Spotting gravitational waves is due to become an almost hourly event in the next decade. Starting around 2023, the Laser Interferometer Gravitational-Wave Observatory (LIGO) will undergo its most significant upgrade since 2015, UK and US funding agencies announced on 14 February.
The US National Science Foundation is contributing US$20.4 million to the Advanced LIGO Plus (or ALIGO+) project, and UK Research and Innovation is providing another £10.7 million (US$13.7 million), with a small contribution from Australia. The upgrades at LIGO’s two sites, in Washington state and Louisiana, will include the addition of a 300-metre-long, high-vacuum optical cavity. That will help scientists to manipulate the quantum properties of the lasers at the heart of LIGO’s detection system, and cut down on noise.
LIGO comprises L-shaped interferometers in Hanford, Washington, and Livingston, Louisiana, each with two 4-kilometre arms. It first operated from 2002 to 2010, and then restarted in 2015 after extensive upgrades.
The observatory made its first detection — the gravitational waves from the merger of two black holes — in September that year. It has now bagged ten black-hole mergers, plus one merger of two neutron stars. LIGO has been undergoing periodic improvements, and is now about to reopen after an upgrade designed to increase its sensitivity by 50%.
But the ALIGO+ upgrades will be more dramatic. If all goes to plan, LIGO will be able to detect neutron-star mergers that occur within 325 megaparsecs (around 1 billion light years) of Earth, says Ken Strain, a physicist at the University of Glasgow, UK, who leads a consortium of British universities that are expected to receive most of the UK money. That would nearly double the design sensitivity of 173 megaparsecs that LIGO expects to reach before the ALIGO+ upgrade.
LIGO is already able to spot black holes billions of parsecs away. By 2022, it should detect about one such event per day, and the subsequent ALIGO+ upgrade should push that to one event every few hours.
The changes will also enhance the quality of observations, not just their frequency, said former LIGO director Barry Barish at a press conference in Washington DC. For example, reducing noise will enable researchers to tell how the black holes were spinning before they merged, which can provide clues to their history. "It gives you the ability to measure things you can’t do now," said Barish, who is a physicist at the California Institute of Technology in Pasadena and shared the 2017 Nobel Prize in Physics.
Turning down the noise
Gravitational-wave interferometers work by continually comparing the lengths of their two arms. They do so by bouncing laser beams between pairs of mirrors at the ends of each arm, and then making the two beams converge on a centre point and overlap. In the absence of gravitational waves, the beams’ electromagnetic oscillations cancel out. But if space-time is disturbed and the arms change length, the laser beams no longer cancel each other out and a sensor begins to detect light.
In practice, the mirrors cannot be kept perfectly immune from thermal and seismic vibrations. Moreover, the laser itself produces noise, owing to the random nature of quantum physics. LIGO scientists have developed elaborate techniques to dampen these sources of noise, and to extract signals from any noise left over.
The LIGO upgrade that is nearing completion includes the implementation of a technique called squeezed light, which is also used by the France-and-Italy-led Virgo interferometer near Pisa, Italy. LIGO’s squeezed-light system will reduce fluctuations in the number of photons that reach the light sensor, but increases how much the beams push the mirrors around. Like the air in a partially inflated air mattress, the quantum noise cannot be completely eliminated, only shifted around.
The major improvement for ALIGO+ — requiring the 300-metre pipes — will introduce ‘frequency-dependent squeezing’. This will enable the interferometers to reduce both the pressure on the mirrors and the photon fluctuations at the same time. Other improvements will include new mirrors with state-of-the-art coatings, which is expected to reduce thermal noise fourfold.
Nature 566, 305 (2019)