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Dark energy from supermassive black holes? Physicists spar over radical idea

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Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#91

Post by raattgift »

codethief wrote: post #68
rolenthedeep wrote: post #44

Here's a question I'm not even remotely qualified to ask: could it be that supermassive black holes cause a contraction of local spacetime in a way that gives the impression that local space is static while distant space expands? I think this mechanism would be largely indistinguishable from classic hubble expansion, at least on local scales.

That's a good question and is not exactly easy to answer. The Einstein field equations of General Relativity are highly non-linear which makes it difficult to talk about superpositions of solutions to the equations and to compare local physics (at the level of black holes) to cosmological effects (e.g. the expansion of the universe) and transfer results and insights between them. Let me explain. When we talk about co…

Preamble: we are largely on the same page. I thought I'd draw your (or our mutual readers') attention to a couple details where we may differ a little.

> [link to wikipedia's de Sitter-Schwarzschild page]

Let's call it Schwarzschild-de Sitter (SdS, for short, and for ease of literature-searching).

More generally there is the McVittie family of metrics of a massive object in a dynamical spacetime. SdS is the limiting case of McVittie where the spacetime is stationary and the central mass is compact, spherically symmetric, 0-angular-momentum, and uncharged. (One could alternatively say that McVittie is a generalized time-dependent SdS.)

> black hole solutions assume a perfect vacuum

Not quite, but this is mainly a specialist quibble, since the most widely known theoretical black holes are vacuum or electrovac spacetimes. See for example the Kerr-Vaidya black hole solutions, which have either a incoming radiation ("null dust") field or an outgoing one (or both) falling onto resp. shining out of ("roughly Hawking") a spinning black hole. There are many other nonvacuum solutions with a compact central mass (which can look more or less black-hole-like), both exact and non-exact.

The important feature of these is asymptotic behaviour, as you touch on in your second-last paragraph, since if the influence of the central mass fades with distance, and the sources are kept distant from each other, that lets us ignore (but see below) the difficulties in combining two or more exact solutions of the Einstein Field Equations into a new exact solution.

Linearized gravity is usually applicable and sufficient, and if not one can obtain corrections using post-Newtonian theory. We don't really need numrel unless mass-ratios are small and compactness is extreme. See the handy diagram at https://en.wikipedia.org/wiki/Post-Newtonian_expansion#/medi...>.

See also Ellis 2010 (Chapter 2, section 3 on inexact solutions, notably his complaint at the bottom of p. 34 to the top of p. 35) https://doi.org/10.1017/CBO9780511622724.002>, which is handily also at s c y h o b. Re his complaint see also Visser 2014 on horizons: https://arxiv.org/abs/1407.7295>.

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#92

Post by inspector-g »

This reminds me of a question I’ve had for a while, and have not been able to formulate it well enough to do any searching for an answer on my own (total layman here).

Do we have any hypotheses of what the matter inside a black hole might be like? I mean “hypotheses” in a strict sense; I do not mean “a good/definitive answer”

To oversimplify a bit: white dwarves are supported by electron degeneracy pressure, and if such pressure is exceeded by gravity then e.g. a neutron star can form. Neutron stars are supported by neutron degeneracy pressure, and if such pressure is exceeded by gravity then a black hole can form.

For each of those “steps” we have an idea of what the matter within must be like to support/exceed such pressures. Are there any hypotheses for what the matter might be like after such known pressures are exceeded and a black hole is formed?

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#93

Post by consilient »

denton-scratch wrote: post #66
empyrrhicist wrote: post #39

Earlier quoted context omitted.

I mean, we already know about existing "dark" matter particles. The Neutrino comes to mind, though it's not massive enough to explain the gravitational phenomena. LCDM really isn't that weird or unexpected, since it's a lot like what we already observe, and we already think more particles should exist.

Hypothetical "dark matter" doesn't interact with ordinary matter, except gravitationally. Neutrinos do interact with ordinary matter; otherwise we wouldn't be able to build neutrino detectors. Therefore neutrinos are not an example of dark matter.

> "dark matter" doesn't interact with ordinary matter, except gravitationally.

It's possible that this is the case, but particles that also interact with the weak force (hence WIMPs: weakly interacting massive particles) are generally considered better candidates.

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#94

Post by wcoenen »

inspector-g wrote: post #92

This reminds me of a question I’ve had for a while, and have not been able to formulate it well enough to do any searching for an answer on my own (total layman here). Do we have any hypotheses of what the matter inside a black hole might be like? I mean “hypotheses” in a strict sense; I do not mean “a good/definitive answer” To oversimplify a bit: white dwarves are supported by electron degeneracy pressure, and if s…

Degeneracy pressure comes from the Pauli exclusion principle.

There is no limit to how high this pressure can go, as there are always higher energy states to occupy. (Except that for electrons at a high enough energy state, they will disappear by fusing with protons.)

