I don't get it either, but this is all kind of like what I was thinking about before I learned that other much smarter people trained in physics and math were also thinking along what I perceive as similar lines.

I'm too curious about this, so I emailed the first author Duncan Farrah. Let's see what he says.

I got a nice, detailed response, but unfortunately not so much about the main point :

This still leaves the question "how could a tiny blob of vacuum energy affect the dynamics of the universe?" The arguments here are pretty subtle (I am also mostly an observer :-)) but the basic idea is that anything relativistic will be in some way linked with the dynamics of the universe. The most famous example is of course photon redshift, but black holes should, in some models, do it too as they are also relativistic objects. This paper:
https://ui.adsabs.harvard.edu/abs/2007PhRvD..76f3510F/abstract
Wws among the first to propose the basic idea, though the theory was worked out in much more detail in
https://ui.adsabs.harvard.edu/abs/2022PhRvD.105h4042C/abstract
https://ui.adsabs.harvard.edu/abs/2019ApJ...882...19C/abstract

(one small addition - I've heard a few people call this a 'backreaction' model. It definitely is not though as the motivations are fundamentally different, as well as the physical origin)

Not sure how far I'll make it through those papers but I'll give it a try.

It seems that as they imply here and state more explicitly elsewhere, it’s not that dark energy is making black holes grow, it’s the other way around : black holes are dark energy.

That same statement is at the start of the article:

“What that means, though, is not that other people haven’t proposed sources for dark energy, but this is the first observational paper where we’re not adding anything new to the universe as a source for dark energy: black holes in Einstein’s theory of gravity are the dark energy.”

Ahh yes, so it does. I read too many articles and thought I'd seen that statement somewhere, and forgot it was in the very one I chose to link. :P

Dark energy is for explainging that space expands uniformly (Hubble constant), right?
This in itself would seem to not fit with any localized source of expansion.

Well dark energy is specifically for explaining the acceleration of the expansion rather than the expansion itself, but the point stands : this is a literally universal process, and not at all obvious how any local source could be responsible.

We use the gravitational assist of a large body to accelerate our spacecraft by navigating close to it and leaving on a different trajectory. Models of solar systems generally fling the small bodies away from the star after a few orbits. Stable orbital configurations are seemingly rare. Is the whole universe behaving like that?
Asking for a friend. ;-)

@Brian Fitzgerald AFAIK no. The expansion of space as described by the hubble constant is literally that: space is expanding. It's not about things moving away from each other (as in speed/velocity) but rather the distance between them literally growing.

This is the reason why we posit an "observable universe" much smaller than the expected size of the universe : light from beyond a certain distance will never reach us, because the distance it has to pass is growing at more than c.

Heh, Brian your idea might fit if there's one or more of universes flinging around a extra-super-massive extra-universe black hole like thing. 🙂 But then of course the expansion would probably be unequal in different directions, more elongated. Maybe this is the thing that makes the whole universe flat.

Maybe the black holes are sinking into spacetime, and spacetime is like a big dough, so when some parts are pushed down, all the other parts rises?

Universe as a raisin bun.

If protons in the stellar winds pass near a black hole but are not captured, might they be accelerated?

Anything at all that passes close to a black hole without being captured would be accelerated.

I believe this is what "black hole jets" are.

might they be accelerated?

You cannot accelerate something faster than the speed of light. Photons are already moving at that speed.

Some here may argue that FTL is possible, but it has NEVER, EVER been experienced or witnessed by science, thus far.

Can anything travel faster than the speed of light? | Live Science

Interesting article/video, but the answer is still yes/no/maybe/we don't really know.

Protons are not photons, and are unlikely to go quite as fast, as you say..

Ah... read your post wrong. I seem to have made that r an h in my own head.

No worries.

Anyway I don't think accelerating some protons along past some few regions of space is going to make space(time) itself expand....

Although I have been assured by respected mathematicians that space-time expands, I remain unconvinced that this is true. How sure are you?

I read through the the first of their recent papers, which is a letter so it's nice and short. But I still have only hints as to what the answer might be.

So far as I can tell - and there's a lot here I don't understand at all - the crux is the nature of the black holes. In this model, they are a totally different beast to the classical event horizon shrouding an infinitely dense singularity. They now have neither, but are composed of vacuum energy. Apparently, theoretical experiments have tried to remodel black holes in such a way before, but failed. The impression I get from the paper (I don't want to give the idea that this is definitely what they state) is that cosmological coupling helps resolve this issue, making these mathematical inconveniences go away whilst giving results consistent with observations.

In particular, they seem concerned with the fact that classical solutions of black holes are incompatible with cosmological observations when extended to infinity. Now formally the gravitational field of any object extends indefinitely, propagating at the speed of light. So my very tentative guess is that's where the solution lies. Being composed in their model of vacuum energy, although they presumably have incredibly dense interiors of ultra-strong gravitational strength, at infinity they now do something much more funky which in some way provides a negative pressure. And while each individual black hole is still in some sense point-like, collectively, since they arise from the end products of stellar evolution, they're essentially everywhere. So the negative pressure, expanding at the speed of light, could presumably fill the universe.

This is very hand-wavy, and not really satisfying, but it's the best I can gather from the letter. Next week I'll try the longer article.

Thanks for the explanation, @Rhysy, hand waving and all. It just occurred to me that the impression of outward expansion may arise from a collapse of the interior of the universe at the origin. Like other transforms, it would be a way of describing the same result, but in a different framework. I'll stop waving my hands for now and return to stirring the bat wings and newt eyes in my cracked pot.

