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.
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).
[1] https://iopscience.iop.org/article/10.3847/2041-8213/acb704/...
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.
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.
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.
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.
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.
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.
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.
« However, this common black hole model is in tension with the overall expansion of the universe »
...why would those have anything to do with each other, a priori?
Unfortunately, yes. The black holes in question are not the textbook ones, they need to be full of something which acts like dark energy.
https://www.youtube.com/@SabineHossenfelder
She's a theoretical physicist, and covers topics such as this from the point of view of a real expert, and doesn't "talk down" to the audience at all. (Though I must say, she does engage in clickbait-style video titles and thumbnails, but the video content is much better than that implies)
I guarantee she will have something to say about this topic in her next video :)
I find that most physicists in educational roles shy away from interjecting with opinions and even mild speculation more than they should but she tends to overcorrect in the other direction. This tends to attract a particular type of fan as well, the kind that likes to feel like they're in on the secret knowledge and loudly have opinions about things they don't truly understand
That's probably the best description of her style of argument I've read, and it's honestly why I can't stand reading/watching her anymore. Much too much of it comes off to me as "What the physics establishment doesn't want you to know!!" type of stuff, so it then attracts the "yeah, she's speaking truth to power, I knew it was a conspiracy!" crowd.
I mean, there are plenty of folks that have expressed a ton of skepticism about string theory without the semi-conspiratorial angle.
- as the universe expands black holes gain mass because of the expansion.
- as black holes gain mass, they create more repulsive force.
- that repulsive force expands the universe even more at an accelerated rate.
I would think about this differently. What if the actual space inside these black holes does not stretch as the Universe itself does? The density relative to that of the Universe would increase, but they just maintained the density they originally had.
Thinking about a piece of fabric stretched with a mass on, as you drag the corners out, the curve (gravity) on the fabric (space) caused by the mass increases. If you locked the mass depth, it would look like it has gotten heavier.
I didn't get a sense of how near or far from the black hole that the expansion variations were present.
https://www.universetoday.com/wp-content/uploads/2011/12/Rot...
It’s analogous to the situation with dark matter. Again we hypothesise some additional factor that causes behaviour we wouldn't otherwise expect. We attach the terms energy and matter to them, because those are the things that generally cause the kind of effects we observe, but they’re placeholder terms and it’s always possible the observations or our theories are incorrect or incomplete.
I'm fine with wild speculations like there are new universes inside blackholes that look like white holes to the people inside (or "Big Bangs") and I love to think about all of it, and higher dimensions, and all of that, but I'm just a nerd. I'm not putting it in a journal or a press release or trying to justify funding on the basis of it.
-------- Reading the paper, this is the best summary I can make. Note that I'm an engineer, not an astrophysicist.
The basic thought is that in 1963, a guy named Kerr seems to have come up with the best approximation of black holes. Many observations have been made of various black holes, and they seem to line up with his proposals. The issue is that this solution has a nasty singularity in it, which is very very extreme and doesn't really "match" the rest of nature. However, it's the only plausible explanation for the behavior seen in black holes.
People have been trying to solve this for ages. A bunch of people have different ideas for how we can resolve the singularity issue - maybe the event horizon is moving with the universe's expansion, or something funky happens to physics at high density (like how quantum mechanics gets weirder as you get smaller), or maybe the mass is somehow moved forward/backward in time and this merely appears to be a singularity from our vantage point.
However, all these are flawed because they don't take into account the fact that black holes are spinning. When you make the black hole spin, these theories all fail in one way or the other - they give the wrong results in short timescales, or they give the wrong results in long timescales.
In 2019, 2 guys named Kevin Croker and Joel Weiner demonstrated that the universe's expansion rate varies based how heavy the space next to it is. (That is a link to a summary of the paper.) This 2019 paper basically solved some questions about Einstein's equations, and importantly it also possibly answers some of the questions around singularities - even spinning ones. However, it didn't delve too deep into those questions, saying they should have a follow-up study.
This new paper is the follow-up study of that paper. It basically holds that "yes, that theory does solve the issue of singularities." They go on to say that the stress that a black hole puts on an object (its gravitational pull) can vary based on how quickly the space near the black hole is expanding.
Because the space near the black hole is expanding at different rates relative to seemingly "minor" (on the scale of the black hole) sizes, you get fluctuations to the gravitational pull that appear to be shifted through time. The paper's authors liken this to how redshift works with light; further away objects are more red than closer objects just because the light's wavelength increases with distance. The difference is that the change in gravitational pull is shifted based on time instead of distance (remembering that time is intrinsically linked to space and that we already know black holes distort time).
