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LIGO can detect gravitational waves that are generated when two black holes collide. Credit: The SXS Project

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After a three-year hiatus made longer by pandemic troubles, the search for gravitational waves — ripples in space-time that are the hallmarks of colliding black holes and other cosmic cataclysms — has resumed.

The Laser Interferometer Gravitational-Wave Observatory (LIGO), which has two massive detectors in Hanford, Washington, and Livingston, Louisiana, is now restarting with improved sensitivity after a multimillion-dollar upgrade. The improvements should allow the facility to pick up signals from colliding black holes every two to three days, compared with once a week or so during its previous run in 2019–20.

Gravitational-wave observatory LIGO set to double its detecting power

The Virgo detector near Pisa, Italy, which has undergone its own €8.4-million (US$9-million) upgrade, was meant to join in, but technical issues are forcing its team to extend its shutdown and perform further maintenance. "Our expectation is we’ll be able to restart by the end of summer or early autumn," says Virgo spokesperson Gianluca Gemme, a physicist at Italy's National Institute for Nuclear Physics in Genoa.

KAGRA, a gravitational-wave detector located under Mount Ikenoyama, Japan, is also restarting on 24 May. Its technology, although more advanced — it was inaugurated in 2020 — is being fine-tuned, and its sensitivity is still lower than LIGO's was in 2015. Principal investigator Takaaki Kajita, a Nobel Prize-winning physicist at the University of Tokyo, says that KAGRA will join LIGO's run for a month and then shut down again for another period of commissioning. At that point, the team will cool the interferometer's four main mirrors to 20 kelvin, Kajita says — a feature that sets KAGRA apart from the other detectors that will serve as the model for next-generation observatories.

Gravitational waves are produced by large, accelerating masses, and the waves cyclically stretch and compress the fabric of space as they travel. Starting with LIGO's historic first detection in 2015, most of the 90 or so gravitational-wave events recorded so far have been from the spiralling motion of pairs of black holes in the process of merging into one; a handful have been produced similarly by the merger of two neutron stars or a neutron star and a black hole.

LIGO, Virgo and KAGRA are all based on the same interferometer concept, which involves splitting a laser beam into two and bouncing the resulting beams between two mirrors at either end of a long vacuum pipe. (At LIGO, the two ‘arms’ of the interferometer are each 4 kilometres long; at Virgo and KAGRA, they are 3 km.) The two beams then come back and are made to overlap at a sensor in the middle. In the absence of any disturbances to space-time, the beams’ oscillations cancel one another out. But the passage of gravitational waves causes the arms to change in length with respect to each other, so that the waves don't overlap perfectly, and the sensor detects a signal.

The LIGO detector in Livingston, Louisiana, is one of a pair based in the United States.Credit: Xinhua/Caltech/MIT/LIGO Lab

Typical gravitational-wave events change the length of the arms by only a fraction of the width of a proton. Sensing such minute changes requires painstaking isolation from noise coming from the environment and from the lasers themselves.

In upgrades carried out before the 2019–20 run, LIGO and Virgo tackled some of this noise with a technique called light squeezing. This approach deals with inherent noise caused by the fact that light is made of individual particles: when the beams arrive at the sensor, each individual photon can arrive slightly too early or too late, which means that the laser waves don't overlap and cancel out perfectly even in the absence of gravitational waves.

"It's like dropping a bucket of BBs [lead pellets]: it's going to make a loud hiss, but they all hit randomly," physicist Lee McCuller explained while showing a prototype of the LIGO interferometers at the Massachusetts Institute of Technology (MIT) in Cambridge. Light squeezing injects an auxiliary laser beam into the interferometer that reduces that effect. "Its photons arrive more regularly, with less noise," said McCuller, who is now at the California Institute of Technology in Pasadena.

The implementation of light squeezing has helped LIGO and Virgo to improve the detectors’ sensitivities to higher-frequency gravitational waves.

But because of the bizarre rules of quantum mechanics, reducing the uncertainty in the arrival time of the photons increases random fluctuations in the laser waves’ intensity. This causes the lasers to push on the interferometer mirrors and make them jitter, adding a different type of noise and potentially reducing their sensitivity to low-frequency gravitational waves. This is a "beautiful manifestation of nature", says MIT experimentalist Nergis Mavalvala, who helped to lead the development of the squeezing technology. "You cannot make an infinitely precise measurement: you have to pay the price somewhere else," she says.

What 50 gravitational-wave events reveal about the Universe

To deal with this issue, an important change in the most-recent upgrades of both LIGO and Virgo has been to build extra 300-metre-long vacuum pipes with mirrors at the ends, to store the auxiliary ‘squeezing’ beam for 2.5 milliseconds before injecting it into the interferometer. The role of these pipes is to shift the waves of the auxiliary laser by distinct amounts depending on their wavelengths. This means that squeezing will be selective: it will decrease the noise at high frequency while also reducing mirror jitter at low frequencies.

MIT physicist Victoria Xu was part of the team that fine-tuned the new squeezing system at LIGO's Hanford laboratory, and she recalls the pleasant surprise when it was first turned on last November. "Things worked almost exactly as you might expect," she says.

With the improved sensitivity of the detectors, researchers will be able to extract more-detailed information about the spiralling objects that produce gravitational waves, including how each spins around its axis and how they revolve around each other. This means putting Albert Einstein's general theory of relativity — which predicts the existence of both black holes and gravitational waves — to stricter tests than ever before. The sheer number of observations will improve the big picture of how, and how often, black holes form from massive stars that collapse in on themselves.

Astrophysicists also anticipate that gravitational waves will reveal distinct types of signal in addition to those from black-hole mergers. One major hope is to pick up the gravitational signal of a collapsing star before it manifests as a supernova explosion — a feat that will be possible only if the collapse occurs somewhere in the Galaxy. Another ambition is to sense the continuous gravitational waves produced by ruggedness in the surface of a pulsar, a spinning neutron star that emits pulses of radiation.

The family of interferometers is due to expand by the end of the decade. The Indian government has announced that it will fund LIGO-India, a replica of the US observatories to be built in part with LIGO's spare components.

Nature 618, 13-14 (2023)

doi: https://doi.org/10.1038/d41586-023-01732-4

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