[2bitencryption]

In my imagination, I always thought we could put a black box around a black hole, and it would be indistinguishable from any other mass - that is, any other mass that can be treated as a point mass.

I.e. put a black hole with solar mass 1 in a black box. Put a star with solar mass 1 in another black box. From a gravitational point of view, you couldn't tell the difference, yes?

But this result implies that the black box with the black hole will gain mass over time, even without adding any mass into the black box? So you could distinguish it from another mass?

Or do I have that wrong? My understanding is as someone who is interested but has no real education on these topics.

[pdonis]

> I always thought we could put a black box around a black hole, and it would be indistinguishable from any other mass - that is, any other mass that can be treated as a point mass.

Yes, that's what the standard theory of black holes says.

> this result implies that the black box with the black hole will gain mass over time, even without adding any mass into the black box?

Sort of. First, it's important to note that the paper is talking about a special type of "black hole", an object that has "vacuum energy" inside it (which means something that acts like a cosmological constant in the Einstein Field Equation)--which isn't a standard black hole (those have zero stress-energy inside). The claim is basically that the total vacuum energy inside such an object can increase as the universe expands.

However, this does not mean that the ordinary "mass" of the black hole would increase. Vacuum energy doesn't work like ordinary mass. The effect that this model is claimed to account for is the accelerated expansion of the universe due to dark energy; basically this model is supposed to provide a mechanism for how dark energy could come into existence as a result of black hole formation (but, again, it's a special kind of "black hole", not the ordinary kind).

[pmontra]

If I understood the paper [1] correctly, the idea is that all black holes don't contain a singularity. They have vacuum energy instead and that leads to the increase of mass and dark energy.

[1] https://iopscience.iop.org/article/10.3847/2041-8213/acb704/...

[pdonis]

> the idea is that all black holes don't contain a singularity

More precisely, theoretically, we can construct models of compact objects that look like standard black holes, but don't have a singularity (and also don't have an event horizon, they only have apparent horizons). Any such compact object must contain "vacuum energy" or something equivalent, i.e., something that looks similar to a cosmological constant in the Einstein Field Equation--that is the only way to evade the conclusions of the various singularity theorems that apply to standard black holes. That type of compact object is what is being hypothesized in the paper under discussion.

[dtgriscom]

> all black holes don't contain a singularity.

Do you mean "not all black holes contain a singularity"?

[kahnclusions]

An important thing to keep in mind is that the singularity is more of a mathematical dead end than a real thing that’s supposed to exist.

The singularity existing in the math suggests that our theories are incomplete, and I would say it’s not surprising that new theories of black holes would do away with the singularity.

[raattgift]

> The singularity existing in the math suggests that our theories are incomplete

No, it's a feature of a mathematically complete model (the Schwarzschild solution) that crops up in extensions (with angular momentum; with electric charge; formed through gravitational collapse rather than eternal) pretty reliably. There is no incompleteness in the Schwarzschild, Kerr, etc. exact solutions. They may not correspond well with things in our universe though, and do not correspond fully to them because our universe (or at least its population of stellar-black-hole-generating galaxies) as far as we can tell is not already infinity years old or full of only vacuum.

(Further efforts which describe somewhat more physically plausible compact objects which grow as matter falls inwards and which are well behaved in deep inter-galaxy-cluster space where expansion is relevant also tend to have singularities if they form by gravitational collapse. Some of these only non-exactly solve the Einstein Field Equations (see the weak <https://en.wikipedia.org/wiki/Non-exact_solutions_in_general...> or the numrel link further below)).

The problem with the singularity is that given a 3d hypervolume (e.g. a set of every point where one would measure an identical average temperature of the cosmic microwave background) which contains all the positions and momenta and other values at every point in the 3d space, one cannot recover the whole set of values from earlier slices, and in particular not the whole set from before the singularity arose.

