B Repeating Big Bang

[dvdpitch]

TL;DR Summary

Colliding Black Holes

Per various articles the universe will die out in 10 to the 78 power years. With burned out stars and black holes.
Obviously black holes can collide which will increase their gravitational pull. I assume this could pull in more and exponentially more black holes.

Is it possible this would become so unstable with so much pressure and energy that it "exploded" and is essentially a future Big Bang.

Meaning that just keeps repeating itself over trillions of years.

Bang... matter is spread over the universe, gases attract, stars form, planets form, stars die, black holes converge to the point of destabilization and then Bang again?

 

[PeroK]

A black hole has no more gravity than the object from which it formed.

 

[dvdpitch]

A black hole has no more gravity than the object from which it formed

Understood, but I am suggesting multiple combined black holes. Equating to a large percentage of the gravity/matter from the universe.

 

[PeroK]

Understood, but I am suggesting multiple combined black holes. Equating to a large percentage of the gravity/matter from the universe.

The matter in the universe is generally moving further apart. The current model shows eternal expansion.

 

[Bandersnatch]

The leftover black holes in the heat death scenario are spread apart by the expansion of the universe. They don't all coalesce together. At best, you get as much mass in a single black hole as there is in one cluster or supercluster of galaxies - whatever is the structure in a given place that is decoupled from expansion.
But not all black holes in a cluster will collide. Some - most - will be ejected from the gravitationally bound system by the same orbital interactions that make them collide, so you get less mass that in the original cluster.
And at the same time, while you're waiting those aeons for the collisions to happen, the holes are evaporating. In the end you're left with just faint radiation in empty space, maybe some husks of stars floating alone in the void.


BTW, I don't think there's any way for a black hole to explode, no matter how large.

 

[Ibix]

No, black holes don't accumulate like that. Outside the radius of where the star (or whatever formed them) used to be their gravity is the same as the star was. Almost everything is gravitationally unbound from any given hole, so unless it's fantastically unlucky enough to get near one by chance it will not fall in.

 

[Ibix]

BTW, I don't think there's any way for a black hole to explode, no matter how large.

Arguably the end point of evaporation is an explosion? But larger holes take longer to do that, and they will, as you say, be enormously more spread out than they are today by the time it happens. If it happens at all, of course.

 

[javisot]

The leftover black holes in the heat death scenario are spread apart by the expansion of the universe. They don't all coalesce together. At best, you get as much mass in a single black hole as there is in one cluster or supercluster of galaxies - whatever is the structure in a given place that is decoupled from expansion.
But not all black holes in a cluster will collide. Some - most - will be ejected from the gravitationally bound system by the same orbital interactions that make them collide, so you get less mass that in the original cluster.
And at the same time, while you're waiting those aeons for the collisions to happen, the holes are evaporating. In the end you're left with just faint radiation in empty space, maybe some husks of stars floating alone in the void.


BTW, I don't think there's any way for a black hole to explode, no matter how large.

A question about this, is it reasonable that the probability of black holes merging decreases in the later phases of the universe, during the "heat death", as well as the probability of accreting matter in general. In that scenario, wouldn't they become increasingly similar to vacuum solutions?

More specifically, I ask whether black holes that form later in the history of the universe, and thus enter further into the heat death phase, will become increasingly similar to the classic vacuum solutions of EFEs.

 

[Jaime Rudas]

Is it possible this would become so unstable with so much pressure and energy that it "exploded" and is essentially a future Big Bang.

No, even if such an event were possible, its outcome would be very different from what we know of the Big Bang.

 

[phinds]

TL;DR Summary: Colliding Black Holes

Is it possible this would become so unstable with so much pressure and energy that it "exploded" and is essentially a future Big Bang.

You seem to have a bad misunderstanding of what the Big Bang WAS. It was most emphatically NOT an explosion in space such as you are describing.

 

[PeterDonis]

Per various articles the universe will die out in 10 to the 78 power years. With burned out stars and black holes.

If black holes evaporate, then the state you describe is not the final end state. The final end state is one consisting of nothing but very diffuse radiation, from evaporation of black holes and quantum tunneling of things like burned out stars into radiation. And no, such a state would not produce another Big Bang.

 

[PeterDonis]

I ask whether black holes that form later in the history of the universe, and thus enter further into the heat death phase, will become increasingly similar to the classic vacuum solutions of EFEs.

A black hole is a vacuum solution.

 

[javisot]

A black hole is a vacuum solution.

A vacuum solution describes an entire empty universe. The real black holes in the universe are not in an empty universe. My question is about the last "real" black holes to form in the universe that will go deeper into heat death, and therefore are in a context increasingly similar to vacuum(?).

 

[Ibix]

A vacuum solution describes an entire empty universe. The real black holes in the universe are not in an empty universe. My question is about the last "real" black holes to form in the universe that will go deeper into heat death, and therefore are in a context increasingly similar to vacuum(?).

I think I see what you are getting at, and have wondered along similar lines.

