Researchers from the LIGO collaboration who last week announced they had detected the first ever gravitational waves – spewed out from two merging black holes – have also picked up a second possible gravitational-wave event. Although the signal from "LVT151012" is much weaker than the confirmed “GW150914” event, the LIGO team says it most likely has an astrophysical source and arose from two coalescing black holes. The researchers have in addition spotted “several even less significant events in the data, most likely just due to some disturbance at the detectors", which they are now analysing to see if any are from gravitational waves. Their conclusions, expected over the course of this year, will see the new era of gravitational-wave astronomy finally start.

While LVT151012 is the next most interesting candidate event, the data for it are currently not statistically significant enough (about 2σ) for it to be declared a “detection” and the team will need to analyse it further to say if it is a true event or noise. Despite this, LIGO scientist Amber Stuver, who is based at the LIGO Livingston Observatory in Louisiana, US, told that the signal from the candidate event, which was detected last October, was similar to that from GW150914 and was “clean and clear”. These events suggest that the rate of binary-black-hole mergers is higher than expected, between six and 400 per cubic gigaparsec per year.

Indeed, Stuver points out that the stellar-mass black holes that merged in the GW150914 event are themselves surprising. Astronomers previously thought that such stellar-mass binaries would either not form at all or, if they did, they would be too far apart to merge within the age of the universe. LIGO’s detection has showed that this is untrue, prompting what Stuver hopes will be a revolution in astronomy.

First black-hole signals

James Hough from the University of Glasgow in the UK agrees with Stuver, pointing out that LIGO’s discovery is also the only direct evidence we have for the existence of any black holes. Astronomers had previously obtained only indirect evidence in the form of X-rays from matter falling into other black holes and the distortion of the orbits of stars at galactic centres that host supermassive black holes.

Hough says the team can be sure that the waves in the GW150914 event came from two merging stellar-mass black holes because the waves are directly related to the size of the system. The radius of the objects is such that they must be black holes for the mass that they have, he explains. “I think that there is no doubt about that at all.”

He adds that the signal from GW150914 was so perfect and clear that it almost needed no sophisticated data analysis to tease it out of LIGO’s data. The signal lasted in the detector for nearly 0.2 s, sweeping from about 30 Hz to 150 Hz, almost exactly as was expected for such a wave.

Matching the templates

According to Stuver, LIGO has a large “template bank” containing detailed simulations and predictions for every possible type of merger – be it binary black holes or neutron stars – with many different permutations and combinations of possible masses. Each template produces a unique gravitational-wave signal and the researchers’ computer system actively looks for a high correlation between it and an incoming signal. If the two are close, a detection is flagged. For GW150914, this correlation was extremely clear and immediately noticeable.

LIGO uses another method of detecting “burst” gravitational waves from an unknown source for which we have no models (such as supernovae) that involves looking for a “statistically significant anomaly” in the data. Both computational methods easily picked up the GW150914 signal, boosting the chances of it being a detection right from the start. Still, the team spent the next four months confirming its find. “We tried everything to show that it was not an actual signal, but it passed every hurdle,” she says.

As the detection was so clear, the researchers were able to tease out information such as the final black hole’s spin. LIGO spokesperson Gabriela González, from Louisiana State University in the US, explains that this spin distorts the gravitational waveform, leaving a stamp on it that LIGO was able to detect. “We produce all kinds of waveforms with all kinds of spins, all kinds of masses,” she says. These are then matched to the data to see which fit best.

González adds that the spin information from GW150914 is also interesting as the spin is not large. The “spin parameter” for the final black hole was found to be just 0.67, which is quite low as high-mass black holes are expected to have a spin near the maximum value of 1. González says that finding out why from LIGO's data will be fertile ground for theorists and astrophysicists to dig into. "We'll just keep detecting waveforms and gathering data," she adds.

Gravitational waves may also contain key information about the nature of dark matter. Although it is too soon to say for sure if the current detection will reveal any information about dark matter, Hough thinks there is a good possibility that "we may see something in the future, about the way the signals are distorted when they reach us".

Physicist Jim Gates Jr is delighted by the gravitational wave discovery, which was on his personal bucket list of physics discoveries. In the audio clip below, he talks to us about the future of gravitational wave astronomy.

Jim Gates on the implications of LIGO having detected gravitational waves
Jim Gates talks to Physics World's Margaret Harris, at the AAAS meeting in Washington, DC
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