Black holes behaving badly ?

Recently there's been a fascinating suggestion that the source of dark energy is nothing more exotic than the boring old black holes we're all familiar with from innumerable sci-fi movies.

I'm not going to do a blow-by-blow breakdown of this, but for a decent summary, see this press release. Very briefly, we've known for a century that the Universe is expanding. On large scales all galaxies are moving away from all other galaxies, though on smaller scales, like that of groups and clusters, gravity dominates and the expansion is negligible. But for a few decades we've known that the acceleration is, contrary to expectations, accelerating. This is "dark energy", the need for some source of impetus beyond the initial Big Bang that can sustain and increase the rate of expansion. Many other interpretations of the data have been attempted but none have stuck; the acceleration appears to be real.

The suggestion that black holes are the source of this goes completely against my intuition. How does a small, ultra-dense source of inescapable gravity somehow cause acceleration of the largest structures in the Universe ?

In this post I'm not going to delve into the observational details and reasoning behind this conclusion as I've not read the main paper about that aspect, though I will link to others who have. Instead, I just want to grapple with this idea that black holes can do the exact opposite of what we've all been taught to expect them to do.

Now understand that I do this as a layman. Relativistic physics is thought to play little or no role in galaxy evolution, any any understanding of general relativity I might have once had is long since gone. So what I'm looking for is an intuitive framework to understand this, not a mathematical one.

To cut to the chase, I don't have one. But maybe it's interesting to look at all the same.

Let's start with the press release :

If mass growth of black holes only occurred through accretion or merger, then the masses of these black holes would not be expected to change much at all. However if black holes gain mass by coupling to the expanding universe, then these passively evolving elliptical galaxies might reveal this phenomenon.

“Here’s a toy analogy. You can think of a coupled black hole like a rubber band, being stretched along with the universe as it expands,” said Croker. “As it stretches, its energy increases. Einstein’s E = mc² tells you that mass and energy are proportional, so the black hole mass increases, too.”

Right, okay, with you so far. An expanding universe can make black holes grow in mass, because handwaviumrelativitytheoryhandwavium is all very complicated. I’m willing to accept that the expansion of the universe can do weird things on small scales, especially, as Kevin Croker says in an earlier interview (content starts at 18 minutes), with only noticeable effects on “relativistic” materials - black holes and neutron stars, basically. Not much at all would happen to other objects. 

Croker also says that the properties of dark energy could be mimicked by a distribution of objects that gets more dispersed but more massive, so as to keep its density constant. That’s a bit trickier, but it makes sense. Observations inferring the acceleration show it's consistent with a constant energy density, and black holes necessarily become more dispersed due to expansion (like everything else), but, unlike everything else, it seems they've been growing in mass too*. So I can believe that their varying mass-energy density would cancel out and remain constant overall.

* This is a key part of their argument from the view of a necessary (but not sufficient) correlation. If dark energy has a constant energy density then its source should as well, which is apparently what black holes do, whereas normal matter just decreases in density over time. This is not direct evidence for a causal connection but it's at least an important consistency which other sources don't provide.

Where I come really unstuck is this :

Croker added, “This measurement, explaining why the universe is accelerating now, gives a beautiful glimpse into the real strength of Einstein’s gravity. A chorus of tiny voices spread throughout the universe can work together to steer the entire cosmos. How cool is that?”

I could buy the claim that it’s not that dark energy is making black holes grow, but actually the claim is the other way around : black holes provide dark energy. But if anything I would expect the opposite. With the galaxy/BH distribution being approximately uniform on the largest scales, if each one gets more massive, I would expect more gravity and therefore a deceleration.

Initial attempts to find a good explanation of this (I would think very obvious) objection weren't very productive. Even Ethan Siegel wasn't much help; his skeptical write-up is needless to say very good, but doesn't address this fundamental point. So I emailed the first author of the latest papers, Duncan Farrah, and he gave a nice response, but still without much about the bit that most interests me. For those who want them, technical (read : incomprehensible) papers about this "cosmological coupling" can be found here, here and here. 

But of course, a mathematical explanation isn't what I want. Farrah did also link to other papers though; being purely observational the main paper about inferring black hole growth rates doesn't deal with the main issue at all, but the second one (which is a letter, so nice and short), does. Well, a bit. 

