LIGO’s current version is known as “advanced LIGO” or aLIGO, which began operations in September 2015. Upgrades to nearly every aspect of the experiment will increase its sensitivity tenfold over the next few years—which means the observatory can “see” a volume a thousand times greater than previously, encompassing a much bigger chunk of the universe. At present, aLIGO can spot colliding pulsars at a distance of 260 million light-years away; when the upgrades are done, that distance will be more like 650 million light-years. (For comparison, the nearest large galaxy to us, Andromeda, is 2.5 million light-years away.) Black holes can be more massive and therefore much “louder” than pulsars, so they can be detected at much greater distances—as the new LIGO announcement attests.
The “loudness” of a gravitational wave depends on what’s producing it, and how close the source is to Earth. Strong gravity makes for loud waves, so objects like binary pulsars are the good sources, and black holes are best because they are even more extreme. The collision LIGO spotted consisted of two black holes respectively 36 and 29 times the mass of the Sun, but much denser. Despite its sun-dwarfing mass, the larger of the two black holes is less than 300 miles across.
When those two black holes crashed together 1.3 billion years ago, they sent out an amazingly powerful burst of gravitational waves, loud enough for LIGO to detect from that astounding distance. If you could hear the waves, they would start on a low note and rapidly sweep up the scale to higher and higher pitches (technically known as a “chirp”, since it resembles the sound many birds make) as the black holes spiral inward, sweeping toward each other inexorably, all the while increasing in volume until the actual collision—and a phenomenally intense burst of waves.
Since the signal itself doesn’t say “I was caused by black holes!,” LIGO scientists had to compare it to various “templates.” A gravitational-wave template is calculated theoretically, based on a few simple assumptions: what sort of objects (black holes, pulsars, etc.) are making the waves, how massive they are, and so forth. Once the best match between the signal and the template is made, researchers can identify the source and even how far away it is. What they can’t do is tell exactly where it is in the sky: LIGO is not very accurate as a telescope.
Since calculations show that collisions of black holes like this are rare, many thought they weren’t great sources for LIGO. That this was the very first thing LIGO ever saw means either we were lucky, or black-hole collisions are more common than we thought. Either is a fascinating possibility, which LIGO researchers will sort out in the coming years.
The road to this discovery has been a long one. Einstein first proposed gravitational waves in a presentation he gave to his fellow scientists in 1913, two years before he finished work on the general theory of relativity. Once he had completed the theory, he wrote a full article on gravitational waves published in 1916, which means the LIGO announcement comes a full century after Einstein first published on them.