A bizarre type of black hole could solve three cosmic mysteries in one

Space-time is receding. Every second that passes, the universe expands faster and faster. But what’s driving this dramatic acceleration is a mystery—scientists have known about it and searched 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 they make up about 68 percent of the universe.

However, it would be reasonable to assume that this mystery has nothing to do with black holes: monsters so gravitationally powerful that once something is pulled in beyond a certain point, it can never escape. They pull matter towards them, so how could they control the expansion of the universe? Yet that’s 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 vicinity, but add up all the black holes in the universe and it starts to add up to something that could inexorably push everything away from everything else.

This wild idea started on the fringes and has appeared in many iterations over the decades. But in the past few years, more and more cosmologists have been paying attention to it—as it turns out that it offers a potential explanation for not one, not two, but three mysteries of the universe. “It’s not the edge anymore,” he says Kevin Crockercosmologist at Arizona State University. “It’s highly controversial, but it’s not marginal.”

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

Black hole singularity

It all comes to a point at the center of the black hole, where gravity is so strong that matter is compressed to infinite density. Known as the astrophysical singularity, it has always been seen as something of a placeholder for physics we don’t yet understand. “No one believes in the singularity,” he says Gregory Tarlea University of Michigan cosmologist and astrophysicist who is a prominent figure in the study of these cosmologically bound black holes, so called because they would be linked to the large-scale behavior of the universe. In fact, he says, something is preventing the singularity from forming. “What will stop it if the matter causing this collapse somehow turns into dark energy.

No one knows exactly how this 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 universe cooled and much of this radiation coalesced into matter. Inside cosmologically bound black holes, this process would be reversed. However, this would not affect their gravitational force, which is based on energy density, not mass specifically.

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

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It has long been agreed that black holes can only really affect their immediate surroundings. “The idea was sort of like ‘what happens in Vegas stays in Vegas,’ but that’s not true,” says Croker, one of the pioneers of the concept of cosmologically bound black holes. “People like to argue causality: why would these things affect things that are so far away? But it’s not just one of them, there’s tons of them and they’re everywhere. It’s this cumulative effect.”

If you throw a bunch of matter into a single cosmologically bound black hole, it may not affect the grand universe, he says. On the other hand, if you had a fleet of space dumpers dumping matter into these black holes all over the universe, you could speed up their expansion. It’s a bit like a balloon filled with many smaller balloons: inflate the smaller ones and the big one will have to expand as well. 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 bound black holes

And it’s not all 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 small balloons actually appear to be expanding: black holes across the universe. they seem to be growing at an unexpectedly high rate. Even what Croker calls “maximum boring” supermassive black holes, which should barely grow, are keeping pace with the expansion of the universe. “It was the first time we saw anything significant that said that once black holes form, they create this dark energy and then [dark] energy increases as the universe expands,” says Tarlé.

Perhaps the biggest objection to this hypothesis is that we have no idea what cosmologically bound black holes would look like or exactly how they would behave. “The problem is that we don’t have a mathematically exact solution that describes these objects—we have an average,” says Rinaldi. Without this solution, for example, it is impossible to say whether the merging behavior of cosmologically bound black holes would match the observations we have of the process. “This task is very, very difficult because the equations are terrible, but at some point there may be a breakthrough – it just takes time,” he says.

In the years since the idea was first developed, time and intense research have taken it from something that many serious cosmologists rejected to something that is at least considered acceptable. One reason is that it seems to match some puzzling recent results from the Dark Energy Spectroscopic Instrument (DESI) in Arizona.

DESI results

DESI measures the location of millions of galaxies in the universe and creates a precise map of how the distances between them have changed over the course of the universe’s history. These distances allow us to calculate how fast the universe was expanding at 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 first saw the data, our mouths dropped a little,” says Tarlé. “It was very clear that dark energy changes over time.

However, if the effects of dark energy come from a cosmological connection to black holes, the DESI results make sense. The formation of black holes follows the same trend as star formation, which peaked about 10 billion years ago and has been steadily slowing since then. Not only this explain shrinking the amount of dark energy suggested by DESI would also help explain another great cosmic mystery.

The Hubble tension is related to a discrepancy between two main ways of calculating the expansion of the universe, one based on measurements of relatively nearby objects and the other based on using the Standard Model of Cosmology to extrapolate forward from measurements of the light left over from the Big Bang. Adding cosmologically bound black holes to our model of cosmology may not completely solve this problem, but it greatly eases the tension by providing an explanation for why the two methods give conflicting results: the times in the history of the universe that probes have probed would have had different expansion rates.

There are several other proposed explanations for the Hubble tension and the apparent weakening of dark energy, but they tend to rely on exotic hypothetical phenomena that are beyond our standard understanding of physics. “[The idea of cosmologically coupled black holes] it relies on general relativity and nothing else—and that’s a plus,” says Rinaldi, which perhaps surprisingly makes it a fairly conservative proposition in the context of these two problems.

Tarlé, Croker and a group of colleagues have now joined further evidence to what they call a “three-legged stool” of observations that match their predictions. This last part is a bit different from the other two in that it is a particle physics mystery. The behavior of the universe allows cosmologists to create a budget for how much matter it contains, which can then be used to calculate the mass of each type of particle.

That’s all well and good, except for neutrinos, tiny—but certainly not massless—particles that interact so rarely with other matter that they’re sometimes called “ghost particles.” Taking the new DESI data into account, the neutrino would have to have a negative mass for the budget calculations to work. Since it should not be allowed to be negative, it must be zero.

But if matter inside black holes turns into dark energy, it affects the balance of the universe. Cosmologically bound black holes would free up space in the mass budget by converting ordinary matter into dark energy. It turned out that they would create just enough space for the neutrino to not only have a positive mass, but one that matches experimental measurements.

Are these three lines of evidence sufficient to fully bring the hypothesis of cosmologically bound black holes out of 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 funny, but I hope at some point other people will jump on board.”

That has already started to happen. Previous research on cosmologically bound black holes has been done by small research groups, each with just a handful of collaborators, but the latest paper on neutrino masses has 50 co-authors.

As is always the case with this kind of controversial proposal, what researchers really need are better models—in this case, solutions to the “horrible” equations—and more data. At least the latter is coming. DESI is still collecting more observations of galaxies, and several other large surveys of space are underway. “It’s a detective story: there’s an obvious suspect who’s behaving very suspiciously, and there’s an obvious crime,” says Afshordi. With three clues that black holes may be behind the accelerating expansion of the universe, more and more detectives are on the case. “But of course, the hardest part is making that connection.

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