Do black holes gain mass as the universe expands?

In 2015, the LIGO facility carried out the first ever detection of gravitational waves - ripples in the fabric of space-time itself. These are produced in cataclysmic events, most commonly collisions between black holes, and astronomers can work backwards from the waves to calculate the masses of the two objects involved in the merger. And they noticed something odd.

The most common type of black hole, and the type the team expected to see involved in most mergers, are stellar black holes. These are formed by the collapse of massive stars and are expected to have masses between five and about 30 times that of the Sun. But the LIGO team has detected several black holes with much higher masses - for example, the most massive collision ever recorded. it was between two black holes with masses of 65 and 85 Suns.

So how did these black holes get that big? The most common explanation is that they grow by engulfing matter, including dust, gas, stars, or other black holes. But the researchers of the new study have now proposed a fairly extravagant alternative: that the masses of black holes could grow as the universe expands, in an effect the team calls cosmological coupling.

The In fact, this idea is not entirely new, the possibility, in fact, is implicit in Einstein's theory of relativity, and light is already a kind of cosmological coupling, as it loses energy as the universe expands, fueling that same expansion. "We thought we would consider the opposite effect," said Duncan Farrah, co-author of the study. "What would LIGO observe if black holes were cosmologically coupled and gained energy without the need to consume other stars or gases?"

For the new study, the researchers ran simulations that took this expansion into account. They simulated millions of pairs of stars through their birth, life and death to form black holes and, above all, they linked the masses of black holes to the size of the simulated universe. This meant that these pairs of black holes became more massive over time as they spiraled towards each other and eventually collided. Researchers say the new model works well because it doesn't it requires no modification to our current understandings of how stars form, live and die. But, of course, the question is far from resolved: the current idea of ​​black holes devouring matter and one another could still be the simplest explanation.