Damage delays restart of Italy’s giant gravitational wave detector

Later this month, physicists will resume their hunt for astrophysical monsters: black holes and neutron stars going bump in the dark and emitting ripples in space called gravitational waves. But one of the three detectors that have spotted such waves—Virgo, near Pisa, Italy—has run into technical problems that will delay its restart, 3 years after all the facilities shut down for maintenance and upgrades. For the next few months, just the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), in Louisiana and Washington state, will take data, making it harder to pinpoint sources on the sky.

The problem appears to originate not in the upgrades, but in older parts that are creating noise that would drown out many signals, says Fiodor Sorrentino, a physicist with Italy's National Institute for Nuclear Physics (INFN) and Virgo's commissioning coordinator. "But we cannot be 100% sure" before opening the detector, he says. Daniel Holz, an astrophysicist at the University of Chicago, says such hiccups are normal, although LIGO and Virgo had dodged them. "We’re owed this kind of bad luck because our excessive good luck had to run out."

The good luck began in 2015, when the LIGO detectors first sensed ripples produced when two massive black holes swirled into each other and merged. Two years later, LIGO and Virgo spotted a nearby merger of two neutron stars, which set off an explosion called a kilonova that was viewed by myriad telescopes as well. So far, the three detectors have tallied more than 90 mergers of black holes and two of neutron stars.

Each detector is a huge L-shaped optical device called an interferometer. Light bounces between weighty mirrors at the ends of each arm of the L. Some light leaks through the mirrors at the elbow, and the two light beams interfere, either canceling or reinforcing each other, depending on the arms’ relative lengths. A passing gravitational wave generally stretches one arm more than the other, causing light to warble out of the device in sync with the wave.

To spot the minuscule stretching, the arms must be long. LIGO's extend 4 kilometers and Virgo's, 3 kilometers. The detectors must also squelch other vibrations to stabilize the length of each arm to 1 femtometer, the width of a proton. So the whole rig resides in a vacuum chamber, and an elaborate suspension system supports each mirror. Virgo's problems seem to have arisen in the suspension and the mirrors.

Each of its 40-kilogram mirrors hangs from a pair of thin glass fibers. In November 2022, a fiber supporting one mirror broke. Although the mirror fell a minimal distance, the jolt appears to have loosened one of four magnets affixed to the mirror and used to stabilize it, Sorrentino says. The magnet's movements generate a smidgen of heat—literally vibrations in the glass. Also, a mirror in the other arm that suffered a similar fall in 2017 now appears to have small internal crack that is growing and generating heat. The noise limits Virgo's sensitivity to roughly half what it was at the end of the last run.

The problems became apparent only recently because some of the upgrades took longer to commission than expected, says Gianluca Gemme, a physicist at INFN and spokesperson for the 850-member Virgo team. Rather than restart the detector, researchers will open its vacuum chamber to remove the loose magnet from one mirror and replace the other mirror. That work should be completed by July, Gemme says. Tuning the instrument would take a few months more. "If everything goes well and there are no additional hidden sources of noise, we should be able to join [LIGO] in the fall," Gemme says. Still, Sorrentino cautions, "This current situation is a bit scary because you never know what will happen when you put hands on your [mirrors]."

The two LIGO detectors are performing well and should be ready for the 24 May restart, says Patrick Brady, an astrophysicist at the University of Wisconsin-Milwaukee and spokesperson for the LIGO collaboration. But the temporary loss of Virgo will limit the science that can be done. Three detectors can pinpoint a source in the sky to within a few dozen square degrees. With two, the localization is far worse.

Virgo proved the value of such triangulation in August 2017 when it and the LIGO detectors spotted the first neutron star merger. Coordinates were quickly dispatched to astronomers, enabling radio dishes, optical telescopes, gamma ray detectors, and other instruments to home in on the explosion and detect the heavy elements it forged—making it literally a cosmic gold mine.

But even with just LIGO, the 18-month run should yield plenty of science, Brady says. LIGO's detectors are already 30% more sensitive than before and should spot a black hole merger once every 2 or 3 days, bagging about 270 in total. That haul should nail down the distribution of black hole masses and may reveal unusual mergers, such as between black holes spinning in different directions. That information could help reveal how black hole pairs form, Brady says. "The excitement is going to come in more than just increasing the number, but in getting these exceptional events."