In just four years, the Laser Interferometer Gravitational wave Observatory (Ligo) will be so sensitive it will be capable of detecting around 1,000 massive black hole collisions a year.
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The prediction, published in the journal Nature, is based on computer simulations of more than a billion evolving binary stars.
"The main thing we find is that what Ligo detected makes sense," said Daniel Holz, assistant professor in physics and astronomy at the University of Chicago and a co-author of the Nature paper. The simulations predict the formation of black-hole binary stars in a range of masses that includes the two already observed.
The paper's lead author, Krzysztof Belczynski of Warsaw University, said he hopes the results will surprise him, that they will expose flaws in the work. Their calculations show, for example, that once Ligo reaches full sensitivity, it will detect only one pair of colliding neutron stars for every 1,000 detections of the far more massive black-hole collisions.
"Actually, I would love to be proven wrong on this issue. Then we will learn a lot," Belczynski said.
The Nature paper, which includes co-authors Tomasz Bulik of Warsaw University and Richard O'Shaughnessy of the Rochester Institute of Technology, describes the most likely black-hole formation scenario that generated the first Ligo gravitational-wave detection in September 2015. That detection confirmed a major prediction of Albert Einstein's 1916 general theory of relativity.
"Here we simulate binary stars, how they evolve, turn into black holes, and eventually get close enough to crash into each other and make gravitational waves that we would observe," Holz said.
The simulations show the formation and evolution of a typical system of binary stars results in a merger of similar masses, and after similar periods of time, to the event Ligo detected last September. These black hole mergers have masses ranging from 20 to 80 times more than the sun.
Ligo will begin recording more gravitational-wave-generating events as the system becomes more sensitive and is used for longer periods of time. Ligo will go through upgrades over the coming years, and is expected to reach its design sensitivity by 2020.
Ligo has detected big black holes and big collisions, with a combined mass greater than 30 times that of the sun. These can only be formed out of big stars.
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"To make those you need to have low metallicity stars, which just means these stars have to be relatively pristine," Holz said. The Big Bang produced mainly hydrogen and helium, which eventually collapsed into stars.
As these stars burned they forged heavier elements, which astronomers call "metals." Those stars with fewer metals lose less mass as they burn, resulting in the formation of more massive black holes when they die. That most likely happened approximately two billion years after the Big Bang, before the young universe had time to form significant quantities of heavy metals. Most of those black holes would have merged relatively quickly after their formation.
Ligo would be unable to detect the ones that merged early and quickly. But if the binaries were formed in large enough numbers, a small fraction would survive for longer periods and would end up merging 11 billion years after the Big Bang (2.8 billion years ago), recently enough for Ligo to detect.
"That's in fact what we think happened," Holz said. Statistically speaking, "it's the most likely scenario." He added, however, that the universe continues to produce binary stars in local, still pristine pockets of low metallicity that resemble conditions of the early universe.
"In those pockets you can make these big stars, make the binaries, and then they'll merge right away and we would detect those as well."
Ligo is not yet able to precisely measure black hole spin alignment, "but we're starting to get there," Holz added. "This study represents the first steps in the birth of the entirely new field of gravitational wave astronomy. We have been waiting for a century, and the future has finally arrived."