I wonder if there will be a Hossenfelder vid about this one...

@Andreas Geisler Briefly, here? https://www.youtube.com/watch?v=ENGJA1cUe3M

Thanks indeed!

That goes along with more of my previous naive thinking where I've thought that at least some of the mass that was compressed into a black hole must certainly have been converted into energy of some form. After all isn't it basically gravity compressing matter together that, at some density, inevitably starts fusion reactions thus converting some of that matter into energy and kickstarting a star? It seems logical that this particular reaction, i.e. converting mass into energy, could/should be "turtles all the way down" ito the infinite(?) gravity in a singularity.

@Greg A. Woods I think, not being an actual physicist, that all the available energy is released as matter falls into the black hole. Not like it matters, because it all falls in anyway.

A star burns because turning hydrogen into helium releases energy -- putting protons and neutrons together is a lower energy state that free-floating hydrogen atoms.

Consider a neutron star. There's no further binding energy to be released by any further compression or reconfiguration. In fact, adding more matter to a neutron star makes it somewhat less stable, but gravity still wins.

I'm no physicist of any kind either...

I wasn't thinking of "available energy" though, whatever that may be.

Indeed in a neutron star they say there's no thermonuclear fusion occurring in the core, but apparently there might be something called pycnonuclear fusion reactions occurring. In any case neutron stars are obviously not massive enough to collapse into a singularity. On the other hand I just found this: https://en.wikipedia.org/wiki/Quark-nova

In any case my thought was simply that there might be some (other) kind of matter to energy conversion happening under the "infinite" gravity forces of a singularity, and this could then account for this "vacuum energy" that generates the negative pressure to cause acceleration of the expansion of the universe.

@Brian Fitzgerald Ah, the old, "space isn't expanding, matter is shrinking !" malarkey... I think pretty much everyone has that idea at some point, it's a rite of passage. Here's the inestimable Fraser Cain on why that doesn't work : https://phys.org/news/2015-02-universe.html

Watched a few more videos and summarised everything as best I could : https://llittlephysicists.blogspot.com/2023/02/black-holes-behaving-badly.html
TLDR : it's a complicated relativistic effect and the outreach departments are underfunded.

I didn't even know that black holes had outreach departments...

@Andreas Geisler Perhaps that's because such a department is composed of dark matter, and therefore we can't see its members? -)

There definitely feels like webcomic potential in terrifying outreach departments.

Just had our regular journal club which includes people who study general relativity for for a living. Turns out... they don't understand how black holes create negative pressure at large distances either, and hardly anyone thinks this is at all credible.

The authors really missed a trick by not explaining this basic point. I willing to buy the claim in the UT interview that such an effect does follow from the equations, but they need to actually explain it at least to the level where other theorists can understand it. Otherwise it's like continental drift : it turned out to be right, but without that basic mechanism, it seems mad as trousers.

(And of course, this is more likely to turn out to be mad anyway. All revolutionary ideas may be greeted with skepticism, but not all ideas greeted with skepticism are revolutionary. Most are just bonkers.)

Just watched a video posted by @John Wehrle by Dr. Becky, who explained it.
As far as I understand, it's ... expansion also expands black holes, causing them to gain vacuum energy, which is the same as gaining mass, and then preservation of energy means they're depleting that vacuum energy, and ... I lost track around there.

Also, this is not pressure in the usual sense, I suppose? Might be helpful if that conceptoid were explained as well?

Yes, Dr Beck's video is linked in the blog post above. She does a good job of explaining the observational difficulties, but in IMHO, the UT piece with Fraser Cain is better from the theoretical perspective (same link but direct version here : https://www.youtube.com/watch?v=-8CIUzPkigQ&ab_channel=FraserCain).

more interesting is Person's explicit statement that according to general relativity, the negative pressure arising from the black holes would be averaged everywhere : we would not, he states categorically, expect to see inhomogeneous expansion as a result of this model. Black holes are truly coupled with the fabric of spacetime; I almost get the impression that they are a nonlocal phenomena. The solution, he says, is in the mathematics but is completely non-Newtonian. At least this is honest enough to bluntly state (in effect), "yes, the negative pressure is not intuitive, but it's frickin' hard to explain how this works without complicated maths".

So it's a bit like a raisin bun, with the raisins being pushed down into the dough, thereby pushing up the dough everywhere.

They should use baking metaphors more for cosmology I think.

First time seeing this post, somehow I often miss Rhysy's. Time zone difference?

Thanks Andreas, although I somewhat disagree with your summary of Dr. Becky's summary. My version would be this:

Vacuum energy creates a small uniform positive energy density throughout space even when no matter is present. But since nothing escapes event horizons vacuum energy within one is lost to the universe. If conservation of energy holds there must be an equal amount of negative energy outside all event horizons. (Me: perhaps a mechanism like Hawking radiation?) That negative energy could explain acceleration of expansion. Otherwise unexplained black hole growth would be evidence for this idea because reasons I didn't follow.

Is this summary of Becky's summary accurate? Not sure. It's how I understood her. Is Becky's summary of the paper's position accurate? IDK but I'd trust it a lot more than my own summary.

Oh, the UT interview looks interesting. Thanks!

This theory... It has me thinking about loop quantum gravity and the weird things it does to space.