Deborah Ferguson (UT Austin), Bhavesh Khamesra (Georgia Tech) and Karan Jani (Vanderbilt University)/LIGO
Space-time is being driven apart. Every second that passes, the universe is expanding faster and faster. What drives this dramatic acceleration, however, is a mystery – one scientists have known about and searched for for decades. Yet we are no closer to understanding it. We call it dark energy, but we know next to nothing about what it is or where it comes from. Yet it makes up about 68 percent of the universe.
However, it would be reasonable to assume that this mystery has nothing to do with black holes: gigantic giants so gravitationally powerful that once something is pulled in past a certain point, it can never escape. They pull matter towards them, so how can they drive the expansion of the universe? Yet that is exactly what a small group of astrophysicists is proposing.
The story goes like this: all matter that falls into black holes goes through a process that turns it into some kind of radiation. This in turn exerts a force on the space around it. Such an effect would be too small to notice in the immediate surroundings, but add up all the black holes in the universe and it starts to rise to something that can push everything inexorably away from everything else.
This wild idea began on the fringes, and has appeared in many iterations over the decades. But more and more cosmologists have been paying attention to it in recent years – as it turns out to provide a potential explanation for not one, not two, but three mysteries in the universe. “It’s not fringe anymore,” says Kevin Croker, a cosmologist at Arizona State University. “It’s highly controversial, but it’s not fringe.”
Black holes offer themselves as a potential source of dark energy precisely because they are so puzzling. “Most of the structures in the universe, such as galaxies and clusters, have very little effect on dark energy. But there has always been one possible exception,” says Niayesh Afshordi, a cosmologist at the University of Waterloo in Canada. “Black holes are (after all) much more mysterious than anything else.”
Black hole singularity
It all comes down to the point at the center of a black hole where gravity is so strong that matter is compressed to infinite density. Known as an astrophysical singularity, this has always been seen as something of a placeholder for physics we don’t yet understand. “No one believes in a singularity,” says Gregory Tarlé, a cosmologist and astrophysicist at the University of Michigan who is a prominent figure in the study of these cosmologically coupled black holes, so-called because they would be coupled with the large-scale behavior of the cosmos. In reality, he says, there is something preventing a singularity from forming. “What’s going to stop it is if the matter causing this collapse somehow turns into dark energy.”
No one knows exactly how that would happen. But Tarlé compares it to the early moments of the universe, when everything was a hot soup of radiation. In the moments after the big bang, the cosmos cooled and much of that radiation coalesced into matter. Inside cosmologically coupled black holes, that process would happen in reverse. However, this will not affect their gravitational force, which is based on energy density, not specific matter.
“If you’re trying to understand how a single dust particle can turn into radiation, it’s not known,” says Massimiliano Rinaldi, a physicist and cosmologist at the University of Trento in Italy. “But we guess it could happen—this conversion isn’t as crazy as it sounds.”
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For a long time it has been agreed that black holes can only really affect their immediate surroundings. “The idea was sort of ‘what happens in Vegas, stays in Vegas,’ but that’s not true,” says Croker, one of the pioneers of the cosmologically coupled black hole concept. “People like to make a causality argument: why can this affect things that are so far away? But it’s not just one of them, there’s tons of them, and they’re all over the place. It’s this overall effect.”
If you threw a bunch of matter into a single cosmologically connected black hole, it might not affect the cosmos very much, he says. On the other hand, if you had a fleet of cosmic dumpers pouring matter into these black holes all over the universe, you could speed up the expansion. It’s a bit like a balloon filled with many smaller balloons: blow up the smaller ones and the big one will also be forced to expand. If these black holes are real, then as a population they must be inextricably linked to the overall structure of the cosmos.
Evidence for cosmologically coupled black holes
And not everything is theoretical either. The first evidence that black holes might be cosmologically connected came in 2023 with the revelation by Croker, Tarlé and their colleagues that the tiny balloons actually appear to be expanding: black holes across the universe appear to be growing at an unexpectedly high rate. Even what Croker calls “maximum boring” supermassive black holes, which should hardly grow at all, are keeping pace with the universe’s expansion. “It was the first time we saw anything significant that said when black holes are formed, they create this dark energy, and then that (dark) energy grows as the universe expands,” says Tarlé.
