Twin Supermassive Black Holes on Verge of Collision

September 22, 2015 | Sarah Tse

artist's impression of two merging black holes
Photo credit: NASA

Astrophysicists recently discovered that two supermassive black holes are headed on a crash course. The cataclysmic event will provide the rare opportunity for scientists to witness never-before-seen gravitational waves and to uphold Einstein’s theory of general relativity.

A pair of supermassive black holes are spiraling towards an explosive fusion that will reverberate throughout the cosmos, according to new predictions by astronomers at Columbia University. The black holes lie at the heart of a distant galaxy within the Virgo constellation, and they are separated by a mere light-week. Once these black holes collide, they could produce gravitational waves with as much energy as 100 million supernova explosions.

They were discovered earlier this year when computational astronomers from Caltech created an algorithm for discerning characteristic light signals produced by black holes. These signals, called quasars, are emitted as a black hole gobbles up the surrounding dust and gas. They normally flicker at random intervals, but when the scientists notice a regular pattern instead, they can deduce that two black holes are headed toward an epic collision.

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The algorithm found 20 pairs of binary black holes that displayed these steady pulses of light. One of the systems, named PG 1302-102, appeared to be moving the fastest — its light emissions increased by 14 percent each year, which hinted that the two black holes were separated by less than a tenth of a light-year. The closest pair of black holes to be discovered up to that point were separated by 20 light-years, and scientists had believed that binary black holes could not overcome the massive energy barrier required to get any closer — a dilemma called “the final-parsec problem,” named after that final parsec of distance separating the two black holes.

So, Dr. Zoltan Haiman and his colleagues, the authors of this latest study, set out to build a model that would explain the behavior of the quasar’s rhythmic beams of light. If the two black holes were truly that close, then the pair must consist of one smaller black hole revolving 7 percent of the speed of light around a second larger black hole. The light emitted by the smaller black hole would become brighter as its orbit brings it closer to observers on Earth, in the same way that the Doppler effect amplifies the siren of a police car as it approaches the listener. The model predicted that the quasar’s ultraviolet emissions would vary in intensity over a five year cycle — which is exactly what the scientists found when they analyzed ultraviolet data from NASA’s Hubble and GALEX space telescopes.

Haiman’s team calculated the masses of the black holes to predict when they would merge. Their best estimate is that the black holes will converge 100,000 years from now — eons in terms of human lifetimes, but a split second on the cosmic timescale.

The most thrilling aspect of watching the progress of this imminent collision is that astrophysicists may finally detect gravitational waves, the wrinkles in space-time predicted by Albert Einstein’s theory of general relativity. He predicted that only the most violent events can release enough energy to send these vibrations throughout the universe.

So far, scientists have yet to find any tangible evidence of gravitational waves. But that may change, thanks to the unprecedented proximity of these black holes. As they inch ever closer together, the increasing interactions of their colossal masses and steep gravitational fields should begin to throw off gravitational waves that will disrupt other galactic phenomena. We will see these anomalies in our data much sooner than the impending merger — perhaps within the next few years.

The researchers also developed new tools for detecting pairs of black holes and predicting their collision. Within the next decade, we may witness a different collision and its accompanying gravitational waves. According to the study's lead author, Daniel D'Orazio, a graduate student at Columbia, "the detection of gravitational waves lets us probe the secrets of gravity and test Einstein's theory in the most extreme environment in our universe — black holes. Getting there is a holy grail of our field."

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