HomeScience and ResearchSpaceVirtual Universe Sheds New Light On Black Holes Development

Virtual Universe Sheds New Light On Black Holes Development

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With the help of machine learning and supercomputers, it is now possible to figure out how black holes grow over time and find their event horizons and what lies beyond them.

The event horizon is a mysterious, unseen layer that surrounds black holes and is the boundary beyond which nothing, including matter, light, or information, may pass. Every trace of the black hole’s history is absorbed by the event horizon.

“Because of these physical facts,” adds co-author Peter Behrooze.

Behroozi and Haowen Zhang, a doctoral student at Steward, led an international team that used machine learning and supercomputers to figure out how black holes grow over time. This was like peeling back their event horizons to see what was beyond.

Millions of artificial “universes” were simulated, and the results showed that supermassive black holes evolve at the same pace as their host galaxies. For two decades, this connection had been hypothesized, but until recently, it had never been proven. The team’s results were published in the Royal Astronomical Society’s Monthly Notices.

“If you go back to earlier and earlier times in the universe,” adds the co-author, “you find that exactly the same relationship was present.

“So, as the galaxy grows from small to large, its black hole, too, is growing from small to large, in exactly the same way as we see in galaxies today all across the universe.”

It is believed that a supermassive black hole is present at the heart of the most, if not all, of the galaxies distributed across the universe. Many of these black holes have masses that are millions or even billions of times larger than that of the sun. How these behemoths expand as quickly as they do and how they develop in the first place has been one of astronomy’ most puzzling mysteries.

To discover answers, Zhang, Behroozi, and his colleagues developed Trinity, a platform that employs a revolutionary kind of machine learning capable of producing millions of universes on a supercomputer, each of which conforms to a distinct physical theory of how galaxies should form.

The team developed a system where computers make suggestions for how supermassive black holes should expand over time.

They then “observed” the virtual world to see whether it corresponded with decades of actual observations of black holes across the real universe. They utilized those principles to mimic the creation of billions of black holes in the virtual universe.

The computers finally landed on the rule sets that best reflected observed data after millions of suggested and rejected rule sets.

“We’re trying to understand the rules of how galaxies form,” Behroozi adds. “In a nutshell, we make Trinity guess what the physical laws may be and let them go in a simulated universe and see how that universe turns out. Does it look anything like the real one or not?”

Researchers say that this method works just as well for anything else in the universe as it does for galaxies.

The name “Trinity” comes from the fact that the project focuses on three main things: galaxies, their supermassive black holes, and their dark matter halos, which are huge cocoons of dark matter that can’t be seen but are needed to explain how galaxies behave physically.

Millions of galaxies and their dark matter halos were simulated using an older version of the researchers’ framework, the UniverseMachine, in prior studies.

The team found that as galaxies grow in their dark matter halos, the mass of the halo and the mass of the galaxy grow in a very specific way.

In this new experiment they just added black holes to this relationship and then, as explained by Behroozi “asked how black holes could grow in those galaxies to reproduce all the observations people have made about them.”

The simulations also shed light on another mysterious phenomenon: supermassive black holes, like the one at the center of the Milky Way, grew most rapidly in their early stages when the universe was only a few billion years old, but then their growth rate dramatically slowed down over the subsequent 10 billion years or so.

“We’ve known for a while that galaxies have this strange behavior, where they reach a peak in their rate of forming new stars, then it dwindles over time, and then, later on, they stop forming stars altogether,” Behroozi adds. “Now, we’ve been able to show that black holes do the same: growing and shutting off at the same times as their host galaxies. This confirms a decades-old hypothesis about black hole growth in galaxies.”

However, the outcome raises further issues, he adds. Black holes are significantly more compact than the galaxies they inhabit. The supermassive black hole in the Milky Way would be the size of the period at the end of this sentence if it were shrunk to Earth’s size.

For the black hole to double in mass within the same timescale as the bigger galaxy, gas fluxes on dramatically different scales must be synchronized. It is still unclear how black holes and galaxies work together to strike this delicate equilibrium.

The really innovative aspect of Trinity, according to Zhang, is that it gives us a mechanism to determine what kinds of linkages between black holes and galaxies are compatible with a broad range of various datasets and observational techniques.

“The algorithm allows us to pick out precisely those relationships between dark matter halos, galaxies and black holes that are able to reproduce all the observations that have been made,” adds the lead author.

“It basically tells us, ‘OK, given all these data, we know the connection between galaxies and black holes must look like this, rather than like that.’ And that approach is extremely powerful.”

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