People love to imagine that the things they don’t understand.
are somehow connected to each other.
For example, quantum mechanics and consciousness, or aliens and pyramids, or black holes and dark matter,
or dark matter and dark energy, dark energy and black holes. Usually there’s no real
relationship whatsoever, but this last pair—black holes and dark energy being the same thing—has
received some recent hype in the press. Let’s see if it might actually be true.
Here’s the idea in a nutshell. Black holes are apparently made of dark energy. As the universe
expands, black holes in that expanding space gain mass proportional to the increasing volume
of the universe. Which results in two things: one, black holes should be growing faster than we
expect based only on what they eat. And two: black holes could actually be the
cause of the accelerating expansion of the universe. This is the claim in an article
by University of Hawaii astrophysicists Duncan Farrah and Kevin Croker and their team.
Now to really assess whether we should dismiss this or take note, we have to follow the paper trail back.
Way, way back. The seed of this idea came from a little paper from the mid 60s by Russian physicist
Erast Gliner. someone you’ve probably never heard. In fact not many people have heard of Gliner,
but this work led to an alternative way of thinking about the nature of nothing,
and set the stage for our theories of both cosmic inflation and dark energy.
So a little science history—stuff we’ve covered before if you’re interested in more detail.
Soon after Einstein first came up with his general theory of relativity, another Russian physicist
Alexander Friedman solved its equations for the entire universe. The resulting Friedman
equations predicted that the universe must be expanding or contracting—which Einstein famously
disliked, and so he added something called the cosmological constant to general relativity so
that the universe described by Friedman's solution had the potential to be static.
The universe was of course subsequently shown to be expanding, so the idea of
the cosmological constant was largely ignored for years.
But Erast Gliner realised that the cosmological constant could be interpreted as an energy in the
fabric of space itself—a vacuum energy—and that this stuff would have bizarre properties. For example
possessing negative pressure. Gliner’s energetic vacuum became a fundamental part of the science
of cosmology. It’s the mathematical mechanism behind cosmic inflation—the extreme expansion rate that
many cosmologists believe kickstarted the Big Bang—and behind dark energy,
the mysterious influence that’s currently causing the expansion of the universe to accelerate.
Which, by the way, tells us that the cosmological constant is in some sense real,
even if Einstein added it to his equations for the wrong reasons.
Gliner’s 1966 paper changed cosmology with this new way of thinking about the cosmological
constant. But there was another rather minor note in that paper. One that sent a handful
of scientists down the unusual train of thought leading to the result we are discussing today.
Gliner speculated that his energetic vacuum could be an endpoint of gravitational collapse.
If so, perhaps this stuff also describes the interior of black holes.
Now, in regular general relativity, a black hole occurs when an object reaches such a high density that an
event horizon forms around it—a surface below which escape from the object’s gravitational
field is impossible. In standard GR, matter that collapses into a black hole ends up crushed into
an infinitesimal point of infinite density—the singularity at the black hole’s center. But
the prediction of the singularity in general relativity is slightly awkward, because it brings
the theory into hopeless conflict with quantum mechanics, for reasons we’ve discussed before.
Many people have tried to find ways around this issue,
for example with wormholes or the fuzzballs of string theory.
Gliner’s insight gives another way to rescue
collapsing matter from breaking the laws of quantum mechanics.
If that matter can transmute to a raw energy of the vacuum then the resulting negative
pressure can potentially counteract gravitational collapse, preventing singularity. A number of
versions of this idea have been proposed over the years. For example we have the dark-energy
star of George Chapline and Robert Laughlin, or the gravastar of Pawel Mazur and Emil Mottola.
The general name for these things is the GEODE - generic object of dark energy.
GEODEs function as black holes in many ways—including being compact, intense
sources of gravity that are almost completely invisible. They also differ in subtle ways that
may one day be detectable. For example, there may be signatures in the gravitational waves they emit
when they merge. But currently we don’t have any direct observational evidence that black holes
are anything other than the things predicted by general relativity. That evidence is hard to come
by because the laws of physics make it literally impossible to see beneath the event horizon.
But if black holes are GEODEs there may be a sneaky way to reveal their nature.
This is the aspect of the new work that’s the most controversial.
Regular black holes are fundamentally isolated. Outside, we can only sense their gravitational
field and electric charge. A black hole only knows about what falls into it. It
certainly doesn’t know what the universe is doing very far from its event horizon.
But according to the scientists behind this new work, a GEODE is connected to the universe on
the largest scales. In a 2021 paper led by Kevin Croker, they find that a new way to look
at the Friedman equations they claim demonstrate something very surprising—compact regions of dark energy
are coupled to the expansion of the entire cosmos. They call this cosmological coupling,
and they claim that as the universe grows, so does a GEODE—that their mass should be
proportional to the volume of the universe. OK, so if that’s true, then “black holes” —we’ll
just call the GEODEs black holes from now on—should have grown enormously since the early universe.
And that’s the assertion of the new paper by Farrah and collaborators. In particular,
they claim that the supermassive black holes at the centers of galaxies in the modern
universe are way too big. All galaxies have these “SMBHs”. They probably began as medium
sized black holes from the gigantic first generation of stars in the early universe,
and grew by merging with other black holes and devouring or “accreting” gas
until they reach millions and even billions of times the Sun’s mass.
The team claims that their sample of SMBHs grew too much over the
last 10 billion years or so to be explained by accretion or mergers alone.
More precisely, they claim that galaxies now have SMBHs that are much bigger relative to
their mass in stars than galaxies in the past. In general, bigger galaxies have bigger SMBHs.
But this team claims that the ratio between black hole mass and galaxy mass has changed,
with the black holes outpacing the growth of galaxies. And that this extra growth is
exactly the amount expected from “cosmological coupling” — so due to being connected to the
expansion of the universe. They claim 99.98% confidence in excluding “zero coupling”.