The paper claims that the necessary outcome of this is that you now have a physical object ("relativistic material" in science words) that must be intrinsically linked to the universe's expansion rate - as the expansion rate changes, that material also changes (or perhaps vice versa). They call this a "cosmological coupling" between everyday physics and the universe's expansion rate.
You can use the strength of this coupling (i.e. how intensely some mass is tied to the universe expansion rate) and plug it into the old 1963 Kerr equations and suddenly they work without needing weird singularities. You still get a singularity at 0 (i.e. no relation between universe expansion rate and mass), but since we know that there is a link we know that it should always be > 0 (i.e. no singularity).
They predict that for black holes you can expect that number to be about equal to 3, give or take, and such a result lines up with the 2019 paper.
Now that they have an idea of a mechanism, they can use the scientific method to see if they can experimentally replicate their hypothesis. There should be a detectable difference between the "classic" singularity approach and a "not a singularity but pretty close" approach, and they are trying to detect this by looking at how black holes gain mass.
Specifically, they're looking at supermassive black holes which seem to grow in mass as they age, even though there shouldn't be a link between time and black hole mass. Because these old galaxies are "dead", the black holes have no way to gain mass by "eating" the stuff around them, and so science currently doesn't know why these black holes appear to be growing with time - they must be growing because of some other mechanism.
The paper goes on to say they're going to do an experiment to see if that "cosmological coupling" factor actually ties in to the size of the black hole, and if the expansion of spacetime local to the black hole may explain why the black hole appears to be gaining mass when it shouldn't.
They do some experiments, blah blah blah, traditionally if there was no link between expansion and ages they "should" get the number 0 according to the 1963 model. Instead they got a value of about 3, consistently, no matter how bad the redshift was. There's a graph, it's probably closer to 2.96 than 3.14 so don't get your hopes up for some weird cosmological coincidence. They can say with 99.98% confidence that the number is not 0 like the 1963 model assumes.
They go on and say this validates their hypothesis, that a singularity explanation is not needed, and that black holes will always grow at a constant rate of about 3, using the equation a3.
They say this means black holes are made of "vacuum energy" and because of the law of conservation of energy black holes cause spacetime to dilute at a-3 , meaning this constant growth rate is causing the universe to expand (or maybe vice versa - but they appear to be related).
They do more math to prove this also holds with everything we know about universe expansion so far and that the rate of universe expansion matches what we should expect with the number of black holes we think there are.
They are careful to say this doesn't prove anything, it just demonstrates a probable link with high confidence. They give examples of further experiments that could potentially disprove their theory:
Checking the cosmic microwave background radiation to see if the numbers still line up
Checking to see if black holes reduce the energy of gamma ray bursts by an amount predicted by their theory
Checking that when two supermassive black holes collide, the result appears to gain more mass than what traditional science would expect (but would be in line with this theory, i.e. a factor of 3)
Stare at a pulsar orbiting a black hole for a decade or so and see if you can see the pulsar's orbit change according to their theory
Their theory implies that there are more massive black holes than what we observe, so someone should check to see if there's a reason why black holes aren't getting as big as this theory suggests (is there some constraint that blocks black holes from growing?)
They don't have the exact formula, only that an exact formula should exist. Someone should work it out. There is a competing theory that solves issues with quantum mechanics that may not line up with this theory; someone should check
Take more measurements and replicate this experiment to verify the numbers are correct with a larger sample size
Check quasars with a redshift of 6 and see if the math still maths
And then they say thank you and do more math. Again, I'm not an expert here so maybe I misunderstood some things, but hopefully that makes things easier to understand. It seems like the 2019 study was more impactful, and this mostly affirms the 2019 study.
https://bigthink.com/starts-with-a-bang/black-holes-dark-ene...
I think the requirement for coupling from spin is due to frame-dragging, where there's basically a shear force to accommodate conservation of momentum on the stress-energy tensor.. or smth, a lot over my head. This coupling in theory goes out to infinity, but wouldn't be significant for small gravitational objects. Very large BHs spinning near lightspeed would couple very far.
From there, the intuition that helps me went like "imagine a rubber band, being pulled apart. Then imagine a pinched point somewhere along its length. As you continue to pull the whole thing, the interior pressure on the pinched part will increase." That's why the interior energy of the BH increases in connexion with the expanding space.
This could be Nobel Prize research if it holds up in the coming years.
