Whatever gave birth to this monster can be real proud. The biggest black hole in the universe weighs in with a respectable mass of 18 billion Suns, and is about the size of an entire galaxy. Just like in the Arnold Schwarzenegger and Danny Devito flick “Twins”, the massive black hole has a puny twin hovering nearby. By observing the orbit of the smaller black hole, astronomers are able to test Einstein’s theory of general relativity with stronger gravitational fields than ever before.
The biggest black hole beats out its nearest competitor by six times. Fortunately, it’s 3.5 billion light years away, forming the heart of a quasar called OJ287. Quasars are extremely bright objects in which matter spiraling into a giant black hole emits large amounts of radiation.
The smaller black hole, which weighs about 100 million Suns, orbits the larger one on an oval-shaped path every 12 years. It comes close enough to punch through the disc of matter surrounding the larger black hole twice each orbit, causing a pair of outbursts that make OJ287 suddenly brighten.
General relativity predicts that the smaller hole’s orbit itself should rotate over time, so that the point at which it comes nearest its neighbor moves around in space. This effect is seen in Mercury’s orbit around the Sun, on a much smaller scale.
In the case of OJ287, the tremendous gravitational field of the larger black hole causes the smaller black hole’s orbit to precess at an impressive 39° each orbit. The precession changes where and when the smaller hole crashes through the disc surrounding its larger sibling.
About a dozen of the resulting bright outbursts have been observed to date, and astronomers led by Mauri Valtonen of Tuorla Observatory in Finland have analysed them to measure the precession rate of the smaller hole’s orbit. That, along with the period of the orbit, suggests the larger black hole weighs a record 18 billion Suns.
So just how big can these bad boys get? Craig Wheeler of the University of Texas in Austin, US, says it depends only on how long a black hole has been around and how fast it has swallowed matter in order to grow. “There is no theoretical upper limit,” he says.
The most recent outburst occurred on 13 September 2007, as predicted by general relativity. “If there was no orbital decay, the outburst would have been 20 days later than when it actually happened,” Valtonen told New Scientist, adding that the black holes are on track to merge within 10,000 years.
Wheeler says the observations of the outbursts fit closely with the expectations from general relativity. “The fact that you can fit Einstein’s theory [so well] … is telling you that that’s working,” he says.