The Virgo observatory in Italy has detected its first gravitational waves just two weeks after the upgraded facility was switched on. The signal came from the merger of two distant black holes and was also seen by the two LIGO detectors in the US, which have already detected three black-hole mergers on their own.
Detecting the same event at three separate locations on Earth gives astronomers a much better idea of where in the sky the gravitational waves were produced. Telescopes can quickly be pointed at that part of the sky to search for electromagnetic radiation given off by the same event – an emerging discipline called multimessenger astronomy.
Located in the flat countryside near Pisa, Virgo is an interferometer comprising two 3 km-long arms in a perpendicular configuration. The LIGO detectors are of similar design but have 4 km arms. Laser light is reflected multiple times between mirrors suspended at the ends of each arm and then combined at a detector. A gravitational wave is a ripple in space–time and when it passes through an interferometer, it can change the distances between the mirrors. This is detected as a change in how the laser light interferes at the detector.
In a spin
The first gravitational waves to be detected by both Virgo and LIGO passed through Earth on 14 August 2017 and the event has been dubbed GW 170814. Physicists working on the LIGO–Virgo collaboration think that the signal was created by the merger of two black holes with masses about 31 and 25 times that of the Sun. The merger occurred about 1.8 billion light-years away and created a spinning black hole of about 53 solar masses. During the merger, a huge amount of energy, equal to about three solar masses, was radiated as gravitational waves, a tiny proportion of which eventually passed through Earth.
Seeing the event in three (rather than two) geographically-separated detectors makes it easier to pinpoint where the gravitational waves came from. This is done by comparing the delay between signal arrival times at the detectors and by comparing the relative strengths of the signals at each detector. The distance to the source is calculated from the intensity of the gravitational waves.
A LIGO–Virgo measurement reduces the volume of the universe likely to contain the source of a detected gravitational wave by a factor of 20 compared with a detection by LIGO alone. The area of sky that is likely to contain GW 170814 can be limited to about 60 square degrees, about 10 times smaller than possible with data from just LIGO.
Restricting the source to a smaller area of the sky allowed astronomers to train 25 telescopes at the source of GW 170814 in the hope of seeing electromagnetic radiation from the black-hole merger. None was spotted, which is not surprising for this type of astronomical event.
Other sources of gravitational waves, such as merging neutron stars, are expected to emit significant amounts of electromagnetic radiation. While such events have yet to be detected by LIGO–Virgo, Imre Bartos of the University of Florida points out that the longer duration and higher frequency of a signal from a neutron-star merger would make its location easier to pinpoint than GW 170814.
Having a third detector also provides more information about the intrinsic angular momenta (spins) of the black holes before they merge. Astrophysicists had thought that binary black holes form from binary stars, which means that their spins should be aligned. Like the previous three detections, however, the spins of the black holes that created GW 170814 appear to be misaligned. Bartos says that this suggests that the black holes formed independently, perhaps in the dense centres of galaxies, and then paired-up before merging.
Built in the early 2000s, Virgo ran in from 2007–2011 without detecting any gravitational waves. It then underwent a lengthy upgrade that lasted until this year.
“The Virgo upgrade to Advanced Virgo had an ambitious objective: to significantly improve the sensitivity of our detector, in order to maximize the probability to detect gravitational wave signals,” says Federico Ferrini, director of the European Gravitational Observatory, which operates Virgo.
Both Virgo and LIGO have now shutdown for further upgrades and Bartos is hopeful that all three detectors will be up and running in autumn 2018.
Physics World visited the LIGO Livingston detector in the US recently and you can read about it in “The great detector“.
Bartos and Marek Kowalski of Humboldt University and DESY in Germany have written the Physics World Discovery book Multimessenger Astronomy, which is free to read.