After two years of upgrades, the Laser Interferometer Gravitational-Wave Observatory (LIGO) is almost ready for its next operational run, which will begin in March 2023.
During this fourth run (O4) LIGO– which consists of two detectors, one in Washington and one in Louisiana – will be joined by two other gravitational wave observatories: the Virgo interferometer in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan.
Gravitational waves are space-time ripples triggered by extremely violent cosmic events, such as mergers between neutron stars, black holes or a combination of the two. The phenomenon was first predicted by Einstein the theory of general relativity.
Related: How do gravitational waves work?
Although the events that postpone these ripples in space time are extremely powerful, gravitational waves are very weak – in fact so weak that Einstein predicted that we would never directly detect them. We ended up discovering them too first direct discovery of gravitational waves occurred in September 2015. Now the new improvements of LIGO increase its sensitivity, allowing it to detect weaker signals than ever before and, as a consequence, more events than previously possible.
For this upcoming run, LIGO scientists aim for the instrument to be sensitive enough to detect gravitational waves from binary neutron star mergers as far away as 522 million to 620 million light-years from Earth. Larger and more violent events, such as collisions and mergers of black holes, should be visible to LIGO at an even greater distance. (The first direct discovery of gravitational waves observed two massive black holes that fused 1.3 billion light-years from Earth.)
Scientists have since discovered a number of gravitational waves. Virgo and LIGO discovered ripples launched by the fusion of binary neutron stars 2017, which helped locate this catastrophic cosmic event. KAGRA joined the LIGO-Virgo network in 2020, and in January 2020, the three observatories spotted gravitational waves from a merger between a black hole and a neutron star. This observation was the first time this type of mixed merger was discovered and serves as an enticing promise of things coming from collaboration.
LIGO completed operational run 3 in March 2020 and was shut down for upgrades. The operators of the three facilities coordinate carefully so that they will start O4 together after the covid-19 impact that delayed the start of the fourth run from December 2021.
One month before the start of O4, the detectors will carry out a technical run to test the upgrades in real time and also check the systems that allow LIGO, Virgo and KAGRA to work in unison. But even during such a technical run, the gravitational wave detectors can deliver exciting science. In fact, LIGO's historic first discovery of gravitational waves came during such a test period.
O4 is planned to last a full year, with a month-long maintenance break around the middle of this period. However, KAGRA will not work with LIGO and Virgo for a full 12 months, as the Japan-based detector is expected to go offline for its own sensitivity upgrades at the end of O4.
“Everyone at LIGO and Virgo and KAGRA are eager to start discovering gravitational waves again and share our discoveries with the world,” said LIGO operators at the California Institute of Technology in a statement. “We look forward to March 2023.”
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