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Space-time is being driven apart. Every second that passes, the universe expands faster and faster. What is propelling this dramatic acceleration is an enigma, though – one scientists have known about, and searched for, for decades. Still, 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. Nevertheless, it makes up about 68 per cent of the universe.

It would be reasonable, however, to assume this mystery has nothing to do with black holes: behemoths so gravitationally powerful that once something is drawn in past a certain point, it can never escape. They pull matter towards them, so how could they be driving the universe’s expansion? Yet that’s exactly what a small group of astrophysicists is suggesting.

The story goes like this: all matter that falls into black holes goes through a process that turns it into a 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 together all the black holes in the universe and it starts to mount up to something that could be pushing 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 over the past few years – as it turns out to offer a potential explanation for not one, not two, but three mysteries of the universe. “It’s not fringe any more,” says , a cosmologist at Arizona State University. “It’s highly controversial, but it’s not fringe.”

Black holes offer themselves up as a potential source of dark energy precisely because they are so perplexing. “Most of the structures in the universe, like galaxies and clusters, have very little effect on dark energy. But there has always been one possible exception,” says , a cosmologist at the University of Waterloo in Canada. “Black holes [after all] are much more mysterious than everything else.”

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Black hole singularity

It all comes down to the point at the centre 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. “Nobody believes in a singularity,” says , 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 behaviour of the cosmos. In reality, he says, something prevents a singularity from forming. “What’s going to stop it is if the matter that’s causing this collapse somehow turns into dark energy.”

Nobody knows exactly how it would happen. But Tarlé compares it to the very 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. This wouldn’t affect their gravitational pull, though, which is based on energy density, not specifically matter.

“If you try to understand how a single particle of dust can turn into radiation, that’s not known,” says , a physicist and cosmologist at the University of Trento in Italy. “But we assume that it can happen – this conversion is not as crazy as it sounds.”

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For a long time, the consensus has been 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 throw a causality argument: why could this stuff here affect things that are so far away? But it’s not just one of them, it’s tons of them, and they’re all over the place. It’s this aggregate effect.”

If you threw a bunch of matter into a single cosmologically coupled black hole, it might not affect the cosmos writ large, he says. On the other hand, if you had a fleet of cosmic dump trucks pouring matter into these black holes all over the universe, you could speed up its expansion. It is a bit like a balloon filled with many smaller balloons: inflate the smaller ones and the big one will be forced to expand as well. If these black holes are real, then, as a population, they must be inextricably tied to the overall structure of the cosmos.

Evidence for cosmologically coupled black holes

And it’s not all theoretical, either. The first piece of evidence that black holes may be cosmologically coupled came in 2023 with the revelation from Croker, Tarlé and their colleagues that the small balloons do, in fact, seem to be expanding: black holes across the universe . Even what Croker calls “maximally boring” supermassive black holes, which should barely be growing at all, are keeping pace with the universe’s expansion. “It was the first time we saw something significant that said that once black holes are formed, they create this dark energy, and then the [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 how exactly 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, it is impossible to tell, for example, if the behaviour of cosmologically coupled black holes as they merge would match observations we have of that process. “The task is very, very difficult because the equations are horrible, but there might 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 shifted it from being something rejected by many serious cosmologists to become something that is at least seen as plausible. One reason for this is that it appears to match up with some puzzling recent results from the Dark Energy Spectroscopic Instrument (DESI) in Arizona.

The DESI results

DESI is measuring the locations of millions of galaxies across the universe, building a precise map of how the distances between them have changed over the course of cosmic history. Those distances allow us to calculate how fast the universe expanded across various epochs. And over the course of the past two years, the first results have been released. They suggest that dark energy may be weakening over time, which was a bombshell: the standard model of cosmology requires dark energy to be constant. “Seeing the data for the first time, our mouths kind of dropped open,” says Tarlé. “It was very clear that dark energy was changing in time.”

But if the effects of dark energy come 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 been steadily slowing since then. Not only would this  amount of dark energy hinted at by DESI, it would also help account for another major cosmic mystery.

The Hubble tension relates to a discrepancy between the two main ways of calculating the universe’s expansion, one based on measurements of relatively nearby objects, and another based on using the standard model of cosmology to extrapolate forwards from measurements of light remaining from the big bang. Adding cosmologically coupled black holes to our model of cosmology may not entirely solve this problem, but it significantly eases the tension by providing an explanation for why the two methods deliver conflicting results: the times they probe in cosmic history would have had different rates of expansion.

There are several other proposed explanations for the Hubble tension and the apparent weakening in 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 upon general relativity and nothing else – and that’s a plus,” says Rinaldi. Perhaps surprisingly, that makes it a relatively conservative proposition in the context of these two problems.

Now, Tarlé, Croker and a group of colleagues have added  to what they call a “three-legged stool” of observations that line up with their predictions. This final leg is a bit different from the other two, in that it is a mystery in particle physics. The behaviour of the universe allows cosmologists to create a budget for 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 are sometimes referred to as “ghost particles”. Taking into account the new DESI data, neutrinos would need to have a negative mass for the budget calculations to work. As it shouldn’t be allowed to be negative, it must be zero.

But if matter is turning into dark energy inside black holes, that affects the balance of the cosmos. Cosmologically coupled black holes would make room in the mass budget by converting regular matter into dark energy. It turns out they would create just enough leeway for neutrinos to not only have a positive mass, but one that lines up with experimental measurements.

Are these three pieces 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 the three legs. We think we can sit on it,” says Croker. “Other people in the community may think it’s dangerously janky, but my hope is that, at some point, some other people will jump on this as well.”

That has already started happening. The earlier research on cosmologically coupled black holes was done 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 this sort of controversial proposal, what researchers really need are better models – in this case, solutions to the “horrible” equations – and more data. The latter, at least, is forthcoming. DESI is still gathering more observations of galaxies, and several other large surveys of the universe are under way. “It’s a detective story: there is an obvious suspect that is acting very suspiciously and there is 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.”