There was some hope that a collapsed star's singularity would last into the infinite future, or that (since that may not be the case) Hawking radiation would not be thermal noise, so that one could recover all the values of an arbitrary 3d volume after the singularity arose, or at least excise/not-care about the relevant values (see <https://en.wikipedia.org/wiki/Numerical_relativity#Excision> for example). However, now a merely extremely long-lived singularity means that one cannot recover a whole values surface in the far future either.

This causes problems when using the very handy <https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...>.

It is in that sense a model with an evolving black hole is incomplete if it has a singularity. But we know that because General Relativity is a mathematically complete theory, with basically the only open-ended questions living in the mechanisms that generate the stress-energy tensor (i.e. the microscopic behaviour of matter).

The discussion's topic article P.R.s the latest installment in a programme that hopes nature will always generate stress-energy in the interior of a collapsed star in a way that evades the formation of a singularity while (the authors and fellow-travellers hope) preserving the external features of a more standard singularity-containing collapsar. Their model isn't mathematically complete in that they do not have an exact solution to the Einstein Field Equations (§4.6, <https://iopscience.iop.org/article/10.3847/2041-8213/acb704>).

Another mathematically complete theory which may admit non-eternal singularities which frustrate everywhere-determined values is Navier-Stokes. And it's the microscopic behaviour of the fluid matter which may let one recover the missing values.

Mathematical completeness, everywhere-uniquely-determined values, and reasonable physical relevance are three different things.

[rbanffy]

From the article I understood it as “no black holes contain singularities”.

[zwkrt]

The way that I understood it is that what you call an 'ordinary' black hole this paper claims is actually a 'naive' black hole, and that more nuanced solutions to general relativity allow for things that act like black holes that we know but that don't contain singularities.

I only point this out because we haven't been inside a black hole, so we don't really know what an 'ordinary' one looks like.

[rbanffy]

> we haven't been inside a black hole

It’s kind of like that, starting from where we are, black holes have no “inside”, since it takes an infinite amount of time to cross the event horizon.

[lamontcg]

No, if you fall into a black hole, it happens in a finite amount of time to you.

It is the observer at infinity that never sees you fall into the black hole, but real physics is local, you have to use the coordinate system of the person falling into the black hole to determine what happens to them.

[raattgift]

> It is the observer at infinity that never sees you fall into the black hole

We don't even need an observer to be at infinity, thanks to the expansion of the universe. With some future telescope our descendants may observe something on a trajectory to enter a black hole in an early-universe galaxy that is just crossing that observer's (cosmological) horizon.

I think it's relevant to raise this since the article at the top is about embedding black-hole-like collapsed stars in an expanding universe and the research which directly discusses the observable consequences.

> real physics is local

Yes, absolutely. You still get spaghettified if you fly into a black hole which is the only other appreciable mass left in the far far future of our universe. Nobody needs to see your last moments.

> you have to use the coordinate system of the person falling into the black hole to determine what happens to them

No, you can use any coordinates you want (or no coordinates at all), but you have to be aware that there are quantities which are invariant under changes of coordinates (e.g. the curvature scalars) and quantities which are coordinate-dependent, and that some systems of coordinates make the latter difficult or even impossible to calculate.

Indeed the infaller can use any set of coordinates she or he wants. Some time coordinate (wristwatch? distant pulsars?) and spatial spherical coordinates with the infaller always at the spatial orgin, East-North-Up coordinates originating on the (spinning) black hole, etc. are all (pardon the pun) attractive in these circumstances.

Also, defining exactly where "falling in" happens is tricky, even for the infaller. Visser 2014 on horizons: <https://arxiv.org/abs/1407.7295>, second sentence third paragraph of the Introduction section ("These distinctions even make a difference when precisely defining what a "black hole" is -- the usual definition in terms of an event horizon is mathematically clean, leading to many lovely theorems [20], but bears little to no resemblance to anything a physicist could actually measure.")

