“Basically, the more massive the holes, the faster gravitational waves drive them together, and we do require them to be as massive as allowed to be,” he said in an email. For their model to hold up, the larger of the black holes has to be a billion solar masses or more.

(Because the galaxy is so far away, the cataclysm might well have already happened more than three billion years ago, but the news still won’t reach us for another 100,000 years or so. So it is still in the future, as far as earthlings — whoever they will be by then — are concerned.)

E. Sterl Phinney, a Caltech astronomer and expert on supermassive black holes, agreed that Dr. Haiman’s model explained the quasar variations. “So Occam’s razor makes it attractive,” he said in an email, referring to the long-held principle that physicists should adopt the simplest theory that fits the facts. But it was surprising, he said, to find two supermassive black holes that have gotten so close.

Black holes, predicted by Albert Einstein’s general theory of relativity, are objects so dense that not even light can escape from them. Every galaxy of note seems to have a supermassive black hole, weighing millions or billions of times as much as the sun, burping sparks of half-eaten stars and gas.

When galaxies merge, their resident black holes are sent into forced marriages, orbiting each other. But without gravitational interactions with stars or interstellar gas, supermassive black holes can’t get close enough to each other to go into a rapid death spiral, a situation known as the “final parsec” problem. (A parsec is the astronomical standard of distance, 3.26 light-years.)

So, as Dr. Phinney explained, unless hundreds of millions of solar masses of gas accompany the black holes, “there are not very convincing ways of getting them to smaller separations” like the black holes in PG 1302-102.

At least that is the theory. If such systems are common, Dr. Phinney said, the gravitational waves emanating from them should sweep the universe and disrupt the timing of signals from pulsars, an effect that could be detected within the next few years by various continuing programs to time pulsars.

“A scientific theory is only as good as the tests which it has passed,” Mr. D’Orazio said in an email. Although general relativity has passed all of the observational and experimental tests thrown at it so far, some of its predictions can be tested only in the most extreme gravitational environments, namely black holes. “Detection of gravitational waves,” he said, “is a direct probe of this region and hence the secrets of gravity.”