Perhaps the biggest objection to this hypothesis is that we have no idea what cosmologically coupled black holes would look like or exactly how they would behave. “The problem is that we don’t have a mathematically precise solution that describes these objects—we have an average,” says Rinaldi. Without that solution, for example, it is impossible to say whether the behavior of cosmologically coupled black holes when they merge would match observations we have of that process. “The task is very, very difficult because the equations are terrible, but there may be a breakthrough at some point – it just needs time,” he says.
In the few years since the idea was first developed, time and intensive research have changed it from something dismissed by many serious cosmologists to something that is at least seen as plausible. One reason for this is that it seems to agree with some puzzling recent results from the Dark Energy Spectroscopic Instrument (DESI) in Arizona.
The DESI results
DESI measures the locations of millions of galaxies across the universe, building a precise map of how the distances between them have changed over cosmic history. These distances allow us to calculate how fast the universe expanded over different epochs. And over the past two years, the first results have been published. They suggest that dark energy can decay over time, which was a bombshell: the standard model of cosmology requires dark energy to be constant. “When we saw the data for the first time, our mouths kind of dropped,” says Tarlé. “It was very clear that dark energy was changing over time.”
But if the effect of dark energy comes from cosmological coupling with black holes, the DESI results make sense. The formation of black holes follows the same trend as star formation, which peaked around 10 billion years ago and has steadily declined since then. Not only would this explain the diminishing amount of dark energy that DESI suggests, it would also help explain another great cosmic mystery.
Together with dark energy, the pattern of dark matter in the universe (shown above) shapes the structure of the universe
VOLKER SPRINGEL/MAX PLANCK INSTITUTE OF ASTROPHYSICS/SCIENCE PHOTO LIBRARY
The Hubble tension relates to a discrepancy between the two main ways of calculating the expansion of the universe, one based on measurements of relatively nearby objects, and another based on using the Standard Model of cosmology to extrapolate forward from measurements of light left over from the big bang. Adding cosmologically coupled black holes to our model of cosmology may not completely solve this problem, but it eases the tension considerably by providing an explanation for why the two methods produce conflicting results: the times they probe in cosmic history would have had different expansion rates.
There are several other proposed explanations for the Hubble voltage and the apparent weakening of dark energy, but they tend to rely on exotic hypothetical phenomena beyond our standard understanding of physics. “(The idea of cosmologically coupled black holes) relies on general relativity and nothing else — and that’s a plus,” Rinaldi says. Perhaps surprisingly, that makes it a relatively conservative proposition in the context of these two issues.
Now, Tarlé, Croker and a group of colleagues have added yet another piece of evidence to what they call a “three-legged stool” of observations that match their predictions. This final stage is slightly different from the other two in that it is a mystery in particle physics. The behavior of the universe allows cosmologists to make a budget of how much mass it contains, which can then be used to calculate the mass of each type of particle.
That’s all well and good, except when it comes to neutrinos, tiny – but, crucially, not massless – particles that interact so rarely with other matter that they’re sometimes referred to as “ghost particles”. Considering the new DESI data, neutrinos must have a negative mass for the budget calculations to work. Since it should not be negative, it must be zero.
But if matter turns into dark energy inside black holes, it affects the balance of the cosmos. Cosmologically coupled black holes would make room in the mass budget by converting ordinary matter into dark energy. It turns out that they would create just enough leeway for neutrinos to not only have a positive mass, but one that is consistent with experimental measurements.
Are these three lines of evidence enough to fully bring the hypothesis of cosmologically coupled black holes in from the cold? “Right now the stool of evidence that we’ve offered has three legs. We think we can sit on it,” says Croker. “Other people in the community may think it’s dangerously jealous, but my hope is that at some point someone else will jump on this as well.”
It has already started to happen. The previous research on cosmologically coupled black holes was carried out by small research groups, each with only a handful of collaborators, but the latest paper, on the neutrino masses, has 50 co-authors.
As is always the case with these kinds of controversial proposals, what scientists really need are better models — in this case, solutions to the “terrible” equations — and more data. At least the latter is coming. DESI is still collecting more observations of galaxies, and several other major surveys of the universe are underway. “It’s a detective story: there’s an obvious suspect who’s acting very suspiciously, and there’s an obvious crime,” says Afshordi. With three clues that black holes may be behind the universe’s accelerating expansion, more and more detectives are on the case. “But, of course, the hard part is making that connection.”
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