[rbanffy]

Good point, but what would that observer perceive as they cross the horizon after the end of time?

[raattgift]

First two preliminaries:

The crossing is not at a straightforward conception of "the end of time" in an expanding universe, since most possible observers are carried away from the final fall-in by the expansion of the universe, so there's nobody orbiting "at infinity" who could in principle see the infall take "an infinite time".

Horizons are part of the causal structure of the entire universe, black holes, planets, toads, warts, and all. The horizon is dominated by the central mass and spin, but not fully determined by it. The horizon in a close black hole binary (or triple) gets very complicated. ("The horizon" is not even necessarily physically measurable, and with black hole evaporation might not even exist, although there are other features which can be indicative of the point of no return for an infaller).

Preliminaries done, there is the "no drama" conjecture. Given a large enough black hole in a quiet enough setting a freely-falling infaller will not know she or he has passed the point of no return, perhaps for several minutes according to his or her wristwatch.

That's because the tidal curvature at the point of no return gets very small as we take the mass of a slowly-spinning black hole above millions of stellar masses, and that's the curvature that's relevant in spaghettification, the leading cause of death of astronauts entering isolated black holes.

Of course, most of the black holes we have found are far from isolated (otherwise we probably wouldn't see them with current equipment), so an infaller is likely to be blasted apart by hard X-rays and superhot gas instead of falling straight in.

The observables for something strongly accelerating into a black hole for a faraway orbiting observer can be quite different; unlike for speed there is no maximum acceleration in relativity. One would have to find a limit to acceleration in the behaviour of matter. An astronaut is not going to survive anything like the acceleration needed to make much difference to the distant orbiting observer though.

The distant observer in the not-really-our-universe Schwarzschild model and seeing the infinitely-prolonged final infall is at rest with respect to the central mass. Different observers, e.g. ones shooting themselves into the same black hole, or hovering just above a different black hole, can see qualitatively different things.

Generically, outside observers will see a dimming and shrinking of (practically) any infaller closer to the black hole than the observer. Many such observers will lose sight of the infaller before the infaller has truly hit a point of no return. Consequently some observers could find themselves seeing a presumed-lost astronaut grow brighter and bigger again, and leave the vincinity of the black hole. (Substitute gas, dust, and parts of stars for astronaut in the previous sentence, and that is what the Event Horizon Telescope collaboration, among others, searches for.)

[bmitc]

It is my understanding that, from a gravity-only standpoint, you are right. But I actually thought that black holes slowly evaporate, i.e., lose mass, from emitting Hawking radiation. It isn't clear from the article whether the vacuum energy black holes still have that property.

The article confuses me on something else. It mentions a link between black hole mass and the expansion of the universe, but then it seems to imply that the expansion causes the black holes to gain mass which in turn causes the expansion to accelerate. It doesn't seem to address why the universe is expanding in the first place. But I guess dark energy was proposed as the thing that was doing the expansion acceleration, and not the expansion cause.

[pdonis]

> I actually thought that black holes slowly evaporate

This is believed to be true, but the time scale is something like 60 or more orders of magnitude longer than the age of the universe, so (a) no evidence for this effect exists or is likely to be found any time soon, and (b) it's irrelevant for the dynamics of our current universe anyway.

[justinpombrio]

I thought we had evidence that black holes evaporate in that Earth hasn't been swallowed up yet.

A while back there were concerns (notably not from physicists) about the LHC forming black holes. I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions, but black holes emit more radiation the smaller they are(!), so these tiny black holes evaporate nearly instantly. (Thus the same would happen if the LHC made any.) A tiny black hole that didn't evaporate would be scary because it could grow larger but not smaller.

[andrewflnr]

To be more precise, if LHC was capable of forming black holes, then they would also be regularly formed in the upper atmosphere by cosmic rays... but more likely neither of those is the case. I don't think many serious physicists actually think particle collisions create black holes.

