface and a later "not-expanding universe" spatial surface since due to their different shapes, you can't make them arbitrary close.  
> **username_4**: @username_2 Do you have any references for the hypothetical scenario you describe with Einstein's static universe that go into more detail or elaborate further? I'm really curious about that, it's not immediately obvious to me how that would work and I'd like to dig more into it.  

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