Roughly 1.2 billion years ago a pair of circling black holes swirled closer and closer, shedding shudders of gravitational energy before they collided. Although the black holes had likely orbited each other for billions of years, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) only caught the event’s final 0.2 second. Witnessing the gravitational waves produced during that parcel of time, however, ushered in a new era of astrophysics, and scientists now want to understand how this duo and others like it end up in their circling embrace in the first place. Researchers are setting out on a new quest to pinpoint exactly where these black hole mergers occur throughout the universe by building new theoretical models and new observatories across the world.
For Astrid Lamberts, an astronomer at California Institute of Technology, that journey started over coffee. In February, when LIGO announced its first detection of gravitational waves, she excitedly discussed the details of the event with her colleagues. But she had one burning question: In what type of galaxy did the merger occur? Lamberts suspected that the answer might lie in the heavy masses of both black holes—both roughly 30 times the mass of our sun—which likely formed from the deaths of low-metallicity stars—that is, stars with fewer heavy elements in them, called metals. (The metallicity of a star changes its chemistry, and stars with more metals tend to expel more of their mass when they die, leaving behind less massive black holes.) Low-metallicity stars are known to populate both small nearby galaxies and large distant galaxies, but she did not know which would have the upper hand in producing LIGO’s signal. So Lamberts approached her colleague Philip Hopkins—an expert in galaxy evolution—but he could not come up with an easy answer either. The fact that there was no clear-cut solution compelled both to dig further.
In a paper recently published to the preprint server arXiv and submitted to Monthly Notices of the Royal Astronomical Society Letters, the team determined the most likely characteristics of the host galaxy based on the masses of the two black holes alone. The key came when Lamberts and her colleagues incorporated a second aspect into their model: the time window from when these stars formed to when the black holes merged. “If they formed too recently, they won’t have actually had time to evolve into black holes and merge yet,” Hopkins says. “And if they formed too early, they would have merged long ago in the past.” Instead, there is an array of times at which these stars could have formed. So the team looked at each of those times and determined which types of galaxies existing then contained the most low-metallicity stars.
At the end of the day, Lamberts and her colleagues found two viable possibilities. Either the progenitor stars formed eight billion to 10 billion years ago and merged together in a galaxy as heavy as the Milky Way or they formed between five billion and eight billion years ago and merged together in a dwarf galaxy, some 1,000 times less massive than the Milky Way. Right now the team is unable to choose one scenario over the other, but Hopkins suspects that they will be able to do so with more future detections.
Meanwhile additional gravitational-wave observatories beyond LIGO’s twin detectors in Louisiana and Washington State are being built across the globe to better tackle this very question. The VIRGO detector in Italy should begin its run in early 2017, the KAGRA detector in Japan will commence observations as early as 2018 and another detector built in LIGO’s exact image in India will start as early as 2023. For LIGO’s first two detections (the finding announced in February, and a second black hole collision reported earlier this month), scientists could pinpoint the mergers’ locations to within 600 square degrees of sky—an area so large that there are tens of thousands of galaxies within it that lie roughly 1.3 billion light-years away. But with five detectors online around the world scientists could measure the difference in arrival times of the gravitational waves at the various observatories to pinpoint a merger’s location to within a few square degrees at best, dropping that range by an order of magnitude.
Even then, there will likely be thousands of galaxy candidates that could host a particular black hole collision. To definitely pinpoint a black hole collision’s location to a single galaxy, scientists will likely need to detect an electromagnetic counterpart—that is, a flash of light, be it visible, x-ray or gamma ray—occurring at the same time as the gravitational waves. Scientists would then be able to look to the origin point of that light to locate the host galaxy of the black holes. (Although scientists don’t expect most black hole collisions to produce light, they could be surprised.) For this reason, with every gravitational-wave discovery LIGO scientists immediately send coordinates to tens of observatories across the globe so that they, too, can look for a signal. “One of the major science goals for the gravitational-wave community is the ability to sift through our data almost in real time and rapidly identify [a merger’s] location on the sky,” says LIGO executive director David Reitze. The astrophysicist is particularly excited about how scientists can use future versions of Lamberts’ model to better target follow-up searches. If astronomers know the mass of the host galaxy, for example, they can prioritize those galaxies first, he says.
Future results should shed light on how duos of spiraling black holes formed in the first place. At the moment, there are two leading theories: One is that the two stars that gave rise to the black holes are born, live and detonate together, always locked in a swirling embrace before they finally merge as black holes. The other is that the progenitor stars are not born together; they die and become black holes far from each other, then something—say the gravitational shove of another object—pushes them into a mutual embrace. Determining which formation mechanism is correct will likely come down to identifying the merger’s host galaxy. If these events continually point to galaxies that likely contain high numbers of stellar clusters, for example, where gravitational pushes and pulls can easily shove estranged pairs of black holes together, then the second scenario is more likely. If, however, these events implicate galaxies that contain very few stellar clusters, then it is more likely the stars had been locked together throughout their lifetimes.
The race to discover the location and backstory of these black hole pairs is a thrilling new challenge for the many scientists who have eagerly awaited LIGO’s first gravitational-wave discoveries. “We've been wondering around in the desert—if you'll let me use a biblical analogy—for about 40 years,” Reitze says. “Literally it was about 40 years from when LIGO was conceived to the time we made a detection. And now we've just walked into the promised land. So we've got to explore the promised land. Who knows how big it is. Who knows what we'll find. But I'm sure it's going to be exciting.”