A New singularity theorem?

[windy miller]

There is a new paper on singularities https://arxiv.org/abs/2501.17910
My understanding is that most cosmologists dont take singularities very seriously as we need a quantum theory fo gravity to resolve them. In particular the statement that time ends in a black hole or began at the big bang are dubious until we get a quantum theory of gravity, then we will know.
Is there any reason to think this new paper actually proves there was a singularity? Any thoughts or explanations for a lay person would be much appreciated.

 

[Ibix]

Is there any reason to think this new paper actually proves there was a singularity?

I don't think so. It's still using semi-classical gravity as far as I can see, so even assuming it's 100% correct (and I'm not qualified to comment), it's still subject to "...but a true theory of quantum gravity could say something different".

 

[PeterDonis]

Is there any reason to think this new paper actually proves there was a singularity?

No. As the paper notes, it's not proving a brand new theorem, it's just extending the range of validity of an existing theorem, the Penrose-Wall theorem. From what I can gather, it's basically just showing that the Penrose-Wall theorem holds in any scenario where semiclassical gravity remains valid throughout, including "bounces" and other scenarios that weren't covered by the original proof of the theorem. But of course the whole point of "we need quantum gravity to resolve them" is that semiclassical gravity does not remain valid throughout; that's the expectation of basically everyone in the field. Nothing in this paper changes that.

 

[PeterDonis]

Moderator's note: Thread level changed to "A" to reflect the subject matter.

 

[PeterDonis]

the whole point of "we need quantum gravity to resolve them" is that semiclassical gravity does not remain valid throughout

To give a counterpoint to my comment here, it is true that this paper claims to significantly expand the boundaries of what is considered "semiclassical gravity": instead of assuming the null energy condition (NEC), this new proof only assumes the Generalized Second Law (GSL), which is a much weaker condition. And, unlike the NEC, which is known to be violated by generic quantum field states (for example, the states used in the usual derivation of Hawking radiation and black hole evaporation), there are no known examples of violations of the GSL.

However, it still looks to me like "semiclassical" in this paper means that there are no quantum aspects of the spacetime geometry itself--and that's what basically everyone in the field expects to not continue to be true when spacetime curvatures get large enough (basically in the Planck regime). So it still looks to me like this paper is not changing the basic expectation that quantum gravity will remove actual singularities.

 

[PeterDonis]

Moderator's note: Thread moved to relativity forum.

 

[windy miller]

thanks guys, thought that was the case.