For neutrons, there is no such disappearing effect, so the degeneracy pressure can keep on increasing. So how can this pressure ever be "overcome by gravity"?

Apparently at some point the degeneracy pressure itself starts to contribute significantly to spacetime curvature. This increases gravitational pressure, which compresses the neutrons, which increases the degeneracy pressure, which increases curvature, etc. No balance is possible anymore and collapse happens.

Presumably this process of ever increasing neutron degeneracy pressure continues within the black hole until unknown physics come into play.

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#95

Post by codethief »

pdonis wrote: post #86
codethief wrote: post #77

Earlier quoted context omitted.

> In the standard black hole model, the collapsing matter that formed the hole hits the singularity and is destroyed Sure, but that's a model for which we don't have any observational evidence whatsoever since it's the black hole's interior. That's why I phrased it that way. It doesn't matter, though, even if you say the matter inside the black hole no longer exists (and I'm very happy to entertain that thought): Any…

> that's a model for which we don't have any observational evidence whatsoever since it's the black hole's interior While this is true, it's not a very strong statement. Physicists extend models all the time into domains where we can't directly test them. The singularity theorems of GR guarantee that the key features of that model, the singularity and the collapsing matter getting destroyed in it, must be true as lon…

> While this is true, it's not a very strong statement. Physicists extend models all the time into domains where we can't directly test them.

Sure but it's not every day that we extend models into a domain (near the singularity) where we know our model must eventually break down somehow. So I think you'll see why I'm a bit sceptical about your claim that

> the collapsing matter that formed the hole hits the singularity and is destroyed

I don't even know what "destroyed" is supposed to mean. Is the claim that the matter simply disappears from the spacetime, with all its conserved quantum numbers and other conserved quantities?

As for:

> The singularity theorems of GR guarantee that the key features of that model, the singularity and the collapsing matter getting destroyed in it, must be true as long as the collapsing matter has the equation of state of ordinary matter.

Do they? The Singularity Theorems assume the existence of a trapped surface. Last I checked[0] (I didn't check deeply, though) the mathematical results on when trapped surfaces arise are very few and far between. Is there a clear proof that in realistic models of black hole formation we must have a trapped surface eventually?

[0]: This was in 2020 when the Nobel committee dubiously claimed that Penrose's Singularity Theorems prove that black hole formation and singularities "are a robust prediction of the general theory of relativity".

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#96

Post by codethief »

raattgift wrote: post #91
codethief wrote: post #68

Earlier quoted context omitted.

That's a good question and is not exactly easy to answer. The Einstein field equations of General Relativity are highly non-linear which makes it difficult to talk about superpositions of solutions to the equations and to compare local physics (at the level of black holes) to cosmological effects (e.g. the expansion of the universe) and transfer results and insights between them. Let me explain. When we talk about co…

Preamble: we are largely on the same page. I thought I'd draw your (or our mutual readers') attention to a couple details where we may differ a little. > [link to wikipedia's de Sitter-Schwarzschild page] Let's call it Schwarzschild-de Sitter (SdS, for short, and for ease of literature-searching). More generally there is the McVittie family of metrics of a massive object in a dynamical spacetime. SdS is the limiting…

Thanks so much for the references!

> > black hole solutions assume a perfect vacuum

> Not quite

You're right, I should have been more precise here. I was merely trying to say: The spacetimes in the vicinity of celestial bodies can be modeled sufficiently well by vacuum black hole solutions (obviously as long as we don't get too close and don't need to consider accretion disks, radiation and what not).

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#97

Post by andrei_says_ »

pleb_nz wrote: post #73
andrei_says_ wrote: post #69

What is going on with titles? Do physicists really put on boxing gloves hoping that physical violence will resolve a scientific argument? Is this a new widely accepted approach? I find this level of editorializing ridiculous to the point of insulting. I don’t need the imagery of violence to boost my curiosity about a new discovery.

Spar can also be used to simply mean 'to argue'. Sail boats can also have a spar. It's not soley a boxing term. I think the title of fine.

Thank you. Had Never seen it in context outside of physical fighting.

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#98

Post by pdonis »

codethief wrote: post #95
pdonis wrote: post #86

Earlier quoted context omitted.

> that's a model for which we don't have any observational evidence whatsoever since it's the black hole's interior While this is true, it's not a very strong statement. Physicists extend models all the time into domains where we can't directly test them. The singularity theorems of GR guarantee that the key features of that model, the singularity and the collapsing matter getting destroyed in it, must be true as lon…

> While this is true, it's not a very strong statement. Physicists extend models all the time into domains where we can't directly test them. Sure but it's not every day that we extend models into a domain (near the singularity) where we know our model must eventually break down somehow . So I think you'll see why I'm a bit sceptical about your claim that > the collapsing matter that formed the hole hits the singular…

> it's not every day that we extend models into a domain (near the singularity) where we know our model must eventually break down somehow

Yes, but that doesn't change what the model says. It just affects how likely we think it is that the model is actually realized in our universe. I agree that it's quite likely that the standard black hole model I described isn't realized in our actual universe. But we can still use it if we don't have any better model to replace it with, since even if it breaks down near the singularity, that still leaves the whole rest of the model with plenty of usefulness.

