black-hole breakthroughs will ‘only get better’

Ten years after the historic discovery of gravitational waves, and having spotted hundreds more of these space-time swells since then, physicists say they are only just getting started.

On 14 September 2015, the twin facilities of the Laser Interferometer Gravitational-wave Observatory (LIGO), in Hanford, Washington, and Livingston, Louisiana, sensed the passing of ripples in space-time that had originated more than a billion years ago in the cataclysmic merger of two black holes, many galaxies away.

That milestone took more than four decades of breakthroughs and heroic improvements in experimental techniques. But seeing such black-hole ‘binaries’ has now become routine. The LIGO detectors — alongside their sister observatories Virgo, near Pisa, Italy, and KAGRA, under Mount Ikenoyama, Japan — have roughly doubled their sensitivity over the past ten years, enabling them to monitor a region of the Universe that is twice as wide and contains about eight times as many galaxies. “We see binary black holes every three days on average now, which is pretty amazing to me,” says David Reitze, a physicist at the California Institute of Technology in Pasadena and longtime director of the LIGO observatories. “It’s only going to get better.”

Over the next decade, teams in both the United States and Europe are hoping to build bigger observatories that can spot gravitational waves from anywhere across the observable Universe. Nature takes a look at scientists’ plans for the next generation of gravitational-wave detectors.

Cosmic Explorer

US-based gravitational-wave researchers want to build the Cosmic Explorer (CE), an interferometer similar to LIGO, but with L-shaped arms that are ten times longer, stretching 40 kilometres. If it is built and works as planned, the CE should collect 100,000 black-hole mergers each year, essentially spotting them wherever they happen in the observable Universe. These will include events that happened more than ten billion years ago, when galaxies were at their busiest creating and destroying stars and making and merging black holes, says Reitze. “You really do want to be able to probe farther back.”

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Each year, the CE would also pick up more than one million mergers of lighter objects called neutron stars — a rate of or one every several seconds, says Stefan Ballmer, a physicist at Syracuse University in New York.

But building it will not be cheap. With its arms stretching far beyond the horizon, the CE will run into the curvature of the Earth. If the endpoints are built at the ground level, the middle points will have to reach some 30 metres below the surface. Physicists have been scouting the United States for remote locations that are naturally bowl-shaped, which could reduce the need for digging. “We are winnowing the candidate sites down to a shortlist,” says Ballmer.

LIGO upgrades

Meanwhile, a planned set of upgrades called LIGO A# (A-sharp) could more than double the sensitivity of the existing observatory in the early 2030s, and help to test the cutting-edge technology that will eventually go into the CE. Improvements will include raising the power of the lasers that run down the interferometer arms. The LIGO team also plans to install heavier, more stable and more perfectly reflective suspended mirrors at the ends of those arms. (Interferometers detect gravitational waves by measuring tiny changes in the amount of time it takes for laser light to bounce between the mirrors.)

However, both the continued funding for maintaining LIGO and that for any upgrades — let alone new US-based facilities — could be at risk if President Donald Trump achieves his target of severely downsizing the US National Science Foundation, the agency that has funded most of LIGO’s construction and operation. Congress has hinted that it might prefer less-drastic cuts, however, and researchers remain hopeful. “I think we just have to wait and see,” Reitze says.