The experiment that first directly detected gravitational waves in the spacetime continuum has repeated the feat, scientists announced at a meeting in San Diego on June 15. The achievement establishes the experiment, called LIGO, as the primary tool with which astrophysicists now observe the play of gravity around massive bodies in the universe. It also reposes faith in some of the sophisticated techniques developed by scientists to detect and study gravitational waves, and highlights the challenges in the road ahead.
“This finding confirms the fact that the first detection wasn’t an isolated event,” P. Ajith, leader of the astrophysical relativity group at the International Centre for Theoretical Sciences (ICTS), Bengaluru, told The Wire. “But more importantly, this is the beginning of serious gravitational astronomy.”
LIGO stands for Laser Interferometer Gravitational-wave Observatory. Its two identical detectors are located in Hanford, Washington, and Livingston, Louisiana. In the wee hours of December 26, 2015, they detected gravitational waves originating from a pair of black holes that were about to merge, about 1.3 billion lightyears away. Before the merger, in the inspiral phase, the black holes rapidly spiral in tight orbits around each other, their acceleration sending away ripples of gravitational energy that alternatively contract and expand spacetime (by minuscule amounts) as they move through it – much as a wave passing through a sheet of cloth would.
A technical detection
These ripples are ‘heard’ by LIGO, which measures the expansions and contractions, as gravitational waves. They are typically most pronounced when the black holes collide and merge, releasing a blast of energy as the new combined black hole settles down into a stable form, a period called the ringdown. In the celebrated first detection of gravitational waves, made in September 2015, waves from the merger and ringdown phases were overwhelmingly more prominent than those from the inspiral phase.
In the December event, designated GW151226, however, LIGO logged those waves coming from the inspiral more prominently than those from the merger and ringdown themselves because the black holes were relatively light – weighing 14- and 8-times the mass of our Sun.
“The combined black hole weighed 21 solar masses,” Ajith confirmed, implying that one solar mass’s worth of energy had been released during the merger. However – he continued – “The signal was so weak that we didn’t actually observe the merger and the ringdown. Plus the power radiated by the black holes was spread over a longer period of time.”
Because of the lower power, “this system was deeply buried in the noise,” Karan Jani, a PhD student at the Georgia Institute of Technology and one of the analysts with LIGO, told The Wire. “To find this system, we had to use the match-filtering technique,” which matches the data with the probable parameters of a pair of black holes. “In contrast, in the September event, we saw only about four orbits of black holes, so most of the energy was radiated during the merger or, as we at LIGO call it, the ‘burst’.”
One kind of searching algorithm can pick such ‘bursts’ efficiently from the data. However, the second detection called for a different technique because of its weak output. This involved combining the results from numerical and analytical relativity, according to Bala Iyer, a noted gravitational physicist and one of the principal leaders of the Indian Initiative in Gravitational-waves Observation (IndIGO) Consortium. As a result, the LIGO team was able to study the gravitational waves originating from 55 rotations of the inspiral.
Also read: The Wire speaks to Kip Thorne, LIGO cofounder and mentor to its first numerical relativity groups
The LIGO team for the December event was effectively the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration out of Europe. In all, that’s over 1,200 researchers from at least 15 countries.
The Indian contribution
The crucial match-filtering technique has a part of its origins in India. This technique “really pulled out this event from the data,” said Iyer. “Its techniques were developed at the Inter-University Centre for Astronomy and Astrophysics and made more sophisticated later, led especially by Sanjeev Dhurandhar’s group.” Dhurandhar is a professor at IUCAA.
The successful detection of a second black hole merger also feeds into the estimates of how many such events the LIGO detectors will be able to pick up on every year. “With detections of two strong events in the four months of our first observing run, we can begin to make predictions about how often we might be hearing gravitational waves in the future,” Albert Lazzarini, a senior member of the experimental collaboration, said in a statement.
We have some understanding of the rates at which these events happen in the universe, Ajith holds. Jani explained that, between all the models, the detection rate is 2-600 per year per cubic gigaparsec (1 gigaparsec = 3.26 billion lightyears). However, in the first run, the LIGO detectors ran for a total of 45 days. So: “If we detect one more black hole merger in the next 320 days of operation, the lower limit could be about 9.” The next run is expected to begin in September and is also expected to last about 45 days.
— Shannon Hall (@ShannonWHall) June 15, 2016
According to Ajith, the Indian members of the LSC were also instrumental in inferring the mass and spin of the combined black hole despite the gravitational-wave signal being so weak. The members currently number 36 from nine institutions.
A curious thing about the measurement is that the black hole merger is estimated to have happened at a distance similar to the one at which LIGO had made its first detection, in September 2015 and announced on February 11 this year: 1.3 billion lightyears away. It is unclear if the LIGO scientists had noticed the presence of the second pair earlier. However, Ajith explained that the distance was simply a coincidence: “The error parts on these measurements are quite large – of the order of 300 lightyears either way – and the waves also come from very different parts of the universe.”
A third detector
At the moment, because there are only two detectors, LIGO isn’t very good at pinpointing the exact sources of gravitational waves. Later this year, the Virgo detector in Italy is expected to come online, followed by the KAGRA detector in Japan. More excitingly, a third LIGO detector is set to come up in India in the first part of the next decade. Its addition will greatly improve the ability to zero in on sources and their orientation in the sky. According to Iyer, three sites for setting it up have been shortlisted – one each in Madhya Pradesh, Maharashtra and Rajasthan. “The exercise is in ranking them, and we are working closely with the participating institutions to make this decision,” Iyer told The Wire.
He added that the first detection had helped IndIGO get approval for the Indian detector from the prime minister. Currently, the consortium is busy setting up the ‘core management team’, as Iyer called it, and that more details of their progress should be available in August. For now: “The second detection will help us remain on schedule.”
Additionally, Ajith said that the multi-wavelength satellite ASTROSAT was involved in making follow-up observations of the black hole merger. According to an IndIGO statement, ASTROSAT’s Cadmium-Zinc-Telluride Imager was among the many instruments tasked with finding if there was any electromagnetic release of energy that accompanied the release of gravitational energy. “According to our conventional understanding of astrophysics, we do not expect black hole mergers to produce any electromagnetic signature,” Ajith said. “However, testing our understanding is a fundamental tenet of science.”
The paper by the LSC and Virgo collaborations was published in the journal Physical Review Letters on June 15.