Ponder a quasar called OJ287, about 3.5 billion light-years away from us in the constellation Cancer, relatively close as quasars go, a super bright (for its distance), super small source of optical and radio energy. A quasar is thought to be powered by a massive black hole at the center of a galaxy. Energy is emitted as gas and dust are swept into the dark pit of oblivion. Astronomers have been observing OJ287 for more than a century, long before it was recognized as a quasar -- long before such things as black holes and quasars were known to exist.
OJ287 has the curious property of flaring up at 12-year intervals, with two flashes about 2 years apart. The best way to account for the flare-ups is to assume OJ287 consists of two black holes, one with a mass of 18 billion Suns, the other with a mass of 100 million Suns. The smaller black hole orbits the larger in an eccentric orbit with a 12-year period. As the smaller black hole approaches the larger, it dives through the accretion disk of hot matter surrounding the primary, causing a burst of energy, zips around the primary, then 2 years later plunges through the accretion disk on its way out again, stirring up another burst.
Now if this is what is actually going on, then Einstein's theory of general relativity predicts that the orbit of the secondary black hole should precess -- that is, the axis of the orbit should slowly rotate like the axis of a top. The theory predicts by exactly how much the orbit should precess -- 39 degree per orbit -- which affects the timing of the flare ups.
And so it was that in September of last year astronomers waited expectantly on the predicted day, the first since the theory was applied in detail, for the second of the expected flares in the current cycle. And -- bingo -- OJ287 brightened on schedule, in perfect compliance with general relativity. (See Nature, April 17, 2008, p.851)
Now what thrills me about this story is not just that we live in a universe with orbiting black holes, or that galaxies (including our own) have supermassive black holes at their centers, but that the human mind can formulate mathematical laws that apply to objects 3.5 billion light-years away. Einstein said: "The most incomprehensible thing about the world is that it is comprehensible." His theory of general relativity has so far passed every test.
The next flare up of OJ287 is expected in early January 2016, and then again two years later. Part of the story of general relativity is the emission of gravitational waves by the orbiting bodies, which so far we have no way to detect. By 2016, or soon thereafter, a gravitational wave detector may be available, to confirm (one assumes) in yet another way the marvelous nature of OJ287, and to celebrate the incomprehensible comprehensibility of nature.
(The diagram is from the Tuorla Observatory of the University of Turku in Finland.)