Extreme Events: When Black Holes Collide

New science has come forward from a team of astronomers who, earlier this year, discovered a pair of black holes in a close orbit, heading toward a cataclysmic merger.  The new results suggest that this incredibly powerful collision will occur much sooner than previously thought, as little as 100,000 years from now (A blip on the radar of astronomical timescales). By precisely calculating the individual and relative masses of the black holes, the team was able to predict how the merger would take place, giving a time line for the collision.

A black hole merger is expected to release the gravitational waves predicted by Einstein, but not yet detected. Above, an artist’s conception of waves rippling through space-time. Credit: NASA

The astronomers, from Columbia University, saw bright flashes of light from the Quasar PG 1302-102, and concluded that they were the result of a binary system of black holes that would eventually merge and produce a huge ripple of gravitational waves.

“This is the closest we’ve come to observing two black holes on their way to a massive collision,” said the study’s senior author, Zoltan Haiman, an astronomer at Columbia. “Watching this process reach its culmination can tell us whether black holes and galaxies grow at the same rate, and ultimately test a fundamental property of space-time: its ability to carry vibrations called gravitational waves, produced in the last, most violent, stage of the merger.”

Supermassive black holes live in the center of massive galaxies, including our own Milky Way. They tend to weigh millions of solar masses and dominate the galaxies in which they reside.  When one such black hole is accereting material rapidly, the surrounding accretion disk is heated to extreme temperatures, causing it to shine brightly.  Quasars are galaxies where the energy output of the black hole and its resulting accretion disk outshine the stellar population, and can be seen across the universe.

Usually quasars brighten and dim randomly, but if there is a regular timing to the brightening and dimming of a quasar, it could be an indication of a binary black hole whose orbital period is creating the periodic flashes of light.

A computational algorithm developed by Matthew Graham, at the California Institute of Technology, was tasked with finding this periodic variation of light in a population of 247,000 quasars.  The algorithm found 14 candidate binaries, and in a January study in Nature, the algorithm showed that PG 1302-102 appeared to brighten by 14 percent every five years, indicating the black hole pair was less than a tenth of a light-year apart, closer than any such pair discovered. This summer, Graham and his colleagues found 90 new candidates, adding to the original 14 he found. This increase in binary black hole discoveries has given astronomers an opportunity to look for collisions and give a prime target for the first detection of the elusive gravity waves.

It always ends up being a little bit abstract when we talk about objects like black holes and gravity waves.  Because of the immense energy and forces involved, we have to go beyond the ordinary physics of human experience to the stranger concepts that explain the universe. Sometimes, the more we learn, the stranger the universe gets.

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