In a separate paper from 2021 the same team also points out that the merging black holes detected
by LIGO are also way too big — and again, by about the amount expected due to cosmological coupling.
It all sounds pretty exciting, right? But wait, that’s not all. The team also claims that these
black holes might not just be made of dark energy - they might literally be dark energy. As in,
it's the dark energy in black holes that’s causing the accelerating expansion of the
universe, rather than the energy of the vacuum between the galaxies.
Let’s see how that’s supposed to work. The key to the exponentially accelerating power of dark
energy is simply its constant energy density. As the universe expands, regular matter and dark
matter thin out and their gravitational influence diminishes. But dark energy is usually described
as a property of the fabric of space itself, and so the more space you have the more dark energy.
Expanding space actually adds energy into the system. For complicated reasons due to relativity
being weird, this causes accelerating expansion. Check these episodes for the gory details.
OK, so if dark energy isn’t in space itself, but is in the black holes,
AND if the black holes are growing in mass with the expanding universe, then you get the same
effect. Although the black holes get further apart as space expands, their masses increase, so their
mass density and their energy density remains constant—just like regular dark energy.
For that reason they should behave like regular dark energy and cause accelerating expansion.
So could these GEODE black holes be numerous enough to account for dark energy? That’s a tall
order, because dark energy makes up around 70% of the energy of the universe. On the other hand,
regular matter makes up around 5%, and regular matter is where black holes supposedly come from,
via the deaths of massive stars. But there’s a way around this. If you make black holes with
regular matter, and those black holes grow with cosmic expansion, then they can exceed that 5%.
Anyway, Farrah and team calculate the mass in black holes of all types
that should have been created by dying stars over the history of the universe,
then calculate how these should have grown due to cosmological coupling.
They find that it’s not that hard to get the amount needed for dark energy.
So that’s the idea in a nutshell. That black hole interiors contain an energetic vacuum which
is coupled to the expansion of the universe. That means black holes grow in mass with the expanding
universe, which in turn causes them to act like regular dark energy and accelerate that expansion.
Now that we have that straight, let’s tear it apart.
If we can. We need to address both the theoretical side and the evidence.
On the theoretical side, we have 1. the idea that black hole interiors might be dark energy,
and then 2. that this dark energy might be coupled to the universe on cosmic scales.
For 1. There isn’t any evidence, nor is there a particularly compelling
physical mechanism—but it’s at least plausible because we don’t understand
black holes fully anyway. Let’s just give them that one for now.
Now the idea that this internal vacuum energy is coupled on cosmic scales feels like the biggest
reach of this whole idea. Normally, we think of general relativity as a purely local theory. That
means every point in spacetime only knows about what’s happening to its neighbouring points.
That means that the interior and exterior of a black hole only interact via information that can
be transmitted across the event horizon by stuff falling in or by Hawking radiation leaking out.
But these mechanisms do not allow black holes to be influenced by the universe on cosmic scales.
Supermassive black holes are in the centers of galaxies.
The immediate spacetime that these black holes live in is NOT expanding.
The expanding universe solution to the Einstein equations comes from assuming matter is perfectly
smoothly distributed through the universe. It’s an approximation which works on the largest scales.
Standard general relativity seems to deny the possibility of cosmological coupling,
however these researchers claim that their reformulation of the Friedman equations allows
this weird communication between small and large scales. This is extremely speculative,
and no one besides this team has published anything that
properly analyses this conjecture. On the theoretical side, this is the flimsiest part.
However, if black holes really are made of vacuum energy AND this cosmological coupling is right,
then the bit about black holes accounting for dark energy is at least theoretically plausible.
Although it would require that almost all black holes are not in galaxies, but rather distributed
evenly through the universe, rather than being clustered like the stars that they came from.
The authors claim that this is possible, but we don’t have time to pick this apart today.
OK, the final thing to look at is the main claim of the most recent paper—the supposed evidence
that supermassive black holes have grown so much that they must be coupled to cosmic expansion.
For more analysis of this side of it check out Dr. Becky’s excellent video on this. But here’s
my hot take. These guys are not the first to try to measure the growth of black holes over cosmic
history. We’ve known for many years that there’s a tight relationship between black hole mass and
galaxy mass, and for many years people have also been trying to measure the evolution of this
relationship over cosmic time. Some suggest that black holes grew faster than galaxies,
some suggest that galaxies grew faster than their black holes. This team's new
result showing rapid black hole growth does some things differently from previous work. But it’s not clear
that they did the analysis better than the many experts who have been doing this sort of calculation for decades.
The fact is, the evolution of black hole masses is really really hard to measure,
largely because those masses are hard to estimate. It’s also very difficult to know
that the galaxies we’re looking at from an earlier universe, billions of light years away,
are representative precursors to the galaxies we’re looking at in the nearby, modern universe.
Remember we can't actually see a single black hole evolve.
There are many potential biases involved in a study like this, and that fact isn’t properly
acknowledged in this paper. In fact, their claim of “excluding zero cosmological coupling”
99.98% feels dishonest in the absence of a full discussion of the contingencies of the result.
The confidence with which the result is stated is a bit of a red flag.
OK, so that’s my take on the black holes equals dark energy idea. Lots
of interesting stuff in there, and it might be true if the most speculative
parts can be verified by other experts. As with anything this potentially revolutionary,
it’s probably not true. But that doesn’t mean it’s not valuable. In order to find
the next big breakthrough, we need to explore all sorts of weird parts of theory space.
And following the intuitions of Erast Gliner has led to incredible discoveries in the past,
so perhaps they will again. And perhaps black holes turn out to be dark energy,
neatly wrapping together two of the most perplexing phenomena in all of space time.