One of the vacuum tubes in Virgo’s 3-kilometer arms where lasers keep watch for gravitational waves. 

Cyril Frésillon/Virgo/CNRS Photothèque

European gravitational wave detector back in action

Virgo, Europe’s premier gravitational wave detector, today joined forces with the two detectors of its U.S. counterpart, the Laser Interferometer Gravitational-Wave Observatory (LIGO), in the hunt for ripples in space-time following a €24 million upgrade. In March, the two observatories were meant to begin joint observations—which allow researchers to pinpoint the locations of sources of cosmic waves, such as merging black holes, more accurately. But problems with fragile glass-fiber suspensions for Virgo’s mirrors delayed its startup. Engineers were forced to temporarily install wire suspensions, reducing Virgo’s sensitivity; it will now take a supporting role during the last few weeks of LIGO’s current observing run.

Gravitational waves are created by cosmic catastrophes such as a pair of black holes locked in a death spiral before finally merging in a burst of energy. The ripples spread out across the universe, causing space to be minutely squeezed and stretched as the wave passes by. Detectors such as LIGO and Virgo detect the tiny changes, amounting to about 1/100,000 of a nanometer or the width of an atomic nucleus, by comparing the length of a pair of 4-kilometer-long arms with lasers to extreme accuracy. (Virgo’s arms are 3 kilometers long.)

In 2015, after a decades-long quest to find the waves, LIGO finally succeeded, registering the tremor of two black holes that merged in the distant universe some 1.3 billion years ago. LIGO had only recently completed a 5-year, $205 million upgrade of its detectors. It bagged another pair of merging black holes before the end of 2015. These direct observations tantalized astronomers because, with only two detectors, it was impossible to narrow down the location of the sources so that conventional telescopes could learn more about them. For accurate location, they needed a third detector. 

The upgraded Virgo was due to join LIGO during its second observing run, which began in November 2016 but problems with the suspensions holding up the mirrors (these bounce the laser beam back and forth along the arms) led to delays. The run was due to finish in May, but the LIGO team, hoping for at least a brief period of joint observation, has stretched out the timeline as the Virgo upgrade neared completion. Because of Virgo’s loss of sensitivity with the wire suspensions, it will not be able to detect gravitational waves independently, but it will be able to help confirm a potential detection made by LIGO and locate sources in the sky with greater accuracy. LIGO’s run will finally finish on 25 August.

Both observatories will then carry out further upgrades, including restoration of Virgo’s glass-fiber suspensions, before the next run starts in the second half of 2018.