In September 2015, just days after getting switched on, the upgraded Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct detection of gravitational radiation (see Physics Today, April 2016, page 14). Although the source of the historic signal—a pair of coalescing black holes—was unambiguous, the whereabouts of the merger in the sky were not. LIGO’s dual detectors, in Louisiana and Washington State, could localize the position only to an area of about 600 square degrees, the equivalent of about 3000 full moons as viewed from Earth’s surface.
Now, for the first time, another observatory has joined LIGO in spotting a gravitational-wave event, providing an all-important third site for triangulating emission sources. On 14 August of this year, a mere two weeks after the Virgo interferometer near Pisa, Italy, entered science mode, the gravitational radiation from a pair of merging black holes roughly 1.8 billion light-years away swept across the planet. The algorithm that continuously processes LIGO data flagged the event within 30 seconds. Subsequent analysis of LIGO and Virgo data allowed researchers to boost the significance of the detection and—using the differences in signal arrival times at the three detectors—to pin down the merger’s position to a slice of the southern sky about 60 square degrees in area (see diagram). As with the previous detections, follow-up telescope searches of the target region came up empty, but that’s not particularly surprising: There’s little reason to believe that merging black holes emit measurable amounts of electromagnetic radiation.
The new event, unlike the previously observed gravitational waves, yielded information about its polarization. In principle, a comparison of the amplitudes of signals received at the two LIGO detectors could have addressed the polarization of the earlier detections. However, the two L-shaped LIGO instruments have similar orientations, which defeats that strategy. Now that Virgo has joined the fold, the plan has been implemented. The collaboration’s analysis supports a prediction of general relativity that gravitational-wave polarization includes only two of the six modes allowed in generic metric theories of gravity.
The binary black hole discovery is the gravitational-wave community’s fourth in two years, yet it holds extra significance because it confirms that the two experiments can complement and check each other. Although Virgo’s detector, like LIGO’s pair, is a Michelson interferometer, it was built by a separate team of designers and is not simply a carbon copy of the detectors in the US (see Physics Today, September 2003, page 31). Virgo returned to duty on 1 August after a six-year upgrade period; the LIGO and Virgo collaborations have an ongoing agreement to share and jointly analyze their data.
The new detection also sets the stage for the imminent era of multimessenger astronomy. Coalescing black holes may not have an electromagnetic counterpart, but colliding neutron stars, asymmetric supernova collapses, and other desired targets of LIGO and Virgo should glow in many wavelengths. Whenever gravitational-wave researchers flag a potentially observable event, they now should be able to offer their astronomer colleagues far more precise coordinates for where to look.
All three detectors are currently shut down for another round of upgrades. When they’re back up and running, planned for fall 2018, their detection rate should increase eightfold. And with the addition of Japan’s KAGRA experiment expected in 2019 and LIGO-India in the 2020s, more help pinpointing sources is on the way. (B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo collaboration), Phys. Rev. Lett., in press.)
