The universe is expanding. No one with expertise in astronomy or physics disagrees with this fact.
Likewise, no one really disputes the fact that at some point, many billions of years in the future, the universe will expand too far– and running out of energy for further expansion. At that point, something has to change. This is where the disagreement begins.
There’s a new wrinkle in this cosmological argument, and it’s a doozy. According a new study by cosmologists Daniela Pérez and Gustavo Romero, both of the Instituto Argentino de Radioastronomía, the universe has repeatedly expanded and contracted, and large black holes are among the only things that have survived these endless cycles of destruction and renewal. These cycles are part of what cosmologists call a possible “cosmological bounce”.
A mathematical model of a theoretical black hole is the centerpiece of the peer-reviewed study by Péerez and Romero, which was published last month in the scientific journal Physical examination D.
“Our main result is that the solution represents a dynamic black hole that exists in all epochs of the bouncing cosmological pattern,” they wrote.
In other words, Peérez and Romero’s black hole survived even when everything around it was wiped out as the universe collapsed en route to an eventual rebound.
This is a compelling finding. The question of a black hole’s role in a bouncing universe “is clearly interesting,” said Leandros Perivolaropoulos, a physicist at the University of Ioannina in Greece who was not involved in the study, “and this paper can be seen as a first attempt to remedy this.
But beware: there are a lot of assumptions in Peérez and Romero’s argument. It’s possible that the moment a universe bounces from contraction to expansion, all the rules that guide our understanding of physics disappear. We may be trying to fathom the unfathomable.
“General relativity itself collapses at both the black hole singularity and the cosmological bounce singularity,” Perivolaropoulos said. “Thus, any conclusion based on this cannot be taken seriously.”
In other words, the moment the universe collapses to its smallest size just before bouncing back, gravity would cease to function normally. This is what we mean by a singularity: an exception to the laws of physics. We have no idea how a black hole would behave when the rules no longer apply.
Peérez and Romero’s methods “have significant potential for improvement, to put it mildly,” added Perivolaropoulos.
“General relativity itself collapses at both the black hole singularity and the cosmological bounce singularity. Thus, any conclusion based on this cannot be taken seriously.”
— Leandros Perivolaropoulos, University of Ioannina
To be clear, the underlying idea that the universe repeatedly expands and contracts is not new. A cosmological bounce is one of the main theories among cosmologists who study the origin and fate of the universe.
In fact, at least one team of scientists even thinks that our 13.7 billion year old universe is at the end of the most recent phase of expansion, and could start contracting again in about a hundred million years, on the way to a new rebound in a few billion or even tens of billions of years.
Alternative theories for how the universe might end include the universe slowing down and freezing, collapsing in on itself, or splitting into countless fragmentary pocket universes. Of all the options, it’s clear why the cosmological bounce is generating a lot of interest. It’s a fancy way to explain some of the weirdest things we see all around us in space.
For one thing, it might help explain why, in a universe that’s mostly uniformly empty, we have these weird, scattered clumps of stuff. Galaxies. Stars. Planets. People. Irregularities in space that are the by-products of endless expansion and contraction.
The bounce could also make sense of larger black holes. Specifically, the “supermassive” variety that is billions of times more massive than our sun and exerts such a strong gravitational force on the space around them that not even light can escape.
So far, we’ve spotted two of these huge black holes using a new global array of radio telescopes called the Event Horizon Telescope. One was observed at the center of our own Milky Way galaxy. The other was spotted in the heart of Messier 87a galaxy 54 million light-years away.
A black hole contains the closest thing to a singularity – an exception to the accepted rules of physics – that we can observe directly with our telescopes. In the dark and impossibly dense core of a black hole, our understanding of the universe crumbles. As Perivolaropoulos said.
There is something special about such a large and dense object. And that something special could help the biggest black holes survive every time the universe bounces around and everything else is compacted into a kind of smooth paste of matter and energy.
Their survival could be the key. Maybe, just maybe, it’s no coincidence that black holes endure and retain their unique weirdness, when everything around them contracts into homogeneity. Maybe black holes are one of the reasons the universe is able to bounce back return after one of its contractions once every 30 billion years or so.
According to Pérez and Romero, there is reason to believe that large black holes, still intact after a cosmological bounce, are helping the universe to rebuild itself by pumping matter back into space and mixing the newly expanding matter with their energy.
“If black holes cross the bounce, they can produce disturbances that would give rise to structure and the early formation of galaxies in the expansion phase,” they wrote. Black holes can act as engines of creation or recreation, if you will, helping form galaxies, stars, and planets in a bouncing galaxy.
“Perhaps black holes are one of the reasons the universe is able to bounce back from one of its contractions once every 30 billion years or so.”
It’s an attractive idea. Especially in light of another theory that is gaining credence (along with the idea of a cosmological bounce) that there are supermassive black holes at the center of every galaxy. We haven’t found them all yet.
To be fair, Pérez and Romero aren’t the first cosmologists to explore the relationship between a bouncing universe and large black holes. Bernard Carr and Timothy Clifton from Queen Mary University of London, alongside Alan Coley from Dalhousie University in Canada, wrote about black holes have been surviving cosmological bounces for years now. “The math we’ve done suggests it’s possible,” Coley told The Daily Beast.
The difference is that, in the model of Coley and his co-authors, black holes are embedded in the surrounding structure of the contracting universe rather than being contained within. This would make it easier for black holes to survive, even if the elements inside the structure of the universe collapse in on themselves.
In the thought of Pérez and Romero, black holes are on the inside the structure. “They’re looking at a slightly different model,” Coley said. In this conception of the bouncing universe, black holes are even more resilient than anyone previously imagined – and potentially more important to the further expansion of the universe.
If there’s a danger in the corner of cosmology that Pérez and Romero share with Coley and his co-authors, it’s that the hard data on bouncing universes and enduring supermassive black holes is pretty thin. Our space probes are few and far between. We can only see so far with older telescopes.
To better understand a possible black hole-assisted cosmological bounce, we need to find more black holes. Especially the bigger ones at the center of the galaxies. We also need better measurements of the background radiation of the universe. A fine reading of the radiation could indicate cycles of expansion and contraction.
The good news is that these sightings may soon be possible. New BICEP network, a suite of four radio telescopes under construction at the South Pole, could give us good radiation readings from the next few years. And we can expect more images (and even a few movies) of large black holes from the Event Horizon Telescope.
If cosmologists like Pérez, Romero and Coley start finding black holes everywhere and also record the telltale radiation patterns of a bouncing universe, then maybe we should start making peace with the idea that whatever we can see and imagine is far less unique than we previously thought.
In fact, we could be living in the third, hundredth, or thousandth version of the universe after repeated bounces, each fueled in part by ever-larger black holes.
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