The Virgo detector near Pisa will begin observing gravitational waves from the universe again in March 2024, along with the LIGO detectors in the US. The collaboration decided that last week.
By necessity, the Virgo detector has so far not participated in the current O4 measurement cycle of the international gravity wave laboratories LIGO in the US and KAGRA in Japan. O4 began in May this year, after two years of changes and improvements to the detectors. Virgo did not appear to have the desired sensitivity on the start date because of too much noise in the system.
The Virgo gravitational wave detector near Pisa is an interferometer with two angled arms over two kilometers in which laser light bounces back and forth and interferes. Minute distance changes between the mirrors are a sign that a gravitational wave is passing Earth.
In 2015, the two LIGO detectors in the US succeeded in such an observation for the first time. Gravitational waves are vibrations of space and time that, according to the theory of relativity, occur when extremely compact objects such as black holes or neutron stars collide.
Virgo has also since participated in the measurement program, in which many dozens of signals have been detected from a variety of sources, some from astronomical telescopes. By using detectors scattered around the world, the position of gravity wave sources can be determined much more precisely.
Nikhef is an important partner in the Virgo project, which is also carried out by France and Italy. Some of the detector’s recent improvements were developed and built with Dutch science money. Nikhef researchers do work intensively on the analyses of the new observations from O4.
Over the past summer, Virgo engineers have done a lot of work on the hardware, including replacing one of the mirrors in the detector’s north arm. This removed some of the noise, but not the broadband noise that limits the sensitivity of the detector.
The improvements make the detector sensitive up to 35 Megaparsec, a measure of how far into the universe the detector can look for colliding black holes and neutron stars, which make spacetime undulate. Virgo initially wanted to participate in O4 with a sensitivity of 60 Mpc.
That sensitivity of 60 Mpc is still the goal, says spokesman Gianluca Gemme of INFN and Virgo, so the commissioning program will continue until the end of the year. But Virgo does want to contribute to the O4 measurement program of LIGO and KAGRA, and will start measuring in March 2024 anyway. “It is a sensible compromise between the desired sensitivity and being able to participate,” Gemme said in a statement.