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The Search for Gravity Waves Expands

Oct. 9, 2019
A third gravitational observatory will join two in service that have already detected and used gravitational waves.

In Japan, the Kamioka Gravitational-wave Detector (KAGRA) will soon go online in the search for the subtle shakings of space and time that indicate the passing of gravitational waves. It will work with the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) and Europe’s Virgo. Representatives for the three observatories signed a memorandum of agreement that includes plans for joint observations and data sharing.

“At present, KAGRA is in the commissioning phase, after the completion of its detector construction this spring. We are looking forward to joining the network of gravitational-wave observations later this year,” says Takaaki Kajita, principal investigator of the KAGRA project and co-winner of the 2015 Nobel Prize in Physics.

In 2015, the twin detectors of LIGO—one in Washington, the other in Louisiana—made history by making the first direct detection of gravitational waves, a discovery that earned three of the project’s founders the 2017 Nobel Prize in Physics (Caltech’s Barry Barish and Kip Thorne, and MIT’s Rainer Weiss). Since then, LIGO and its partner Virgo have identified more than 30 likely detections of gravitational waves, mostly from colliding black holes.

KAGRA, illustrated at top right, will join a network of gravitational-wave observatories that includes LIGO Hanford (top left), LIGO Livingston (bottom right), and Virgo (bottom left).

The more detectors in the global gravitational-wave network, the more accurately researchers can localize gravitational-wave signals in the sky, and the better they understand the cataclysmic events that produced the signals.

For instance, in 2017, Virgo and the two LIGO detectors together localized a collision of two neutron stars to a patch of sky about 30 square degrees in size, or less than 0.1% of the sky. This was a small-enough patch to let ground-based and space telescopes pinpoint the galaxy that hosted the collision and observe its explosive aftermath in light.

“These findings amounted to the first time a cosmic event had been observed in both gravitational waves and light. It gave astronomers a first-of-its kind look at the spectacular smashup of neutron stars,” says Virgo Collaboration spokesperson Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and Maastricht University in the Netherlands.

With KAGRA joining the network, the gravitational-wave events will eventually be narrowed down to patches of sky only about 10 square degrees, greatly enhancing the ability of light-based telescopes to carry out follow-up observations. For its initial run, KAGRA will operate at sensitivities that are likely too low to detect gravitational waves, but with time, as the performance of the instrumentation is improved, it will reach sensitivities high enough to join the hunt.

Having a fourth detector will also increase the overall detection rate, helping scientists to probe and understand some of the most energetic events in the universe.

KAGRA is expected to come online for the first time in December of this year, joining the third observing run of LIGO and Virgo, which began on April 1, 2019. The Japanese detector will pioneer two new approaches to gravitational-wave searches. It will be the first kilometer-scale gravitational-wave observatory to operate underground, which will dampen unwanted noise from winds and seismic activity; it will also be the first to use cryogenically chilled mirrors, a technique that cuts down on thermal noise.

“These features could supply a very important direction for the future of gravitational-wave detectors with much higher sensitivities. Therefore, we should make every effort, for the global gravitational-wave community, to prove that the underground site and the cryogenic mirrors are useful,” says Kajita.

The new MOA also includes the German-British GEO600 detector. Although GEO600 is not sensitive enough to detect gravitational waves from distant black hole and neutron star collisions, it has been important for testing new technologies that will be key for improving future detectors. In addition, LIGO India is expected to join the network of observatories in 2025, signifying the beginning of a truly global effort to catch ripples in the fabric of space and time.

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