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Astronomers Say Black Holes Are Gaining Mass as the Universe Grows

A new model suggests they're 'cosmologically coupled'.

Astronomers Say Black Holes Are Gaining Mass as the Universe Grows
An abstract depiction of an active black hole. Elen11 / iStock

Sometimes nature unlocks when we examine the way we look at it.

Black holes remain a focal point for cosmic mysteries, one of which asks the question: why do black holes frequently possess more mass than initially predicted? Now, a team of astronomers has suggested a new model to solve this predicament, proposing that black hole growth might be "cosmologically coupled" to the expansion of the entire universe, according to a recent study published in the journal Astrophysical Journal Letters.

And this could lend greater sensitivity to gravitational wave observatories, including future ones set to launch into space.

Black holes and the mystery of excess mass

Back in 2015, the LIGO facility detected gravitational waves for the first time. Gravitational waves are literal ripples racing through the underlying fabric of spacetime, and are generated by unconscionably violent events in the universe, like the merger of two black holes into a larger one. Astronomers know how to reverse the flow of data from these waves to trace their path backward in time and calculate the masses of the two cosmic monsters that merged. And when they did, something weird happened. The most abundant kind of black holes in the universe, which the team expected to find were responsible for the majority of mergers, are ones with masses comparable to the mass of the sun, called stellar black holes. These are roughly 30 times the mass of our host star, but the LIGO team identified multiple black holes with masses that far exceeded this. The most massive collision ever observed happened between two black holes with masses between 65 and 85 suns.

That's more than twice to nearly three times the mass of the sun, much higher than anticipated. But how did it happen? The conventional explanation suggests they got big by swallowing up unimaginable volumes of matter, like gas, dust, entire stars, or even other black holes. But the researchers' new study thought up another, wilder explanation: black hole masses may grow as the universe does, in a phenomenon the research team is calling cosmological coupling. This isn't something they just made up, and the researchers of the new study aren't the first to consider the idea. Einstein's theory of relativity implied this possibility, and the fact is that light's already cosmologically coupled, in a way.

Cosmological coupling could enhance the sensitivity of future gravitational wave observatories

Namely, light loses energy as the universe grows, which re-enforces that expansion like a feedback loop. "We thought to consider the opposite effect," said Duncan Farrah, co-author of the study, in a New Atlas report. "What would LIGO-Virgo observe if black holes were cosmologically coupled and gained energy without needing to consume other stars or gas?" The research team noticed that the normal way to model black holes is within simulated universes that don't take cosmic expansion into account. This was by convention, to simplify the math, but the researchers suspected it might obscure the effects of cosmological coupling. This is what inspired them to run simulations that included the expansion of the universe.

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The researchers simulated millions of binary star systems throughout the entire scope of their lifetime, from birth to death and transformation into black holes. And, since these black holes were linked to the growth inherent to cosmological expansion, they grew even more massive as the eons passed, spiraling inward until the pair merged in a catastrophic collision. This discovery could help test procedures for gravitational wave observatories achieve a higher sensitivity to future events, and do the same for new gravitational observatories like LISA (which may have a better shot in space), yet to join the hunt for these gigantic cosmic waves. It's an exciting time to look at the universe.

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