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Comment by pryelluw | Hacker News Reader

You mean there could be black holes inside other black holes?

We don’t exactly know what the inside of a black hole looks like. It’s infinitely curved space so it’s potentially quite big. One theory is the space inside a black hole expands at the speed of light. You can’t leave because you can’t go fast enough to get out.

We know that if you have a big enough black hole that the space near the singularity no longer shreds matter. But would it matter if a sun came through intact or a sun’s worth of material came in, condensed, and went supernova?

https://www.nationalgeographic.com/science/article/140218-bl...

It depends on what you mean by the "inside" of a black hole. If you mean the event horizon, then for supermassive black holes, you wouldn't even notice that you'd passed it, so at that part of the inside, it looks exactly the same as outside. You might notice a difference if you tried to change your ship's direction and head "away", but you'd see that nothing you could do would let you get away. All geodesics point inwards.

The insides of black holes are not infinitely curved, at least not most of the inside. That might be the singularity, but our understanding of that is definitely incomplete due to a lack of a theory of quantum gravity. And it does seem that nature abhors infinities, so the curvature of spacetime at the singularity is most likely not infinite, but we just don't know what it is.

Mathematically, the universe looks a little bit like a black hole running backwards in time. But only a little bit.

More please.

Ehh, read Steven Hawking. Black holes (singularities) are strikingly similar to what we know of the big bang. If we are in a black hole, it could explain the accelerating expansion of the universe.

my pet science fiction theory is that passing the event horizon removes half a dimension. so outside the universe you can move freely back and forth in time, but here in the universe it is constrained you can only go forward. when you pass another event horizon one of what we call a physical dimension gets constrained. you can't move toward the wall of the event horizon, that dimension is gone.

Dimensions do not disappear after you cross the event horizon. In fact, for very very large black holes, you would most likely notice nothing as you crossed the event horizon (barring there being a huge accretion disk spinning around at a significant fraction of the speed of light and spewing X-rays). That's because it's so far away from the singularity that the gravitational field isn't strong enough yet to meaningfully distort space. But all paths at that point forward lead to the singularity.

exactly there is now not only a direction that you cannot go in, you can't detect it is not there any more. the analogy with time still fits. the only way through time is forward toward the singularity but it will take you forever to get there. the best you can do is stop and that would take an infinite amount of energy.

> there is now not only a direction that you cannot go in, you can't detect it is not there any more

You can still detect that it's there. You just can't go there.

> the only way through time is forward toward the singularity but it will take you forever to get there

That is not true. You will reach it in finite time.

Which book to start with?

In GR, space and time form a single geometry called space-time, and gravity is a curvature of this geometry. Within this geometry, there are 3 spacial dimensions, and 1 time dimension. However, there is no universal axis. In the same way that different observers may disagree on which direction x is, they could also disagree on which direction t is.

In the context of a black hole, we can compare the coordinate system that someone far away would use, with the coordinate system of someone near the black hole. For someone far away, the center of the black hole is just that. A point in 3 dimensional space causing a gravitatational field. However as you aproach the black hole, your natural coordinate system starts to warp relative to a distant observer. Your time axis begins to point more towards the center of the black hole.

At the point you cross the event horizon, your perspective of "future" points entirely in what a distant observer would call a spatial direction: towards the center of the black hole. In particular, the black hole singularity is no longer a point in space, but a point in time. Every observer within the event horizon sees a universe where the future is an infinitly dense singularity that all geodesics will arrive at.

Compare this with our current universe. We see a universe where the past is an infinitely dense singularity that all geodesics originate from.

Black holes have the property that things go from the event horizon to the singularity in a finite amount of proper time. Time reversed, an object shooting out from the singularity will hit the event horizon in a finite proper time. What would happen to us at that point?

All this is not very intuitive to me, but my mind's eye is telling me it would appear as accelerating expansion until you reach the horizon, and reaching the event horizon would feel like being in your own observable universe by yourself.

Like I said, the analogy is stretched at best. Our universe isn't a black hole.

> Mathematically, the universe looks a little bit like a black hole running backwards in time

It really doesn't, not even a little bit.

Schwarzschild has spherical symmetry, Kerr has axisymmetry, neither has homogeneity, and each has isotropy only at a very restrictive set of points in spacetime. These are vacuum solutions of the Einstein Field Equations of General Relativity; comparable metrics that introduce collapsing matter (e.g. Lemaître-Tolman-Bondi in the spherically symmetric case, something like a perturbed LTB or perturbed Oppenheimer-Snyder metric along the lines of Kegeles 1978 https://doi.org/10.1103/PhysRevD.18.1020 in the axisymmetric case; more modern solutions are mostly considered with numerical relativity) have even less isotropy and homogeneity.

Our universe for all practical purposes is homogeneous and looks isotropic from points far outside galaxy clusters, which is most points. There is no universal "centre" around which anything is orbiting. The mathematical description is the expanding Friedmann-Lemaître-Robertson-Walker (FLRW) metric, which is neither axisymmetric nor spherically symmetric, but which is homogeneous and everywhere spatially isotropic.

The Weyl tensor (the tidal part of the Riemann curvature tensor) is very different in the non-vacuum "white hole" solutions from FLRW, with galaxies and galaxy clusters in the former solutions producing a significant redshift-prolation relation for an observer freely-falling away from the white hole. We can also redshift-bucket galaxies and find ourselves (as "Eulerian" observers) collecting sets in planes and annuluses, with a very strongly distinguished set radially through the white hole. It would be attractive to use that set as the cosmological frame, comparable to the attractiveness of the one we use in the standard cosmology.

Time-reversal is interesting. There is a small literature on incoming Vaidya black holes which justifies a tiny handful of papers that treats bulk matter (e.g. galaxy clusters, or the stuff of the Friedmann equations) as the evaporating "shine" on cosmos-originating outgoing Vaiyda white holes. The authors in this area follow some contortions with respect to hiding the central part of such a cosmology (the observable universe must be a truly microscopic wedge in the whole picture in order to hide probably-unphysical observables), and do not appear to have made much progress.

Moreover, this sort of approach approach runs into another problem. "Structure disintegration" at late times (i.e., time reversed structure formation) is tricky, and I think leads you down a path comparable to Einstein-de Sitter 1932, which is now reasonably understood to be a less-than-ideal way of writing down a particular FLRW solution.

Roughly the problem is that you have to evaporate the earliest galaxies (and ultimately the earliest atoms, nuclei, and protons) which is hard to do while preserving the "no drama" condition near a very flat black hole horizon, and even harder to do far from the horizon. Under time-reversed-time-reversal (i.e., back to the white hole spitting out matter in the early universe), how do you recover adiabatic cooling?

We get adiabatic cooling from the metric expansion in FLRW; it's harder to see how BBNS, neutral hydrogen, and the end of the dark ages arise in a white hole solution, at least in the asymptotically flat case. And if you ditch asymptotic flatness for de Sitter (spherical symmetry) or maybe expanding Kasner (axisymmetry), why are you better off than FLRW? Or, back to your point, how does such a universe still look even a little bit like a time-reversed black hole? Accelerated expansion (or accelerated shrinking under time reversal) I think is going to be even harder to lift out of FLRW (with the appropriate value for the cosmological constant) into a cosmology with a central white hole.

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