These gravitational waves are created when two black holes orbiting each other spiral inwards and merge, producing a massive burst of energy. Last week the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced it had found a second such ripple in space-time.

But how do you make a binary black hole in the first place? Start with a binary star, says Krzysztof Belczynski of Warsaw University, Poland.

Belczynski and colleagues modelled how binary star systems could evolve into binary black holes like the source of GW150914, the first LIGO signal. In one example, they started with two stars formed around 2 billion years after the big bang, one 96 times the mass of the sun, the other 60 times.

After around 3.5 million years, the stars got close enough to shift material from the larger to the smaller, eventually collapsing the larger star into a black hole. For a few million years more, the black hole and the star shared an envelope of gas, drawing them closer before the second star collapsed, leaving two black holes of 37 and 31 solar masses. These happily coexisted for another 10 billion years before colliding.

The team calculates that there should be 218 such mergers each year in a particular volume of space, a cube 3.26 billion light years on a side. LIGO’s rate of detection so far implies the universe produces between 9 and 240 black hole mergers in the same time and volume, so the model fits.

They also looked at two other possible histories, one in which black holes wobble slightly after forming, and another in which they spend more time in the common envelope. But these produced too few or too many mergers to match LIGO’s data.

Thrashing about

Not everyone agrees this is the best way to form binary black holes, however. Last week, Carl Rodriguez of Northwestern University in Evanston, Illinois, and his colleagues suggested most binary black holes are created in a different way: from many individual holes thrashing around in dense stellar regions known as globular clusters. They predicted a rate of between 5 and 10 mergers – still consistent with LIGO.

With the scant data collected so far, the competing sides are playing a bit of a game to prove their model was correct all along.

“If globular cluster rates are too low, they will crank their numbers up. I can crank my numbers down. We can all do that,” says Belczynski. Frederic Rasio, also at Northwestern University, points out that in an earlier paper, Belczynski’s team predicted zero black hole mergers.

“The game now is to try and narrow down which classes of models are correct,” says Mark Hannam of Cardiff University, UK. “People are starting to do these studies as we actually have some data to play with.”

It’s likely we’ll never know the exact history of the black holes behind GW150914, but as LIGO gathers more data, physicists will be able to see which formation method dominates in the universe. Statistics on the black holes’ masses and spins should reveal which model is best, as the rate of mergers alone is likely not enough to distinguish between them.

“What we really need is a population of binary black hole mergers from LIGO, so we can start comparing all of our model predictions in detail,” says Rodriguez. “If I were a betting man, I would say the numbers are lower than this current paper suggests. But the nice thing is, after a few more detections, we won’t need to place any bets at all.”

Source: New Scientist