New model suggests black holes gain mass as the universe expands

There are many mysteries surrounding black holes, and now a team of astronomers has proposed a novel solution to one of them – why are so many observed black holes more massive than expected? A new model suggests that their growth may be “cosmologically coupled” to the expansion of the universe.

In 2015, the LIGO facility made the first-ever detection of gravitational waves – ripples in the fabric of spacetime 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 strange.

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

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

It might sound a bit odd, but the idea isn’t entirely unprecedented. The possibility is implied by Einstein’s theory of relativity, and light is already kind of cosmologically coupled, in that it loses energy as the universe expands, feeding that very expansion.

“We thought to consider the opposite effect,” says Duncan Farrah, co-author of the study. “What would LIGO–Virgo observe if black holes were cosmologically coupled and gained energy without needing to consume other stars or gas?”

The team points out that when black holes are modeled, it’s usually done in simulated universes that don’t expand. That’s for simplicity’s sake, but could be obscuring any effects of cosmological coupling. So for the new study, the researchers ran simulations that accounted for this expansion.

They simulated millions of pairs of stars through their birth, life and death to form black holes – and importantly, they linked the black holes’ masses to the size of the simulated universe. That meant these black hole pairs grew more massive over time, as they spiraled towards each other and eventually collided.

Perhaps unsurprisingly, the black holes that formed from the mergers were more massive, but also it seemed to lead to more mergers taking place. And sure enough, the predictions seemed to agree quite well with data from the LIGO-Virgo collaboration.

The researchers say that the new model works well because it doesn’t require any modifications to our existing understandings of how stars form, live and die. But of course, the question is far from settled – the current idea of black holes gobbling up matter and each other could be a simpler explanation.

“Many aspects of merging black holes are not known in detail, such as the dominant formation environments and the intricate physical processes that persist throughout their lives,” says Michael Zevin, co-author of the study. “While we used a simulated stellar population that reflects the data we currently have, there’s a lot of wiggle room. We can see that cosmological coupling is a useful idea, but we can’t yet measure the strength of this coupling.”

The idea could be tested as gravitational wave observatories become more sensitive, and especially as new ones, such as the space-based LISA, join the search.

The research was published in the Astrophysical Journal Letters.

Source: University of Hawaii