[ISL]

Collisions of sufficient energy could potentially create tiny black holes. GP has the right idea though -- evaporation goes power-law faster the smaller the black hole.

No matter what the mechanism for protection from cosmogenic-collision black holes, if they were problematic, the Sun would have been destroyed long ago through a black-hole creation, black-hole capture, solar-collapse process with cosmic rays much higher in energy than anything humans will ever generate. So, as long as you can look outside and see the sun, you need not ever sweat the particle-collision destroys the world hypothesis, no matter whence the particles are generated.

[pdonis]

> A while back there were concerns (notably not from physicists) about the LHC forming black holes.

There were concerns, but they were not well founded in actual physics.

> I remember the response being that tiny black holes frequently form in the upper atmosphere due to high energy particle collisions

I'm not aware of any such response. The response I'm aware of was that events with higher energy than the LHC is capable of creating happen routinely in cosmic ray collisions, and no black hole formation has ever been observed in such collisions, so black hole formation is not going to happen at the LHC either. That is consistent with our best current theoretical prediction, which is that you would need an accelerator capable of reaching the Planck scale, many orders of magnitude higher energy than the LHC, for black hole production to be possible.

[opportune]

Not really, a black hole is not different orbitally than any other object with mass. You could replace all the stars in the Milky Way with a black hole of the same mass and basically nothing would change for us except not having starlight. Black holes suck everything in the same way that stars and planets do - unless you are literally heading right towards it, if the relative velocity is high, you would just move past each other.

A tiny black hole would not have enough mass to pull in a significant amount of matter and would just pass through the earth if it were coming from space. If the black hole were created on earth it would need a lot more mass than a collider could give it to do anything funky.

[bmitc]

That makes sense. I forgot about the timescales for the evaporation. Thanks!

[ikrenji]

evaporation could be relevant for small blackholes, eg the tiniest ones quickly disappear

[AmericanOP]

The expansion of space-time is an observed property of space. It has always been expanding, but at different rates.

My interpretation of this theory is that spacetime beyond the event horizon is also expanding. This expansion increases vacuum space, which contains vacuum energy.

This either correlates or is coupled with vacuum energy in our observable universe.

[DiogenesKynikos]

The "no-hair theorem" says that black holes only have three properties: mass, angular momentum and electric charge.

If a black hole is perturbed (for example, by merging with another black hole or swallowing a star), it will temporarily be more complicated, but then it quickly goes back to having only above three properties. The extra properties (such as the gravitational quadrupole moment) asymptomatically decay, over a relatively short timespan.

[btilly]

The "no hair theorem" is a theorem of classical general relativity.

Attempts to try to model it with some quantum mechanics thrown in show a tremendous amount of additional state that scales with the surface area of the black hole.

This work suggests even more complications to that picture. That it looks very different from the classical theory.

All of this should come with disclaimers and fudge factors because of our lack of a real theory reconciling GR with QM.

[DiogenesKynikos]

Black holes probably have additional state in a full quantum theory, such as lepton number.

However, to an extremely good approximation, they'll still look like objects with just three properties.

[pmayrgundter]

In this paper they're talking about spinning black holes, the "Kerr" solutions to GR. Iiuc, there's a lot more structure with these type.

See the table on this page for the categories:

https://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric

[btilly]

Current thinking is that black holes have a LOT of state, not just a little. Anything else would result in loss of entropy.

See https://physics.stackexchange.com/a/163046/6796.

[raattgift]

That's only current thinking in SYM/string/D-brane circles, surely? Strominger & Vafa (at the link) is explicitly about N=5 AdS_2 x S^3. Ok, nice that you get unitarity above all in that, but it's not at all clear that the picture corresponds with our universe. I do not see how it could possibly correspond with the universe of the linked article.

Croker, Weiner et al. (the authors of the topic paper) are keen first and foremost on their spinning black hole interior solution, which is wholly classical and found at <https://arxiv.org/abs/2107.06643>. In the more recent topic paper they argue that they can make the exterior solution well-behaved too, following the path McVittie paved in 1933 in embedding massive objects in an expanding classical spacetime.

