How Weird Inside Black Holes?

In the last few days, I’ve dived down a rabbit hole inspired by some new astrophysics papers suggesting that dark energy is actually black holes. I think I get it now. So let me explain.

The universe is expanding, and instead of that expansion decelerating as was expected, we found a few decades ago that it is actually accelerating. And according to general relativity, this implies that a big component of the universe must have nearly maximal negative pressure, and be increasing in mass to keep its mass density constant. That is, on the margin this stuff is strongly attracted to itself, rather than repelled by itself. (We are ignoring gravity here, which yes attracts everything to everything, but is already accounted for in general relativity.)

This was surprising because most of the stuff we thought the universe contained, like stars, gas, and light, has positive pressure. And even the dark matter that we posit to explain fast galaxy rotation, and many other things, is assumed to have positive pressure.

Now we do know that very dense complex matter sometimes has negative pressure. For example, solids when stretched, or in certain meta-stable liquids and gasses. Bose-Einstein condensates can have negative pressure, as can quantum chromo dynamic (QCD) plasmas. But those sorts of states seem rare. (E.g., in the buildings and machines around you, far more material by mass is under compression than tension.) And when combined into larger systems, they typically have positive pressures on their surfaces, toward outside stuff, which is what we thought matters for cosmology.

Thus cosmologists have instead assumed new physical stuff with negative pressures, stuff that is very not-dense, and spread out very thinly across the universe. Like a cosmological constant, vacuum energy a-la Casmir effect, or some new very light scalar fields.

Enter Kevin Croker and Joel Weiner, who in 2021 showed in a theory paper (which looks solid to me) that what matters for cosmological expansion, and also for local energy changes, is the local ratio of pressure to energy density. Whatever stuff has a local negative pressure, that stuff would increase locally in mass as the universe expands. And thus the universe expansion could be accelerating because of a few very small but very dense pockets of matter with negative pressure, even if all the other nearly empty volume has zero or positive pressure.

Where could such pockets of dense negative pressure matter be hiding? One obvious candidate is very dense objects, including things that look on the outside like black holes, held together mainly by gravity. When the universe expands in length by a factor of two, and its volume expands by a factor of eight, then max negative pressure objects would increase in mass by a factor of eight. So we might search for such objects in the sky via looking for such mass increases.

Enter Crocker and Weiner’s 17 other co-authors in another paper. It has long been noticed that the big dense objects at galaxy centers have been increasing in mass over the history of the universe at a surprisingly fast rate. These authors focus on elliptical galaxies, which are thought to be mostly spent, and thus have few stars or gas to fuel central object growth. They find that over the last 7-10 billion years, such elliptical galaxy central objects have grown in mass a factor (~7 to 20) which is close to that predicted if they were max negative pressure objects, growing simply due to the expansion of the universe. They also estimate that such central galaxy object growth seems, by itself, sufficient to explain the observed universe expansion acceleration.

So we have two puzzles, the acceleration of universe expansion, and the surprisingly fast growth of galaxy central objects, both of which can be explained if we posit that those central objects are filled with very negative pressure matter of some sort. The ordinary sort of matter that initially fell into such objects was somehow converted quickly into negative pressure matter, which has remained, and since grown due to universe expansion.

General relativity says that such central objects filled with negative pressure stuff don’t actually have to form singularities. And even if the stuff falls behind an event horizon, that stuff still contributes to the growth of the object’s mass and to the universe expansion acceleration. And thus this theory also lets us avoid dealing with such theoretically problematic stuff as singularities.

The main strike against this theory is that we don’t have any particular reason to expect ordinary stuff to, when rapidly crushed into a dense central object, convert to negative pressure matter which then remains stable for billions of years. But nor can we show this to be impossible. It’s just that, among the high density matter arrangements with which we are familiar, net negative pressure versions seem relatively rare. So, just how weird is that stuff inside central galaxy black holes?

I don’t know if this theory is true, but it certainly seems a reasonable candidate to consider. Positing that familiar stuff achieves an unusual but common kind of state seems at least as plausible as positing entirely new unfamiliar kinds of stuff.

Added: Here is 2021 news article on related publication.

Added 11p: Hmm, in the machines around us the heaviest parts under tension tend to be those rotating rapidly. So maybe rapidly rotating central galaxy objects induce negative pressure in their materials?