Federico Ferrini says he's "quite confident" the European lab will be ready this summer to join forces with LIGO, the US-based Laser Interferometer Gravitational-Wave Observatory. It gained fame when it proved the existence of gravitational waves, the ripples in spacetime predicted by Einstein's general theory of relativity in 1916.
Virgo is nearing the end of a five-year, €24 million upgrade, which will allow it to work in tandem with LIGO to pinpoint the sources of these ripples. Last September, after a hundred-year hunt by scientists and its own five-year upgrade, LIGO finally spotted its first gravitational wave.
The discovery is a crucial step in our efforts to better understand gravity – and hence the universe.
The three-kilometer-long Virgo was supposed to join LIGO this March. But last summer the 0.4 mm thick glass fibre suspensions, which support the instrument’s 40-kilogram mirrors in its tunnels, began to snap; the fault was triggered by dust.
Annoying as the delays are, they weren’t totally unexpected, says Ferrini. Virgo is the first large interferometer using the glass fibres. They work perfectly in a vacuum, but as soon as even microscopic dust touches them, it creates miniscule cracks that slowly grow larger and cause the suspension to fail - days or even weeks later.
"We started with the upgrade two years after LIGO's last upgrade, and we plan to finish it two years after LIGO's, so we are totally on track,” argues Ferrini.
For the time being, EGO’s engineers have replaced the suspensions with the steel wires that the instrument had before the upgrade, although they make the detector less sensitive. Ferrini says that glass will be put back after the first scientific run. “It’s just the problem we have to face,” he says. Technical hiccups are not uncommon in projects on such a massive scale, he adds, pointing to the first defective mirror of the Hubble space telescope, and the explosion of magnets at CERN’s Large Hadron Collider.
“A perfect, flawless upgrade is impossible. We - LIGO and Virgo - are working beyond the limits," says Ferrini. "But even with steel suspensions we might detect a gravitational wave coming from a merger of two neutron stars up to 50 megaparsecs (163 million light-years) from Earth.”
LIGO can detect such an event at up to 80 megaparsecs. The hope that once Virgo’s suspensions are replaced with glass again, the instrument will be much more sensitive.
Detecting the waves
To spot the ripples, the detectors use optical devices called interferometers – two vacuum tunnels, each several kilometers long, built in an L-shaped configuration. First, a high-powered laser beam is fired through the tunnels, split in two and sent on separate light paths down the detectors. The two beams get bounced back and forth by mirrors suspended on carefully designed pendulums at the end of each arm, and eventually come back to their starting point, where they are recombined, overlapping perfectly - until a gravitational wave stretches the length of one arm by about 1/1000 the width of a proton. This changes the combined signal – and this is what LIGO managed to detect.
“Interferometers used by LIGO and Virgo are incredibly complex and sensitive, with many, many component parts that have to work together perfectly - of which the suspension system is just one,” says astrophysicist Chris North at Cardiff University and a member of the LIGO collaboration. “It isn't particularly surprising that getting such a complex experiment ready takes longer than planned, but the expectation is that they'll be solved in the coming months and Advanced Virgo will join Advanced LIGO in the search for gravitational waves.”
Each detector is able to spot the passage of a gravitational wave independently. But when three separate detectors work together, the data is analysed simultaneously to help weed out false signals, and to better understand the properties of the sources when a gravitational wave is detected. “A gravitational wave should be seen by all detectors at about the same time - anything that's not, is likely a bit of spurious noise,” says North.
The noise will disrupt the data, and may be caused by anything from ground shaking when a lorry drives past, or an earthquake striking somewhere in the world. If the laser changes in power even in the slightest, data will also be flawed, says North. Every time something like that happens, a detector has to be reset.
“Having three detectors running at the same time increases the chance that there will be at least two operational at any one time, and therefore decreases the chance of missing a real signal,” says North.
Having detectors far apart means that it is easier to rule out ‘local’ effects such as ground motion or laser fluctuations. The two Advanced LIGO detectors are about 3000 km apart, and having a third one in Europe is a big advantage, says North. “The different detectors also pick up gravitational waves at slightly different times - as they can take a fraction of a second to pass through the Earth, and this can help locate the source in the sky. It's similar to triangulation used in navigation, and the location can be determined much, much more accurately using three detectors than just two.”
Once the two machines with their three detectors are working together, the hope is to investigate the structure of neutron stars and even get some indication about the nature of the mysterious dark matter that makes up some 23% of the contents of the universe. “There are many other sources that LIGO and Virgo may be able to detect, such as particular types of exploding stars, and the rotations of individual neutron stars, and there are efforts underway to search the data for all sorts of signals,” says North.