What's of great interest about alternate models for collapsed objects that have dark energy inside, like the Bardeen "black hole", is that they do hold out the promise of being a better model to replace the standard black hole model, that doesn't have a singularity anywhere and so would not be expected to break down the way we think the standard black hole model breaks down near the singularity.

> Is the claim that the matter simply disappears from the spacetime, with all its conserved quantum numbers and other conserved quantities?

The matter disappears, but conserved quantities do not. They remain embedded in the spacetime geometry that is left behind.

> The Singularity Theorems assume the existence of a trapped surface.

Yes, but the alternate "black hole" models with dark energy inside, such as the Bardeen "black hole", also have trapped surfaces, so this doesn't help to distinguish the models.

The singularity theorems also assume energy conditions. Those are the conditions that the models with dark energy inside violate, and which allow those models to not have singularities even though they do have trapped surfaces.

> Is there a clear proof that in realistic models of black hole formation we must have a trapped surface eventually?

I don't know about "proof", but there are plenty of numerical simulations of realistic collapses of massive objects like stars that show trapped surfaces forming. So I would say it's a robust expectation of any such collapse process, even if we don't have an ironclad proof that it must occur in every single case.

> the Nobel committee dubiously claimed that Penrose's Singularity Theorems prove that black hole formation and singularities "are a robust prediction of the general theory of relativity".

That statement was justified. But saying that it's a robust prediction if particular conditions are satisfied is not the same as saying that all of those conditions must be satisfied in our actual universe.

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#99

Post by raattgift »

codethief wrote: post #96
raattgift wrote: post #91

Earlier quoted context omitted.

Preamble: we are largely on the same page. I thought I'd draw your (or our mutual readers') attention to a couple details where we may differ a little. > [link to wikipedia's de Sitter-Schwarzschild page] Let's call it Schwarzschild-de Sitter (SdS, for short, and for ease of literature-searching). More generally there is the McVittie family of metrics of a massive object in a dynamical spacetime. SdS is the limiting…

Thanks so much for the references! > > black hole solutions assume a perfect vacuum > Not quite You're right, I should have been more precise here. I was merely trying to say: The spacetimes in the vicinity of celestial bodies can be modeled sufficiently well by vacuum black hole solutions (obviously as long as we don't get too close and don't need to consider accretion disks, radiation and what not).

... or actual surfaces, which we are allowed in Kerr (or more properly perturbations thereof) or Hartle-Thorne spacetimes where the central body is non-compact, for example.

Where modelling an astrophysical body this way tends to fall down is that as far as we can tell the central mass evolves and is in general not uniform in the sense that it has long-lasting multipole moments (that would quickly bald away for a black hole), frustrating the hopes of matching the object's interior with the exterior solution. In comparison, outer space is practically always empty enough of stress-energy that the non-physicality of the vacuum or lambdavac region is rarely the issue (and can be dealt with perturbatively, for example).

The other major problem is that binary (and triple) systems are surprisingly commonplace, and the N-body problem will drive one towards approximations of GR for tractability. This is the essence of this thread's cautions on the difficulties in superposing solutions to the EFEs. Worse, the two problems above can feed into one another, like when binary stars raise long-lived bumps on each other.

This reminds me to re-read the invigorating https://link.springer.com/article/10.1007/s00190-016-0927-4> which compares the gravitation of Earth and its messy multibody neighbourhood with several exact vacuum solutions and a couple of formalisms. The first author is the redoubtable Michael Soffel https://www.iau.org/administration/membership/individual/733...> A PDF of the paper can also be found at s c y h o b. The kicker is the two paragraphs before section 3.

Re: Dark energy from supermassive black holes? Physicists spar over radical idea

#100

Post by mr_mitm »

jdmichal wrote: post #84
cowl wrote: post #23

Earlier quoted context omitted.

Well the whole idea is to "make it work" under the constrains of our current understanding (General Relativity). After all , all that Dark Energy is by definition is something that is Contrasting the Gravity as described by general Relativity. If we remove that fact than it doesn't need to be reconciled with anything really. Saying that it is a type of energy that doesn't behave according to general Relativity, you n…

Are there MOND theories that attempt to account for dark energy too? Or only dark matter?

Not that I'm aware, but there are modified GR theories. In classical GR, the equations can be derived by applying the principle of least action with the Ricci scalar R being the Langrangian. People are studying what happens if you assume the Lagrangian is a function of R, like R^n or so. It doesn't have the same stigma as MoND, but it's also not that popular. It's also very hard to test for.

https://en.wikipedia.org/wiki/F(R)_gravity

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