B Regular black holes

[javisot]

TL;DR Summary

Regular black holes by infinite corrections

A work published some time ago by several of my compatriots, on regular black holes, without singularity.

https://arxiv.org/abs/2412.02742

I would like to ask some questions but first I want to hear expert opinions on this work.

Sabine has uploaded a video about it,

 

[PeterDonis]

I would like to ask some questions

Then you should ask them.

first I want to hear expert opinions on this work

Any such opinion will be just that, an opinion. Meaning, there is no experimental evidence involved, and without any experimental evidence, no one's opinion carries any real weight anyway.

 

[PeterDonis]

Btw, there is at least one known class of solutions in standard GR (i.e., without the modifications described in the paper referenced in the OP) that, while it has no actual event horizon, looks like a black hole from the outside for a very, very long time, on the order of the Hawking evaporation time. The term "Bardeen black hole" is used to describe this class of solutions, although since they have no event horizons, the term "black hole" is actually a misnomer.

See this post and the paper it references:

https://www.physicsforums.com/threa...n-evaporating-black-hole.1062617/post-7086255

Note, btw, that the paper uses the term "regular black hole" to describe the Bardeen black hole class of solutions--not solutions in a modified theory which has different dynamics from standard GR, as in the paper referenced in the OP.

 

[javisot]

Any such opinion will be just that, an opinion. Meaning, there is no experimental evidence involved, and without any experimental evidence, no one's opinion carries any real weight anyway.

Sorry, I explained myself poorly. I'd like to hear expert opinions on whether this work is theoretically correct and make sense. Assuming it's incorrect, then I wouldn't have any questions.

 

[javisot]

I extract 3 parts of the work that I try to understand:

1-"We have shown that regular black
holes are the endpoint of gravitational collapse in a purely gravitational theory. This is a generic consequence of a theory containing an infinite tower of higher-curvature corrections with only very mild and qualitative conditions on the couplings. This provides a mechanism for the resolution of singularities and the formation of regular black holes in any di-mension D ≥ 5. We believe this is the first time results of such generality have been achieved. Our model affords considerable opportunity to address important problems in the theory of regular
black holes and singularity resolution."

2- "These effects will likely play an
important role in the final stages of regular black hole evaporation. Whether or not the mechanism we have identified is the one responsible for singularity resolution in Nature remains to be seen. What is clear is that it provides a robust mechanism where many long-thought impossible questions can be finally addressed."

3- "We analytically solve the collapse of a thin shell of dust and show that it inevitably experiences a bounce at small radius and that its motion can be extended to arbitrary proper time. The collapse of the shell always gives rise to a singularity-free, geodesically complete spacetime that contains horizons if the total mass is above a critical value. In that case, the shell bounces into a new universe through a white hole explosion. Our construction provides, to the best of our knowlege, the first fully dynamical description of formation of regular black holes, and it suggests that higher-derivative corrections may be the most natural way to resolve the singularities of Einstein’s theory."

 

[PeterDonis]

I'd like to hear expert opinions on whether this work is theoretically correct and make sense.

It's probably too early for experts to have an opinion on that, since the paper is proposing a modified theory of gravity.

 

[PeterDonis]

I extract 3 parts of the work that I try to understand:

Ok, so what questions do you have about them?

 

[javisot]

Ok, so what questions do you have about them?

I want to understand part 3.

So, these regular black holes, as described in this paper, have an event horizon, no singularity, and they transition to white? (I'm not clear about this last part, since the authors dedicate little to it)

 

[PeterDonis]

So, these regular black holes, as described in this paper, have an event horizon, no singularity, and they transition to white?

"Transition to white" in the sense that they "bounce into a new universe", as what you quoted from the paper says. Fig. 2 of the paper shows the Penrose diagram that describes this.

 

[javisot]

"Transition to white" in the sense that they "bounce into a new universe", as what you quoted from the paper says. Fig. 2 of the paper shows the Penrose diagram that describes this.

I see it. I also see that the only part where they discuss it is practically at the end, and they stop explaining what appears to be the description of the white hole after the bounce. It feels like they don't want to talk about it.

"One of these contains the shell, inside of which the spacetime is flat, while the other region corresponds to a “de Sitter core” in which the line r = 0 is fully regular.[77] Ultimately, the shell starts climbing the potential and reaches a turning point at which R˙ = 0 and R = Rmin — this always happens in region III. At that point, a bounce occurs. The shell begins increasing its size, crossing the inner and outer horizons and emerging in a new universe from a white hole. The shell will grow up to r = R0, at which point the process of collapse restarts. If the total mass is below the critical threshold, the shell experiences a bounce as well, but horizons never form."

 

[javisot]

So they start from GR black hole, apply an infinite tower of higher-curvature corrections, hey get a black hole without singularity, and then a bounce (similar to LQC)?

I just noticed that this white hole part isn't mentioned by Sabine in her video either.

 

[PeterDonis]

they stop explaining what appears to be the description of the white hole after the bounce. It feels like they don't want to talk about it.

I can imagine they don't, because of the "new universe" part.

 

[PeterDonis]

they start from GR black hole, apply an infinite tower of higher-curvature corrections

More precisely, they apply "an infinite tower of higher-curvature corrections" to the Einstein-Hilbert action (and hence to the Einstein Field Equation), and then examine the properties of what corresponds to something like the Schwarzschild solution in this modified theory.

they get a black hole without singularity, and then a bounce (similar to LQC)?

That's more or less how they describe it, but what their model actually shows is something a lot weirder: it's like the maximal extension of Reissner-Nordstrom or Kerr, even though their model is spherically symmetric (no rotation) and vacuum (no electric charge). And that maximal extension has some very strange properties, which aren't normally talked about much since nobody believes that actual objects in our universe have those properties, they're just properties of an idealized, physically unreasonable model. But this paper appears to be relying on those same properties and claiming that they are physically reasonable.