[HN Gopher] Black holes as the source of dark energy
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Black holes as the source of dark energy
Author : tristanz
Score : 45 points
Date : 2023-02-17 19:23 UTC (3 hours ago)
(HTM) web link (aasnova.org)
(TXT) w3m dump (aasnova.org)
| chickenimprint wrote:
| 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".
| asplake wrote:
| Now you have three problems
| pfdietz wrote:
| "I've combined black holes and dark energy. Oh great, now I
| have regular expressions."
| hanselot wrote:
| But can you use it to play golf?
| garbagecoder wrote:
| 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.
| thricegr8 wrote:
| Cross-posting /r/bestof
| (https://www.reddit.com/r/worldnews/comments/113casc/scientis...)
| because it really does a nice job with history and breaking down
| the highlights (to me at least:)
|
| -------- 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.
| jiggawatts wrote:
| This is by far the best summary I've seen anywhere!
| 2bitencryption wrote:
| For those with an interest in this topic, but a disdain for
| "science news", I recommend checking out Sabine Hossenfelder's
| youtube channel:
|
| 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 :)
| AmericanOP wrote:
| Similarly I recommend the 'History of the Universe' channel for
| an understanding of cosmology:
|
| https://youtube.com/@HistoryoftheUniverse
| ketralnis wrote:
| 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
| pfdietz wrote:
| She already panned it on twitter.
|
| https://twitter.com/skdh/status/1626113544339980291
| 2bitencryption wrote:
| 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.
| AmericanOP wrote:
| 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.
| bmitc wrote:
| 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 wrote:
| _> 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.
| bmitc wrote:
| That makes sense. I forgot about the timescales for the
| evaporation. Thanks!
| ikrenji wrote:
| evaporation could be relevant for small blackholes, eg the
| tiniest ones quickly disappear
| pdonis wrote:
| _> 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 wrote:
| 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/acb7
| 04/...
| pdonis wrote:
| _> 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.
| DiogenesKynikos wrote:
| 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 wrote:
| 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.
| smath wrote:
| 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.
| ianred wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| Because of the ideas in the article?
| wahern wrote:
| > 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?
|
| It was my extremely uneducated understanding that vacuum energy
| is increasing _everywhere_ , at least according to common
| models (like Lambda-CDM?), as a consequence of cosmic
| inflation. Presumably the model of a blackhole as containing a
| singularity meant it wasn't subject to inflation and thus
| increasing vacuum energy, but in this paper they argue that the
| data comports with vacuum energy increasing within blackholes.
| (And I'm just going to guess that the geometry of blackholes
| might imply more vacuum energy, and more generally that vacuum
| energy density is related to local spacetime geometry.)
|
| I wouldn't get too hung up on what "mass" means here. There are
| many different meanings and models behind that word (e.g. rest
| mass vs relativistic mass), and in some definitions the meaning
| of mass-energy equivalency gets very complex, making it
| difficult to navigate the precise implications without
| developing a better understanding of the math and the models.
| Vacuum energy is a perfect example of this.
|
| Take all of the above with a grain of salt. I never took
| physics past a special university-accredited high school class
| in relativity, and that was before I appreciated the value of
| paying attention in school. And the most memorable thing from
| that class, beyond calculating Lorentz Transformations, was
| that we used the university's NNTP server for class discussion
| and assignments.
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