[leiroigh]

>in a higher-dimensional parent universe

That's incorrect: The parent universe is not higher-dimensional, it's the same good old 3+1 as our universe.

What they propose is: Let's take our good old GR, and start with a (large, dilute) compactly supported spherically collapsing collapsing cloud of matter. During that, you get an event horizon; afterwards, this looks like a normal black hole outside, and you never see the internal evolution again ("frozen star", it's an event horizon). Inside, you have the matter cloud, then a large shell of vacuum, then the event horizon.

Quantum mechanics suggests that degeneracy pressure gives you an equation of state that looks like "dilute = dust" first, and at some point "oh no, incompressible".

They figure out that under various assumptions (and I think approximations), they get a solution where the inside bounces due to the degeneracy pressure. Viewed from inside, they identify that there should be an apparent cosmological constant, with the cosmological horizon somehow (?) corresponding to the BH horizon as viewed from the outside.

All along the article, they plug in various rough numbers, and they claim that our observed universe (with its scale, mass, age, apparent cosmological constant, etc) is compatible with this mechanism, even hand-waving at pertubations and CMB an-isotropies.

This would be super cool if it worked!

But I'm not convinced that the model truly works (internally) yet, too much hand-waving. And the matching to our real observed universe is also not yet convincing (to me). That being said, I'm out of the cosmology game for some years, and I'm a mathematician, not a physicist, so take my view with a generous helping of salt.

(I'm commenting from "reading" the arxiv preprint, but from not following all computations and references)

PS. I think that they also don't comment on stability near the bounce. But I think that regime is known to have BKL-style anisotropic instability. Now it may be that with the right parameters, the bounce occurs before these can rear their heads, and it might even be that I missed that they or one of their references argue that this is the case if you plug in numbers matched to our observed universe.

But the model would still be amazing if it all worked out, even if it was unstable.

[mr_toad]

> with the cosmological horizon somehow (?) corresponding to the BH horizon as viewed from the outside.

That’s not mentioned in the summary. After inflation the event horizon would not exist.

[leiroigh]

I have not really looked at the summary, opted to go straight to the source.

This identification happens in equations 31-34 on page 7f subsection "Cosmic Acceleration" in https://arxiv.org/abs/2505.23877

The justification looks super sketchy and hand-wavy to me, though, which I summarized as "somehow (?)".

"After inflation the event horizon would not exist."

Apparent cosmological constant viewed from the bouncing inside induces a cosmological horizon, which they identify with the black hole horizon viewed from the outside. Super elegant idea, but I don't buy that this is supposed to be true.

[michaelmrose]

Why does this black hole bounce whilst others from the limited info we possess appear to be stable regardless of lack of singularity

[leiroigh]

The bounce is invisible from the outside -- an event horizon means causal decoupling. From outside, the formation of the black hole looks like the good old "frozen star" picture.

There will never be observational evidence on what happens on the other side of any event horizon, you'd have to cross over to the other side to see it for yourself (but you won't be able to report back your findings). There's a fun greg egan short story about that ;)

[IggleSniggle]

What's the story?

[leiroigh]

"The Planck dive", freely available on Greg Egan's website https://www.gregegan.net/PLANCK/Complete/Planck.html