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Black holes as the source of dark energy

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Re: Black holes as the source of dark energy

#61

Post by simonh »

luxuryballs wrote: post #51

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

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.

Re: Black holes as the source of dark energy

#63

Post by Beldin »

hn_throwaway_99 wrote: post #38
ketralnis wrote: post #25

Earlier quoted context omitted.

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…

> "it's so easy, it's just [controversial pet theory] and the rest of science disagrees because they are wrong". 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…

Hear, hear!

I appreciate someone challenging commonly held beliefs - science needs that. But I've grown accustomed to expecting her popular science output to present her point of view as the only valid one, and her the one sane person in physics. It has gone far beyond adding much-needed nuance, and has basically become contrarian-for-contrarian's-sake.

Which is a pity, because challenge to existing beliefs is valuable. But when someone mentions that she has a take on an exciting new physics result, I find myself able to predict the general direction (negative) and the tone (derisive) with high enough accuracy not to have to read it anymore.

Re: Black holes as the source of dark energy

#64

Post by T-A »

phyzome wrote: post #53
chickenimprint wrote: post #2

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".

In particular, this was a very weird sentence: « 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?

Because the "common black hole model" is static, while the standard cosmological model is not. How do you stitch them together without breaking either one, i.e. without violating general relativity?

Re: Black holes as the source of dark energy

#65

Post by T-A »

froeb wrote: post #39

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…

> no new physics needed. Am I missing something?

Unfortunately, yes. The black holes in question are not the textbook ones, they need to be full of something which acts like dark energy.

Re: Black holes as the source of dark energy

#66

Post by JumpCrisscross »

aqfamnzc wrote: post #57
andrewflnr wrote: post #47

Earlier quoted context omitted.

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.)

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.

> 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.

Re: Black holes as the source of dark energy

#67

Post by kahnclusions »

dtgriscom wrote: post #48
pmontra wrote: post #20

Earlier quoted context omitted.

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/...

> all black holes don't contain a singularity. Do you mean "not all black holes contain a singularity"?

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.

Re: Black holes as the source of dark energy

#68

Post by andrewflnr »

aqfamnzc wrote: post #57
andrewflnr wrote: post #47

Earlier quoted context omitted.

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.)

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.

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.

Re: Black holes as the source of dark energy

#69

Post by lamontcg »

rbanffy wrote: post #59
zwkrt wrote: post #40

Earlier quoted context omitted.

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.

> 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.

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.

Re: Black holes as the source of dark energy

#70

Post by froeb »

T-A wrote: post #65
froeb wrote: post #39

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…

> no new physics needed. Am I missing something? Unfortunately, yes. The black holes in question are not the textbook ones, they need to be full of something which acts like dark energy.

The textbook black hole is unphysical though, it assumes spacetime is asymptotically flat. As far as I can tell, the paper's claim is that if you do black holes more realistically, including better boundary conditions, then you can find solutions that have dark energy inside them, purely from GR. I wouldn't call that new physics exactly, I would call that a better understanding of physics we already have.
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