In a non-collapsing ideal FLRW universe, $a(t)\rightarrow\infty$ as $t\rightarrow\infty$, so $\rho(t)\rightarrow 0$. If one naively supposes that the same thing happens in a universe where the ideal fluid is replaced with something with a discrete nature (e.g. a population of galaxies or black holes), you eventually end up with a situation where all other galaxies/holes are behind the cosmological horizon for every individual galaxy/hole. What happens then?

My slightly unformed thought is that each grain always remains in the future domain of dependence of an ever-growing amount of fairly uniform matter. So the naive picture, that we eventually end up with a lot of isolated galaxies each in what appears to be an otherwise empty space, isn't totally implausible. However, I don't think galaxies are distributed uniformly - they form sheets and lines like the boundaries between soap bubbles (because under-dense regions expand and over-dense ones collapse), so on a bit more reflection you might expect something more complex.

I think you'd need to study perturbed FLRW solutions to get an authoritative answer. I'm sure I did find some notes on this at DAMTP in Cambridge, but search is failing me now. There's at least one paper on arXiv if you search "perturbed FLRW".

 

[Drakkith]

Understood, but I am suggesting multiple combined black holes. Equating to a large percentage of the gravity/matter from the universe.

No. Even if all of the matter in our galactic supercluster was turned into a black hole, this black hole would never be able to suck in matter from even adjacent superclusters as expansion is moving all of them away from us.

Or, to put it another way, this black hole would have the exact same mass and gravity as our current supercluster, and if our supercluster is not attracting other superclusters towards it then neither would the black hole.

 

[Jaime Rudas]

If black holes evaporate, then the state you describe is not the final end state. The final end state is one consisting of nothing but very diffuse radiation, from evaporation of black holes and quantum tunneling of things like burned out stars into radiation. And no, such a state would not produce another Big Bang.

Would it not produce one in a state of eternal inflation?

 

[PeterDonis]

A vacuum solution describes an entire empty universe.

That's one possible vacuum solution, but not the only one.

The real black holes in the universe are not in an empty universe.

We do not have a single model that describes both black holes--the vacuum solution that's left behind after a gravitational collapse of a massive star occurs--and a universe with matter in it. The FLRW solutions that describe universes with matter in them do not contain any black holes.

the universe that will go deeper into heat death, and therefore are in a context increasingly similar to vacuum(?).

The universe does not "become increasingly similar to vacuum" in the far future. A universe that contains increasingly diffuse radiation is still not vacuum, and trying to analyze it as if it were "close" to vacuum doesn't work. So I think the answer to your question, to the extent it's a meaningful question at all, is "no".

 

[PeterDonis]

Would it not produce one in a state of eternal inflation?

A state of eternal inflation is not the same as the state I was describing. In an eternal inflation model, once inflation has ended in a given "bubble" of a universe with a Big Bang, there is never another "Big Bang" in that bubble, even if it eventually evolves into the sort of "heat death" state that is being discussed in this thread. In an eternal inflation model that includes the universe we live in, the universe we live in is such a "bubble".

 

[javisot]

We do not have a single model that describes both black holes--the vacuum solution that's left behind after a gravitational collapse of a massive star occurs--and a universe with matter in it. The FLRW solutions that describe universes with matter in them do not contain any black holes.

I know... I'm very worried about not having a model of the universe with enough detail to properly include black holes. There will be a phase in the life of the universe where black holes will be practically the only relevant objects.

As Ibix says, perturbed FLRW solutions exist. But after looking at those solutions, none of them take into account that the environment of a real black hole in the universe isn't the same environment throughout the entire life of the universe. (Black holes closer to the "hot and dense state" should have very rich environments, while black holes closer to heat death should have very poor environments)

 

[PeterDonis]

I'm very worried about not having a model of the universe with enough detail to properly include black holes.

That's a valid worry if the things we call "black holes" really are black holes in the strict technical sense, i.e., regions of spacetime that are not in the causal past of future null infinity. It's extremely difficult to see how such a thing could fit into an overall universe model like an FLRW spacetime.

However, one obvious way around this problem is to consider the hypothesis that the things we call "black holes" aren't true black holes--they're something like a Bardeen "black hole" (which is a misnomer since the spacetime called by that name has no event horizons and no singularities). Things like that are no more difficult to fit into an overall universe model based on something like an FLRW spacetime than ordinary stars and planets. (They also allow including "evaporation" through something like Hawking radiation in the model, without all the problems involved with Hawking radiation added to the standard black hole model with an event horizon and a singularity.) My personal view is that something like this is where we will eventually end up.

In the meantime, however, a model in which we approximate black holes as "world tubes" with a certain mass (and angular momentum if they're spinning) would work just fine, the same way we would do with galaxies if we want to take a step beyond the standard FLRW spacetime that just averages over all the mass. (Recent investigations of possible significant effects of non-uniform density, which have been discussed in other PF threads, are done along these lines.) So I don't think the problem you refer to is actually a significant problem in a practical sense. It's just an open foundational issue that we'll have to resolve sooner or later.