The world was stunned when scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced in February they had finally detected gravitational waves, solving a century-old investigation that began with Albert Einstein.
Well, hold onto your butts — the LIGO superstars have done it again. Mere months after they made the measurements of the first gravitational wave signals, LIGO’s instruments managed to detect gravitational waves a second time — again a result of a pair of black holes crashing into one another — this past Christmas. The findings are published in the latest issue of Physical Review Letters.
At a press conference held by the American Astronomical Society in San Diego today, Gabriela González, the LIGO scientific collaboration (LSC) spokesperson, excitedly praised the ability of LIGO’s detectors — which aren’t yet running at full capacity — to pick up on such faint signals. “Despite these being so tiny, these LIGO instruments on Earth detected very clearly these gravitational waves,” she said. “With this, we can tell you now, the era of gravitational wave astronomy has just begun.”
Other LIGO scientists echoed Gonzàlez’s delight — and surprise — at having detected another pair of binary black holes within one year.
“I would never have guessed that we would be so fortunate to have, not only one, but two definitive binary black-hole detections within the first few months of observations,” said Chad Hanna, an astrophysicist at Penn State University affiliated with LIGO, in a PSU news release.
Gravitational waves are often referred to as ripples in spacetime caused by the presence of mass. They don’t necessarily do anything, but they are an important indicator that gravity, well, exists. Gravitational waves essentially carry information about the nature of gravity, why and how larger masses impose gravitational effects on smaller masses, and more.
The December signal was the result of a pair of black holes fourteen and eight times the mass of the sun, respectively, colliding into one-another to form a single massive black hole about 21 times the mass of the sun all of it happening 1.4 billion years ago. It’s a significantly smaller event than the first black hole merger observed in September — comprising a pair of black holes 29 and 36 times more massive than the sun, respectively, and expelling more energy than all the stars of the universe put together — but that’s not a negative at all.
In fact, observing gravitational waves produced by a weaker celestial event is a pretty encouraging development. If scientists hope to study gravitational waves more in depth, they’ll want to make as many measurements as possible, from all kinds of cosmic phenomena. For LIGO’s instruments to pick up on something less massive is a powerful step forward.
It is very significant that these black holes were much less massive than those observed in the first detection, said González in a news release issued by MIT. “Because of their lighter masses compared to the first detection, they spent more time — about one second — in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe.”
At the AAS conference, David Reitze, the Executive Director of the LIGO project, confirmed plans to increase the sensitivity of the detectors by 15 to 25 percent before the next run this fall. “The future is going to be full of binary black hole mergers for LIGO,” he said. “We’re going to see a lot more of these.” He also hinted at LIGO’s search for events other than binary black hole mergers; the collision of binary neutron stars, he said, could also soon be detected.
The results also suggest black hole mergers are much more common than scientists initially believed.
Gravitational waves are ultra difficult to measure because of how weak they are. Scientists measure gravitational waves through an instrument known as an interferometer, which essentially produces a specialized laser running across very large distances that is sensitive enough to detect the presence of these signals moving through.
LIGO uses two different interferometers (one in Livingston, Louisiana, and one in Hanford, Washington) as a way to both measure the waves and verify that the signal is a gravitational wave and not just an aberration caused by local geological movement or other factors.
Although LIGO has been operational since 2002, the reason we’re starting to actually find gravitational waves is thanks to a major upgrade both interferometers (plus the Italy-based Virgo interferometer) underwent last year. In fact, the first signals were found mere days after the upgrades were complete. Needless to say, those renovations are always exceeding expectations.
Describing LIGO’s future projects, Reitze discussed plans to build another detector in India. “Hopefully, we’ll have five detectors going into the next decade,” he said, also referring to the Hanford and Livingston detectors, Italy’s Virgo, and KAGRA, which is currently under construction in Japan; it’s hoped that having more detectors will allow researchers to not only sweep a larger swath of the sky for gravitational wave events but also better locate them, in a process similar to triangulation.
The new findings aren’t simply an additional dataset to the now-growing catalogue of gravitational wave data. Scientists expect to harness the numbers as part of an effort to form predictions about what kinds of events will produce measurable gravitational waves, where those events have occurred, and when to expect those gravitational waves to reach Earth.
“Certainly we’re going to be seeing a lot more black holes, hopefully binary neutrons, and if we’re lucky, a supernova,” Reitze said at the AAS conference. “Gravitational wave astronomy is real. We’re here.”