They don't come to the end of the path though. As they say in <https://iopscience.iop.org/article/10.3847/2041-8213/acb704> §4.6, their desired combination of interior solution, initial formation, infall/merger, arbitrary angular momentum, and being easy to embed in an expanding Robertson-Walker universe is far from complete (There are "known exact solutions with each [property] ... there is no known solution that possesses all [of them]"), they're just hoping to find one.

It is not at all clear to me that they have a strong idea about no-hair in their compact objects' causal structure. (I guess totally wildly that it will sensitively depend on the details of the embedding. See Visser 2014 <https://arxiv.org/abs/1407.7295>).

Also, it strikes me that their entire idea is to avoid strong gravity in the interior of collapsed stars and in particular avoid the singularity, so one should really think of this as an anti-quantum-gravity approach to black holes, or at least an approach that might evade perturbative non-renormalizability.

No extra dimensions, no boundaries, nothing special in the stress-energy tensor, mute on the subject of entropy (which in any case should be thought about in comparison with the huuuuuuuge entropy from the expanding space. Expansion is after all the focus of the topic paper, and so it's rather distant from anti-de Sitter ideas).

[btilly]

This is officially above my paygrade.

I just know the basic argument that if black holes are simple, then going from a complex thermodynamic arrangement without a black hole to one with a black hole would represent a spontaneous reduction in entropy. And therefore theories where black holes have a lot of entropy are of interest. While this was originally an argument for string theory, it can be used to argue for other theories as well.

I can't opine on your wild guess that how much hair their model of a not-quite black hole is depends on the embedding. But if it is true, I would expect that embeddings that give it a lot of hair are going to be of more interest than the ones that give it no hair exactly for the thermodynamic reason that I gave.

[smath]

You mention point mass. Yes, the volume also matters. If your second black box contains the same mass but over a bigger volume, then the spacetime curvature it will cause will be less extreme than the black hole in the first box. The book I most like on this topic is Kip Thorne's Black Holes and Time Warps. IMO Thorne is a better explainer than Hawking.

[andrewflnr]

I'm pretty sure by the time you're outside the box, assuming it's the same size for both, you can't tell anymore. I'm quite confident this is the case for classical gravity and a spherically symmetric "box", and I don't think tides or relativistic corrections are noticeably different far away from the horizon. (Yeah, you'll feel the black hole's tides, but stars have tides too.)

[aqfamnzc]

I'm not a relativity expert, but couldn't you tell the difference between a point mass (or just significantly smaller volume) and a star in this black box scenario? At a great distance, they would appear the same gravitationally, but as you get closer, the star would appear less massive. Since more mass would be pulling on you at an angle, rather than directly toward the center.

[andrewflnr]

Definitely not for classical gravity. https://en.wikipedia.org/wiki/Shell_theorem Point 1 there says that the physical extent of the mass doesn't matter for a spherically symmetric object, to which a star is close enough.

[JumpCrisscross]

> more mass would be pulling on you at an angle, rather than directly toward the center

Is this true? I don’t think a faraway object accelerates faster than a near one.

[ianred]

Are we talking about the volume of the event horizon? If I understood it correctly, the total of the mass of a black hole is in its singularity. The volume of the event horizon will depend on the total mass of the black hole.

[smath]

Oh I just mean when comparing (A) block hole in a black box, vs (B) a non-black-hole start of the same mass, B will likely be over a large volume, and hence will produce different spacetime curvature.

[fsakura]

Why do you think the black box with the black hole will gain mass over time?

AFAIK:

On the contrary it will lose mass over time due to Hawking Radiation and evaporate eventually (though that might take literally forever).

Also spacetime curvature will be slightly different for point mass vs distributed mass.

[andrewflnr]

Because of the ideas in the article?