Are Two Giant Black Holes About to Collide?

A flickering quasar may be signaling that the most colossal of cosmic smashups could be just a few years away.

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Each of these galaxies, caught in the act of collision by NASA’s NuSTAR telescope, has a giant black hole in its core. Eventually, the black holes will collide in a titanic smashup, sending out a blast of gravitational waves.

A collision between two giant black holes is the most titanic smashup astronomers can imagine. Nobody’s ever seen it happen—but if a new report in Astrophysical Journal Letters is correct, they might not have long to wait.

A rhythmically flickering light near the edge of the observable universe, say the authors, betrays the presence of two enormous black holes, totaling ten billion stars’ worth of mass between them and orbiting each other so tightly that they might have only two decades before the crash happens. “It’s amazing if true,” says NYU theorist Andrew MacFadyen, who was not involved in the research.

The idea that giant black holes could slam into each other isn’t the amazing part. Astronomers are convinced it was relatively common in the early universe, when the cosmos was smaller than it is today and galaxies were packed more closely together. Those galaxies would occasionally collide and merge, and since virtually all galaxies (including our own Milky Way) harbor huge black holes at their cores, the black holes would merge sooner or later as well.

When that happens, says Einstein’s General Theory of Relativity, the collision should release a powerful burst of gravitational waves—ripples in the very fabric of spacetime itself. Physicists have built enormous instruments to detect those waves, which would be a ringing confirmation of the theory. (See Beyond the Big Bang: Einstein’s Evolving Universe.)

The pulsing light that University of Maryland astronomer Tingting Liu and her co-authors saw, known as a quasar, suggests that we might get to see that happen for the first time in the not-too-distant future. That’s what McFadyen considers amazing. Quasars are huge black holes gulping down enormous quantities of gas; the light is generated when the gas heats up to incandescence as it swirls around the black hole in a formation called an accretion disk.

But the flickerings that all quasars tend to have usually happen at random. In this quasar, known as PSO J334.2028+01.4075, the authors claim it’s happening with a regular rhythm, brightening about once every 542 days.

The best explanation, argue Liu and her colleagues, is that the accretion disk of hot gas is swirling around not a single black hole, but a pair that are orbiting each other. “The disk is not symmetrical, for some reason,” says Liu, “so one of the black holes has easier access to the gas.” Once every orbit, she is convinced, that black hole interacts with the disk in some way to cause a flare-up.

Cosmic Carnage

If Liu and her co-authors are right about the 542-day period, the black holes are practically on top of one another in cosmic terms—only 0.02 light-years apart, or about one two-hundredth the distance from the sun to Alpha Centauri, the nearest star. And if that’s the case, they say, the crash itself is only about 21 years away. “A study came out a few months ago in Nature that identified a similar system,” says co-author Suvi Gezari, also at Maryland. “But ours is more fun, because our black holes are even bigger and even closer together.”

The idea that such a long-hoped-for test of Einstein’s theory is just around the corner is exciting for theorists, and it could speed the construction of a new, ultra-sensitive gravitational-wave observatory to watch the cosmic carnage.

But other astrophysicists aren’t convinced it’s quite that imminent. In fact, says Caltech astrophysicist Matthew Graham, lead author of that recent Nature study, “if we analyzed our system the way they did, we’d be expecting a collision in only five years. But we didn’t cite an anticipated merger time because we think it’s far too uncertain. Until you truly understand the mechanism behind the flare-up, you can’t really know what stage of the process you’re in.”

Graham isn’t the only one treading with caution. “We all love this idea,” McFadyen says, but some are more skeptical than others. One reason to be wary, he says, is that the merger process probably stretches out over millions of years. “It’s statistically very unlikely that we’d be lucky enough to catch two supermassive black holes just two decades before the final collision.”

Johns Hopkins astronomer Julian Krolik, meanwhile, another outsider, isn’t convinced that the 542-day period the report's authors claim is actually there. Such patterns, he says, “often turn out to be spurious when you observe over a longer period.”

That’s what Liu and her colleagues hope to do. They’re also looking at observations from the Pan-STARRS sky survey, which turned up PSO J334.2028+01.4075 in the first place, for more examples. They’re also eagerly awaiting the new Large Synoptic Survey Telescope, slated to go into operation in 2021, which will look over a wider and deeper swath of sky. (See Cosmic Vision).

One way or another, says Gezari, “We’ll see in 21 years whether we’re right or wrong. Either way,” she says, “we won’t have long to wait.”

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