Would a single black hole form in the big crunch scenario?

[ilikescience94]

In the big crunch scenario, matter would clump together faster and faster, could a blob of this matter form a black hole, even if it had the necessary energy to maintain fusion or if it had no energy to burn up the matter? If the big crunch were to happen, and a single blob of mass existed at the end, would space time become smaller as well, or would that not shrink, or would spacetime exponentially increase the way that it is in our universe, because matter would be less dense, so there would be more empty space for dark energy to do it's thing, even though it lost the battle? Also, if a black hole did form, with the mass of a few hundred billion galaxies, what would happen to Hawking Radiation?

 

[phinds]

the condition of the universe just after the singularity was a plasma that was hot and dense pretty much beyond human imagining, but it was not a black hole because energy overcame gravity. Probably the big crunch (which is not likely to happen since there's no reason to believe the expansion will stop and reverse) would end up back in the same state.

 

[ilikescience94]

ok thanks, I know that our universe won't experience a big crunch, unless our conception about expansion is wrong, but that plasma makes sense, I wasn't really thinking about that, also, I was wondering if there could be a maximum gravitational pull, because gravity is related to the distance between objects, so if this plasma were say 1 billion miles or so in radius, then would extra matter pilling onto this plasma increase the gravitational pull, or would it just continue the same gravitational pull, because the mass at the center of this plasma ball be so far away that its gravitational pull is negligible for far away objects. I don't really know how to explain it, but I guess it would be like a hollow ball inside of a larger ball as far as gravity is concerned, so that for every mile extra of thickness that is added to the outside of this plasma, a mile of matter on the inside no longer has a significant pull on any outside objects, or would general relativity allow space curvature, so that this inside mass is no longer negligible, because it bends spacetime, which would attract matter based on the bend of spacetime, which is affected by all of the matter rather than traditional Newtonian gravity which relies solely on radial distances between objects?

 

[Drakkith]

The issue is that the universe is believed to be infinite in size, so this black hole would exist everywhere without a gradient. Also, when talking about a big crunch, we are talking about the reverse of the big bang, and in these scenarios we have dynamic spacetime, not static like we have in a regular black hole. (I hope I'm using dynamic and static spacetime correctly here) For example, the very early universe was not believed to be a black hole because the the solutions to GR that lead to a black hole do not include changing spacetime.

 

[bcrowell]

Drakkith's post seems on target to me. Some additional considerations: We could imagine a big crunch with or without a singularity. In the case without a singularity, Tolman ruled out cyclical models on thermodynamic grounds in 1934. In fact, the Hawkin-Penrose singularity theorems tell us that a big crunch would have to be a singularity. In the case with a singularity, it doesn't make sense to talk about what the laws of physics predict to happen after the singularity, because the laws of physics lose their predictive power at such a singularity.

 

[ikjyotsingh]

I think we have to be a bit careful here. The big crunch leading to a black hole singularity is a bit misnomer, since the geometries from a dynamic evolution sense don't match. Our current universe is FLRW which admits a six-dimensional isometry group. Black hole solutions given by the Schwarzschild class of metrics have a four-dimensional isometry group, not to mention that they are also static! From a dynamical sense to show that our FLRW universe would evolve into a Black Hole singularity I believe is very difficult to do.

What is possible, and actually what the Hawking-Penrose singularity theorems show as can be seen from the Raychaudhuri-Ehlers equation is that at late asymptotic times, an FLRW universe can admit a singularity where the expansion scalar becomes 0. But this singularity is NOT a black hole singularity. A Black hole singularity is only defined by the Kretchshmann scalar for the S-metrics.

Also, an important point to note from GFR Ellis in the Relativistic Cosmology textbook: "There is also a possibility of "sudden singularities" and "rip singularities" at late times, but these seem to occur only for hypothetical matter with implausible physical properties."