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 (or at least they certainly don't have a singularity), 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.

When I went into work and discussed the topic with a somewhat more knowledgeable friend, his interpretation was more-or-less identical to mine. Hooray ! But he also raised the good point that at large distances the gravitational field of any object tends to be the same, so it's strange to propose that only black holes are contributing to the negative pressure, especially as the mass of black holes is very much smaller than the mass of normal matter.

Then I went on a video binge hoping that some professional outreacher would have helped clarify things. And I think perhaps they have, though only partially.

First, Sabine Hossenfelder has a short section in this video about the idea. Like just about everyone else, she's skeptical of the observational evidence, but does at least touch on the main issue in her own contrarian way : first, she says that black holes don't contribute enough mass-energy to be important (as already mentioned), and second she says that we shouldn't call it dark energy at all, but rather the "curvature of empty space". In her view, it's not a problem that needs investigating. Space just does this, there's no inconsistency with anything else so it's a "bad problem" to work on (for a more detailed video and discussion about this, see this thread).

I have problems with this. Okay, the observed mass of black holes on small scales is tiny, but does this tell us anything at all about their proposed negative pressure on large scales ? Not necessarily. That point earlier that this coupling might only affect relativistic materials is, I think, probably crucial.

As to the "curvature of empty space", it smacks of just trying to define the problem away rather than solve it. While she says that, "When I say “not a good problem” I do not mean “I don’t want to see an answer”, I mean “not a promising route to progress.”", I am honestly not entirely sure I believe her. A lot of it feels very much like brushing what everyone else thinks of as whacking great problems under a convenient carpet : sure, it could be the case that on large scales things just expand more and more rapidly, but this is a damned odd presumption. When I see something accelerating without an obvious cause, I want to explain it.

The analogy to the case of dark matter seems to be pretty direct here. Sure, we could have found flat rotation curves and declared, "oh well, I guess that's just what gravity does on large scales", but this would have been an awfully strange thing to do. There is an inconsistency here, a massive conflict with everyday notions about how gravity/acceleration work. We should be willing and able to relinquish these, but not at the drop of a hat. Presuming "this just happens" is very strange and very dangerous avenue and doesn't feel at all like a "promising route to progress" to be, but the opposite.

YouTube's recommendation algorithm must have improved of late because it came up with a couple more decent videos. First, Dr Becky has a great discussion on the observational problems of the study, not dismissing the idea but rather preferring that the study adds more evidence for our lack of understanding of black hole growth and galaxy evolution. She also notes that the black holes growing in mass is somehow related to the conservation of energy, that for the black holes to increase the Universe has to give something up. Intriguing, but it's not at all obvious to me how the acceleration increase would be countered by more massive black holes.

Finally there's this excellent video from Universe Today's Fraser Cain, interviewing co-author Chris Pearson. If you only watch one of the videos linked here, choose this one. Again there are some hints of some sort of conservation being at work, that the "tension" in the acceleration is analogous to that of an elastic band. But 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".

And I'm fine(ish) with this. I'd far rather have this than not acknowledging the (apparent) problem at all. If it's going to take some time to translate it to something comprehensible, so be it. Note that this is in stark contrast to a paper which appeared today claiming simply that black holes do not create negative pressure, end-of. It's for this reason, though, that we do need that intuitive, non-mathematical framework to consider; the mathematics may be more powerful, but hardly anyone else will be persuaded if you throw some incompressible gobbledegook at them.

Now as per my own remit, I've not gone into the observational evidence for the theory. Other links herein do that very well, and Pearson does a good job of summing up why they might be wrong as well as the reasons they think they might be right. 

It goes without saying that most papers attempting a revolution is cosmology or any other field are simply wrong. It's right and proper that radical claims are intensely scrutinised, but it's also right and proper that people are making such radical claims in the first place (regardless of what Hossenfelder thinks). "Publish or perish" does not yet dominate as an absolute incentive to mediocrity. So while I'd bet that in a year or two this claim will have faded into nothing, I'd hope that it'd do something more like dark energy and just keep growing. I get the distinct impression that this means reinterpreting not just black holes, but cosmology itself. And that would be pretty neat - at least, if outreach improves to the point where I can understand it.