‘A different answer about how black holes formed in the universe,’ says researcher of origins of black holes

Just exactly how black holes form in the universe cannot be deduced from current measurements, researchers say.

A new study shows that the current catalog of black hole binaries is insufficient to provide insight into how black holes form.

Focusing on the way black holes tilt and spin, before they merge, can provide details as to their origin from a vibrant cluster of stars or a silent galactic disk. Just which one of these theories is more likely is what astronomers hope they will conclude by studying 69 known binaries.

The data can be interpreted differently, however, when viewing models of black hole formation that examine the known binaries and their spins. In the study, appearing in the journal Astronomy and Astrophysics Letters, MIT physicists showed that a black hole's origins can be "spun" in different ways, depending on a model's assumptions of how the universe works.

"When you change the model and make it more flexible or make different assumptions, you get a different answer about how black holes formed in the universe," says study co-author Sylvia Biscoveanu, an MIT graduate student working in the LIGO Laboratory.

Black holes in binary systems are believed to form by two methods. The “field binary evolution” is when two stars merge together and ultimately explode in a supernova resulting in two black holes that continue circling in a binary system. In this method, the black holes have similar spins, which leave scientists to believe that they evolved from a galactic disk.

When two black holes evolve separately with a unique tilt and spin, they are formed in a "dynamical assembly." An unknown astrophysical process then draws the black holes together, forming a binary system, with randomly-oriented spins that would be formed in a globular cluster.

Astronomers have derived the spins of black holes in 69 binaries to date, discovered by a network of gravitational-wave detectors, including LIGO in the U.S., and its Italian counterpart, Virgo. 

Researchers have projected the 69 black holes' properties, including their mass and spin, and found that binaries could have both an aligned spin, along with random spins, meaning black holes could be formed by both galactic disks and globular clusters.

The MIT team concluded, however, that the data also provided different results when input into different theoretical models of the creation of black holes. Thus, astronomers need more information to make a statement that is unrelated to the astrophysical beliefs they make.