There are many mysteries surrounding black holes, and now a team of astronomers has come up with a new 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" with the expansion of the universe.
In 2015, scientists used the laser interferometer gravitational wave observatory (ligo) to discover for the first time the ripples of gravitational waves – the structure of space-time itself. These waves are produced during catastrophic events, most commonly collisions between black holes, from which astronomers can reverse calculate the masses of the two objects involved in the merger. However, they noticed something strange.
The most common type of black hole, and the one the team expects to see in most mergers, is a stellar black hole. These black holes are formed by the collapse of massive stars and are expected to have about 5 to 30 times their masses as the Sun. But ligo's team detected several much more massive black holes — for example, the largest collision on record was between two black holes with masses of 65 and 85 suns, respectively.
So how did these black holes become so big? The most common explanation is that they grow by devouring matter, including dust, gas, stars, or other black holes. But the scientists for the new study have now come up with a rather bizarre alternative — that the mass of a black hole may grow as the universe expands, which the team calls the "cosmological coupling" effect.
This may sound a little strange, but the idea is not completely without precedent. This possibility is hinted at by Einstein's theory of relativity, and light is already a cosmological coupling because it loses energy as the universe expands, providing energy for that expansion.
"We think to consider the opposite effect," said duncan Farrah, co-author of the study. "What would ligo-virgo observe if black holes were cosmically coupled and they gained energy without consuming other stars or gases?"
The team notes that when black holes are modeled, they usually do so in a simulated universe that doesn't expand. This is for simplicity, but may obscure any effects of cosmological coupling. So, in the new study, the researchers ran simulations that took into account this swelling.
They simulated millions of stars, forming black holes through their birth, life, and death —importantly, they linked the black hole's mass to the size of the simulated universe. This means that these pairs of black holes become more and more massive over time as they spiral closer to each other and eventually collide.
Perhaps not surprisingly, the black holes that formed after the merger were more massive, but this also seems to have led to more mergers occurring. To be sure, these predictions seem to match quite well with the data from ligo-virgo collaboration.
The researchers say the new model works well because it doesn't require any modifications to our existing understanding of how stars form, live and die. But of course, the problem is far from solved — the current idea of black holes devouring matter and devouring each other could be a simpler explanation.
"Many aspects of black hole mergers are not well known, such as the major formation environments and the complex physical processes that run through their lives," said Michael Zevin, co-author of the study. "Although we used a simulated stellar colony that reflects the data we currently have, there is still a lot of room for maneuver. We can see that cosmological coupling is a useful idea, but we cannot yet measure the intensity of this coupling. ”
This idea could be tested as gravitational-wave observatories become more sensitive, especially as new observatories such as space-based lisa join the search.
The study was published in the Astrophysical Journal Letters.