The most massive known black hole in the universe has been discovered, weighing in with the mass of 18 billion Suns. Observing the orbit of a smaller black hole around this monster has allowed astronomers to test Einstein's theory of general relativity with stronger gravitational fields than ever before.

The black hole is about six times as massive as the previous record holder and in fact weighs as much as a small galaxy. It lurks 3.5 billion light years away, and forms the heart of a quasar called OJ287. A quasar is an extremely bright object in which matter spiralling into a giant black hole emits copious amounts of radiation.

But rather than hosting just a single colossal black hole, the quasar appears to harbour two - a setup that has allowed astronomers to accurately 'weigh' the larger one.

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, or precess, over time, so that the point at which it comes nearest its neighbour moves around in space - an effect seen in Mercury's orbit around the Sun, albeit on a 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 incredible 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.

A couple of other black holes have been estimated to be as massive, but their masses are less certain, says Valtonen. That's because the estimates were based on the speed of gas clouds around the black holes, and it is not clear whether the clouds are simply passing by the black holes or actually orbiting them.

But Tod Strohmayer of NASA's Goddard Space Flight Center in Maryland, US, says he is not convinced that Valtonen's team has really measured the mass of the large black hole in OJ287 accurately.

That's because only a handful of the outbursts have been measured with high precision, making it difficult to determine if the precession scenario is responsible for the outbursts. "Obviously, if subsequent timings continue to agree with the model, then that would provide further support," he told New Scientist.

Just how big can black holes 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 new research also tested another prediction of general relativity - that the black holes should spiral towards each other as they radiate energy away in the form of gravitational waves, or ripples in space. This radiation affects the timing of the disc crossings and their accompanying outbursts.

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.

The research was presented on Wednesday at a meeting of the American Astronomical Society in Austin, Texas, US.