Yet with a sample size of one, such assertions could only be educated guesses, says Carl Rodriguez, an astrophysicist at Carnegie Mellon University.
Now data from LIGO’s latest catalog show that black-hole binaries are far less common than expected. In fact, the rate of merging black holes now observed could be “entirely explained” by star clusters, according to a preprint paper posted late last month by Rodriguez and his collaborators.
In addition, the new mergers have enabled a fresh approach to the puzzle of where black holes come from. Despite their elusive nature, black holes are very simple. Aside from mass and charge, the only trait a black hole can have is spin—a measure of how quickly it rotates. If a pair of black holes, and the stars from which they form, live their whole lives together, the constant push and pull will align their spins. But if two black holes happen to encounter each other later in life, their spins will likely be unequal.
After measuring the spin of the black holes in the LIGO data set, astronomers now suggest that the dynamic and isolated scenarios are almost equally likely. There is no “one channel to rule them all,” wrote the University of Chicago astrophysicist Michael Zevin and collaborators in a recent preprint outlining many different pathways that together can explain this new and growing population of black-hole binaries.
“The simplest answer is not always the correct one,” Zevin says. “It’s a more complicated landscape, and it’s certainly a bigger challenge. But I think it’s a more fun problem to address as well.”
LIGO and its sister observatory Virgo have grown more sensitive over time, which means they can now see colliding black holes that are much farther away from Earth and much further back in time. “We’re listening to a really big chunk of the universe, out to when the universe was much younger than it is today,” Fishbach says.
In a recent preprint, Fishbach and her collaborators found indications of differences in the types of black holes observed at different points in cosmic history. In particular, heavier black holes seem to be more common earlier in the universe’s history.
This came as no surprise to many astrophysicists; they expect that the universe’s first stars formed from huge clouds of hydrogen and helium, which would make them much bigger than later stars. Black holes created from these stars, then, should also be huge.
But it’s one thing to predict what happened in the early universe, and another to observe it. “You can really start to use [black holes] as a tracer of how the universe formed stars over cosmic time—and how the galaxies that form those stars and star clusters are assembled,” Rodriguez says. “And that starts to get really cool.”
The study is a first step toward using large data sets of black holes as a radical tool to explore the cosmos. Astronomers have created an astonishingly accurate model of how the universe evolved, known as Lambda-CDM. But no model is perfect. Gravitational waves offer a way to measure the universe that’s completely independent of every other method in the history of cosmology, says Salvatore Vitale, an astrophysicist at the Massachusetts Institute of Technology. “If you get the same results, you’ll sleep better at night. If you don’t, that points to a potential misunderstanding.”