Michael Lucy
Over the past two years, gravitational wave detectors have let us eavesdrop on the universe-shaking booms of black holes colliding. Now at last we can see where they came from.
For the first time, astronomers at the Laser Interferometer Gravitational-Wave Observatory (LIGO), based in the US, and the Virgo detector, in Italy, have combined three detectors to triangulate back to the source of the boom. Their results pinpoint the location of a pair of colliding black holes. The findings will be published in a paper in Physical Review Letters.
A gravitational wave detector is like an ear that lets us hear the vibrations of the universe, but on its own it doesn’t tell us where they came from. To pinpoint the direction of a cosmic boom – so you can turn some telescopic eyes on it for a good look – you need to employ some schoolroom geometry. According to the rules of triangulation, that requires at least three detectors working together.
“This really makes clear the value of having more detectors,” says David Blair of the University of Western Australia, who is involved in the LIGO collaboration.
This event – the merger of two black holes, about 1.8 billion light-years away, with masses about 31 and 25 times that of the Sun – is only the fourth confirmed black-hole merger ever observed. The first was observed in September 2015. It is also the first spotted by the recently upgraded Virgo, which came online in April this year.
The signal was observed first at the LIGO detector in Louisiana, then about 8 milliseconds later at LIGO’s Washington detector, and after another 6 milliseconds it reached Virgo near Pisa, in Italy.
Each detector uses a pair of long laser beams at right angles to each other to measure tiny fluctuations – less than a thousandth the width of an atom – in the length of the beam caused by gravitational waves.
Astronomers use the time delays and the phase of the received signals, combined with their knowledge of the position and orientation of the detectors, to triangulate the direction from which the signals came.
The three earlier black hole mergers were observed only by the two US-based LIGO detectors. Two detectors let you narrow the source down to a two-dimensional plane; the addition of a third introduces an extra data point that shrinks the stripe down to a blob.
In this case the blob is still about 300 times the size of the Moon, but that’s a small-enough area to cover with a conventional telescope.
Optical, X-ray and radio telescopes have been turned to look at the area where the signal came from. They saw nothing. This was not unexpected, since colliding black holes are still actually “black”. While their collision creates gravitational waves, there is no visible radiation. So why bother with the telescopes?
Well, other events that reverberate with gravitational waves – such as colliding neutron stars – are predicted to produce large amounts of electromagnetic radiation. Indeed, the astronomy world was set abuzz in late August by rumours that just such an event had been spotted, though no confirmation has yet been forthcoming.
Greater precision in locating the sources of gravitational waves will depend on upgrades to existing detectors – LIGO and Virgo are both shutting down to get ready for another joint round of observations next year – but, more significantly, on building new detectors in other places.
“It’s very important to have that long baseline,” says Blair, meaning the distance between the detectors. Adding a detector in Western Australia – a proposal Blair has advocated for many years – would bring the longest baseline from 8,000 km up to 12,000 km. “It would be particularly good to have a long baseline to the southern hemisphere,” he adds, because this would extend the network further into the third dimension.
