<p>The likely answer is no.</p>
<p>The reason is that the universe expands at a sufficient rate to make gravitationally bound galactic (super)clusters separate from each other, eventually likely becoming totally isolated. There is no way of all the matter in the universe to get together in one place even in infinite time if the expansion continues (technically, there is a cosmological event horizon).</p>
<p>But what about the fate of the superclusters? They will eventually undergo merging processes that merge their black holes and (over long time) either scatter stars and other matter into intergalactic space or absorb them into the black hole. So at the end you do get one big black hole for each former supercluster with much of its mass.</p>
<p>The angular momentum of the supercluster is conserved. <a href="https://astronomy.stackexchange.com/questions/21210/does-the-abell-1689-supercluster-have-angular-momentum">It doesn't look like we have great data on supercluster angular momentum, but it is likely nonzero</a>. The <a href="https://www.nepjol.info/index.php/JIST/article/view/33743" rel="nofollow noreferrer">galaxies seem to have random momentum vectors</a>, but if you sum a growing number <span class="math-container">$N$</span> of random vectors you should expect the total length of the vector sum to grow as <span class="math-container">$\propto \sqrt{N}$</span>. Hence the big black holes are likely going to have a fair bit of angular momentum, and different supercluster black holes are likely to have independent spin.</p>
<p>In some cases two black holes of exactly opposite spin merge pole-to-pole and make a Schwarzschild hole, but this is unlikely. Even two non-rotating holes merging in a non-polar merger will end up as a black hole with angular momentum.</p>
<p>So the predicted end state of the degenerate era is a universe of isolated, randomly oriented black holes plus particles left from proton decay in intergalactic (interhole?) space. As time goes by <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.14.3260" rel="nofollow noreferrer">the angular momentum is slowly shed by Hawking radiation</a> until they evaporate, and all the angular momentum resides in particle spin.</p>

CC BY-SA 4.0
2022-09-24T09:14:15.250Z
2022-09-24T09:14:15.250Z
Anders Sandberg
<p>The answer to the question in the title is probably not. That would be a big crunch, and there is nothing to suggest that the universe is headed toward a big crunch.</p>
<p>The answer to the question in the body is yes: angular momentum is conserved (in some appropriate limit in GR), so a merger of two black holes with opposite angular momenta will give you one with zero angular momentum.</p>

CC BY-SA 4.0
2022-09-23T19:33:44.937Z
2022-09-23T19:33:44.937Z
benrg
<p>I see other similar questions, but mine is more specific: If total merger into one BH is the ultimate endgame,  then if the universe had no net angular momentum in the beginning, then wouldn't the final BH also have no spin?
So can Kerr BHs can become Schw BHs by merging with a Kerr BH of opposite angular momentum?</p>

CC BY-SA 4.0
2022-09-22T21:10:19.963Z
*Welcome to Astronomy SE!* Every site is different; here it's best if you expand a bit on "I see other similar questions" and add links to a few of them in your question, probably at the bottom. Assuming they are from Astronomy SE, all you have to do is paste the short link that you get from the "Share" button under each question, (or the full verbose link in the url bar) and when you save the changes the question title appears nicely formatted. I usually add a bullet (space dash space) in front of each one. *Thanks!*
uhoh
CC BY-SA 4.0
2022-09-22T21:13:56.577Z
basically same thing as the comment under your Physics SE question https://physics.stackexchange.com/q/699795/83380
uhoh
<supermassive-black-hole>
Will the universe eventually merge completely into a single, enormous BH
Paul
# Will the universe eventually merge completely into a single, enormous BH

(Asked by: username_0 on 2022-09-22)

I see other similar questions, but mine is more specific: If total merger into one BH is the ultimate endgame, then if the universe had no net angular momentum in the beginning, then wouldn't the final BH also have no spin?
So can Kerr BHs can become Schw BHs by merging with a Kerr BH of opposite angular momentum?

**Tags:** supermassive-black-hole

> **Comments:**
> **username_1**: *Welcome to Astronomy SE!* Every site is different; here it's best if you expand a bit on "I see other similar questions" and add links to a few of them in your question, probably at the bottom. Assuming they are from Astronomy SE, all you have to do is paste the short link that you get from the "Share" button under each question, (or the full verbose link in the url bar) and when you save the changes the question title appears nicely formatted. I usually add a bullet (space dash space) in front of each one. *Thanks!*  
> **username_1**: basically same thing as the comment under your Physics SE question https://physics.stackexchange.com/q/699795/83380  

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## 2 answers

### Answer by username_2 on 2022-09-24. Score: 2

The likely answer is no.

The reason is that the universe expands at a sufficient rate to make gravitationally bound galactic (super)clusters separate from each other, eventually likely becoming totally isolated. There is no way of all the matter in the universe to get together in one place even in infinite time if the expansion continues (technically, there is a cosmological event horizon).

But what about the fate of the superclusters? They will eventually undergo merging processes that merge their black holes and (over long time) either scatter stars and other matter into intergalactic space or absorb them into the black hole. So at the end you do get one big black hole for each former supercluster with much of its mass.

The angular momentum of the supercluster is conserved. [It doesn't look like we have great data on supercluster angular momentum, but it is likely nonzero](https://astronomy.stackexchange.com/questions/21210/does-the-abell-1689-supercluster-have-angular-momentum). The [galaxies seem to have random momentum vectors](https://www.nepjol.info/index.php/JIST/article/view/33743), but if you sum a growing number $N$ of random vectors you should expect the total length of the vector sum to grow as $\propto \sqrt{N}$. Hence the big black holes are likely going to have a fair bit of angular momentum, and different supercluster black holes are likely to have independent spin.

In some cases two black holes of exactly opposite spin merge pole-to-pole and make a Schwarzschild hole, but this is unlikely. Even two non-rotating holes merging in a non-polar merger will end up as a black hole with angular momentum.

So the predicted end state of the degenerate era is a universe of isolated, randomly oriented black holes plus particles left from proton decay in intergalactic (interhole?) space. As time goes by [the angular momentum is slowly shed by Hawking radiation](https://journals.aps.org/prd/abstract/10.1103/PhysRevD.14.3260) until they evaporate, and all the angular momentum resides in particle spin.


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### Answer by username_3 on 2022-09-23. Score: 1

The answer to the question in the title is probably not. That would be a big crunch, and there is nothing to suggest that the universe is headed toward a big crunch.

The answer to the question in the body is yes: angular momentum is conserved (in some appropriate limit in GR), so a merger of two black holes with opposite angular momenta will give you one with zero angular momentum.


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