The discovery of gravitational waves has ushered in a new era of astronomy, and thanks to data from LIGO – the gravitational wave observatory – we can now study the black hole mergers that cause them.

In a paper published in Nature, an international team of physicists has used advanced numerical simulations to study the history of the black holes responsible for the first detection of gravitational waves. Their models suggest this event involved previously massive stars (40 to 100 times the mass of the Sun) that most likely formed together 2 billion years after the Big Bang.

“The universe isn’t the same everywhere,” Richard O’Shaughnessy from the Rochester University of Technology (RIT), a co-author of the study, said in a statement. “Some places produce many more binary black holes than others. Our study takes these differences into careful account.”

Stars only form when large hydrogen clouds are cool enough to condense and collapse on themselves. Elements such as carbon and oxygen are actually quite good at cooling down gas, so stars lacking heavier elements have to be larger and more massive.

The binary star progenitors in this study must have had less than 10 percent of the heavy elements, referred to in astronomy as metals, that we see in the Sun. This strongly suggests that they formed in the early universe when the interstellar gas was more metal-poor, although it’s possible that they formed more recently in an uncontaminated nebula.

“Because LIGO is so much more sensitive to these heavy black holes, these regions of pristine gas that make heavy black holes are extremely important,” O’Shaughnessy said. “These rare regions act like factories for building identifiable pairs of black holes.”

Once the second-generation LIGO reaches full sensitivity, the researchers expect it to be able to spot 1,000 of these events every year with total masses between 20 and 80 solar masses.

LIGO is made of two L-shaped laser interferometers, which can detect subtle changes due to a passing gravitational wave. In the experimental run that finished in January, the observatory detected two signals – the February one and one announced last week.

The observatories will be restarted later this year and they won’t be alone. Virgo, a European interferometer, will be back online after being upgraded, which means we will shortly be getting a completely new look into black holes.