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.
> 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).
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.
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.
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.
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.
I don't remember this quote exactly, or whom to attribute it to but it goes something along the lines of "throwing two unsolved problems at each other doesn't create a solution".
A lot of crazy sounding physics theories involve some non-trivial tweak to the laws of physics as we know them (e.g. the Alcubierre drive, many exotic dark matter theories, ...) so I usually dismiss them as unlikely. However, from what I have read about this so far, it seems to a natural consequence of black hole solutions in an expanding spacetime, no new physics needed. Am I missing something? If that's really the case, the odds of this being real are much higher than most typical hyped up physics fare. Would love an expert take.
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 :)
She's generally good but in addition to the clickbait she also has a habit of "it's so easy, it's just [controversial pet theory] and the rest of science disagrees because they are wrong".
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
> In one such model, black holes do not contain a singularity, but are instead filled with vacuum energy. These vacuum-energy black holes are intriguing because their growth is coupled to the expansion of the universe: as the universe expands, these black holes gain mass.
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.
Couldn't the proposed variations in spacetime expansion in relative proximity to a SMBH change the curvature of galaxies and perhaps account for the discrepancy between rotational velocities [1] observed vs. expected?
I didn't get a sense of how near or far from the black hole that the expansion variations were present.
isn’t dark energy just a term for something we think must be there yet we can’t find it? go too many layers deep on speculation and it starts to feel meaningless and self-referential
I believe dark energy is the name for something that would be necessary to reconcile the model of an expanding universe with the amount of mass-energy that we can observe (even accounting for missing sources of gravitation, aka “dark matter”).
You would think that the gravitational attraction of all the matter in the universe would slow down its expansion, at least a little. However we observe that the rate of expansion of the universe seems to be accelerating over time, and it seems like something must be causing that. The cause is called dark energy, but you’re right, we don’t really know what it is.
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.
There are lots of plausible explanations for why some of our observations don't match GR predictions. It's been a long time since I studied this stuff so I'm pretty sure I'm way out of date, but dark energy still seems like epicycles to me and I'm not sure it's the Occam's razor explanation either. No, we don't want to abandon GR, but we know it's not the complete theory, and a lot of this stuff might make perfect sense if we have a theory of quantum gravity.
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.
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.
2bitencryption|3 years ago
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|3 years ago
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).
bmitc|3 years ago
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.
AmericanOP|3 years ago
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.
unknown|3 years ago
[deleted]
DiogenesKynikos|3 years ago
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.
smath|3 years ago
fsakura|3 years ago
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.
chickenimprint|3 years ago
phyzome|3 years ago
« 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?
asplake|3 years ago
test6554|3 years ago
froeb|3 years ago
T-A|3 years ago
Unfortunately, yes. The black holes in question are not the textbook ones, they need to be full of something which acts like dark energy.
2bitencryption|3 years ago
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 :)
ketralnis|3 years ago
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
pfdietz|3 years ago
https://twitter.com/skdh/status/1626113544339980291
AmericanOP|3 years ago
https://youtube.com/@HistoryoftheUniverse
habibur|3 years ago
- 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.
bArray|3 years ago
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.
TrispusAttucks|3 years ago
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...
luxuryballs|3 years ago
progrus|3 years ago
simonh|3 years ago
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.
garbagecoder|3 years ago
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.
thricegr8|3 years ago
-------- 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.
pmayrgundter|3 years ago
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.
jiggawatts|3 years ago
ericbarrett|3 years ago
This could be Nobel Prize research if it holds up in the coming years.
vl|3 years ago
Does it expand faster next to heavy object or slower?
unknown|3 years ago
[deleted]