Faint gravitational waves could soon be on LIGO’s radar

A conspicuous “chirp” heralded the first detection of gravitational waves. But some future measurements could be more like hushed murmurs.

Scientists may soon be able to tease out a faint signal of gravitational waves from black hole collisions too distant to be detected directly, scientists with LIGO, the Advanced Laser Interferometer Gravitational-Wave Observatory, report in the April 1 Physical Review Letters. A detection could come in as few as three years — considerably faster than scientists had dreamed possible, the new analysis suggests.
When LIGO detected the stretching and squeezing of spacegenerated by a pair of merging black holes, scientists were wowed (SN: 03/05/16, p. 6). The signal stood out well above spurious bumps and wiggles in the data, which are ever-present in LIGO’s extremely sensitive detectors. It rose swiftly in frequency — when converted to sound waves, it was reminiscent of a bird’s chirp — a hallmark of the black holes’ inward-spiraling cosmic dance.

But such obvious swells are outnumbered by a sea of smaller ripples. With these ripples, “you’re looking at black holes which are much farther away,” says LIGO spokesperson Gabriela González of Louisiana State University in Baton Rouge.

LIGO is not sensitive enough to detect these waves outright, but by comparing the data recorded by LIGO’s separate detectors — one in Louisiana and one in Washington state — scientists could identify patterns revealing the presence of the background waves. Such a measurement would allow scientists to compare black hole populations of different ages and could help nail down the conditions under which black hole pairs form.

“My honest opinion was, ‘I’m going to be lucky if we see this result in my lifetime,’” says physicist Emanuele Berti of the University of Mississippi in Oxford, who is not involved with LIGO. He has changed his tune. “Nature was good to us, and now we think that we’re going to be able to see them pretty soon.”

That’s because new estimates of the rate of such black hole mergers are higher than many scientists expected. Using models of binary black hole populations combined with LIGO data, scientists find that LIGO could be sensitive to nearly 2,000 such black hole mergers a year, meeting their most optimistic predictions.

The possibility shows LIGO’s versatility, González says. “There are good prospects of all kinds — it’s not just detections of single events.”