WASHINGTON — Tremors in the cosmic fabric of space and time have finally been detected, opening a new avenue for exploring the universe.
The historic discovery of those tremors, known as gravitational waves, comes almost exactly a century after Albert Einstein first posited their existence. Researchers with the Advanced Laser Interferometer Gravitational-Wave Observatory, or Advanced LIGO, announced the seminal detection February 11 at a news conference and in a paper in Physical Review Letters. The gravitational swell originated more than 750 million light-years away, where the high-speed dance of two converging black holes shook the very foundation upon which planets, stars and galaxies reside.
“It’s the first time the universe has spoken to us through gravitational waves,” LIGO laboratory executive director David Reitze said at the news conference.
The discovery immediately becomes a likely candidate for a Nobel Prize, and not just because it ties a neat bow around decades of evidence supporting a major prediction of Einstein’s 1915 general theory of relativity. “Gravitational waves allow us to look at the universe not just with light but with gravity,” says Shane Larson, an astrophysicist at Northwestern University in Evanston, Ill. Gravitational waves can expose the gory details of black holes and other extreme phenomena that can’t be obtained with traditional telescopes. With this discovery, the era of gravitational wave astronomy has begun.
The detection occurred on September 14, 2015, four days before the official start of observations for the newly upgraded observatory. Striking gold so quickly raises hopes for an impending flurry of sightings.
The fleeting burst of waves arrived on Earth long after two black holes, one about 36 times the mass of the sun and the other roughly 29, spiraled toward each other and coalesced. If Isaac Newton had been right about gravity, then the mass of the two black holes would have exerted an invisible force that pulled the objects together. But general relativity maintains that those black holes merged because their mass indented the fabric of space and time (SN: 10/17/15, p. 16). As the black holes drew near in a deepening pit of spacetime, they also churned up that fabric, emitting gravitational radiation (or gravity waves, as scientists often call them). Unlike more familiar kinds of waves, these gravitational ripples don’t travel “through” space; they are vibrations of spacetime itself, propagating outward in all directions at the speed of light.
Nearly every instance of an object accelerating generates gravity waves — you produced feeble ones getting out of bed this morning. Advanced LIGO is fine-tuned to home in on more detectable (and scientifically relevant) fare: waves emitted from regions where a lot of mass is packed into small spaces and moving very quickly. These black holes certainly qualify. Their tremendous mass was packed into spheres about 150 kilometers in diameter. By the time the black holes experienced their final unifying plunge, they were circling each other at about half the speed of light. On September 14 at 4:50 a.m. Eastern time, the gravity waves emitted by the black holes during their last fractions of a second of independence encountered the two L-shaped LIGO detectors.
LIGO’s detectors in Hanford, Wash., and Livingston, La., newly reactivated after five years of upgrades, each consist of a powerful laser that splits into two perpendicular, 4-kilometer-long beams. When the gravitational waters of spacetime are calm, the beams recombine at the junction and cancel each other out — the troughs of one beam’s 1,064-nanometer waves of laser light completely negate the crests of the second beam’s waves.
But the gravitational disturbance from the black hole pair distorted spacetime, slightly squeezing one arm of the detector while stretching the other (SN: 1/8/00, p. 26).When the beams recombined, the light no longer matched up perfectly. The detectors sensed that crest missed trough by the tiniest of distances, about a thousandth the diameter of a proton.
The LIGO facilities registered the signal just 7 milliseconds apart, indicating a light-speed pulse from deep space rather than a slower-moving vibration from an underground quake or a big rig rumbling along the highway. Physicists used the combined measurements to estimate a distance of 750 million to 1.8 billion light-years to the black holes. At least one more detector, preferably two, would be needed to triangulate the precise location of the black holes in the sky.
While the rendezvous was millions of years in the making, only the final two-tenths of a second produced gravity waves with the requisite intensity and frequency for detection by Advanced LIGO. Those two-tenths of a second told quite a story. At first, the black holes were circling each other about 17 times a second; by the end, it was 75. The gravity wave frequency and intensity reached a peak, and then the black holes merged. The show was over.
Combining the wave measurements with computer simulations, the scientists determined that a pair of 36- and 29-solar-mass black holes had become one 62-solar-mass beast. The missing three suns’ worth of mass had been transformed into energy (Einstein again, E=mc2) and carried away in the form of gravity waves. The power output during that mass-energy conversion exceeded that of all the stars in the universe combined.
The observed signal matches what physicists expected from a black hole merger almost perfectly. Ingrid Stairs, an astrophysicist at the University of British Columbia not involved with LIGO, says she and her colleagues were “bowled over by how beautiful it was.” Translated into sound, the signal resembled a rumbling followed by a chirp. “It stood out like a sore thumb,” says Rainer Weiss, one of the primary architects of LIGO. “We didn’t expect anything this big.” Weiss had visited Livingston just days before and almost shut down the detector to fix some minor problems. Had he done so, “we would have missed it.”
Despite the seeming no-doubt signal, LIGO researchers conducted a series of rigorous statistical tests. The signal survived. “I have great confidence in the team as a whole and everything they’ve done with the data,” Stairs says.
LIGO’s announcement falls between two very relevant centennials: Einstein’s introduction of general relativity (November 1915) and his prediction of gravitational waves (June 1916, though he had to fix the math two years later). Russell Hulse and Joseph Taylor Jr. won the 1993 Nobel Prize in physics for deducing gravity wave emission based on the motion of a stellar corpse called a neutron star and a closely orbiting companion. Now Advanced LIGO has sealed the deal with the first direct measurement.
The observatory achieved what its predecessor, which ran from 2001 to 2010, could not because of a five-year upgrade that enhanced sensitivity by at least a factor of three. Increased sensitivity translates to identifying more distant objects: If the search area of first-generation LIGO included all the space that could fit within a baseball, Advanced LIGO could spot everything inside a basketball. The comparison to everyday-sized objects ends there. Advanced LIGO’s range extends up to 5 billion light-years in all directions for merging objects about 100 times the mass of the sun, project leader David Shoemaker of MIT says. That extended reach, plus an extra boost in sensitivity at the wave frequencies associated with black holes, enabled the historic detection.
This ability to examine black holes and other influential dark objects without actually “seeing” them with light has scientists excited about the gravitational wave era. Black holes gobble up some matter and launch the rest away in powerful jets, scattering atoms within and between galaxies; pairs of neutron stars, also targets of Advanced LIGO, may ultimately trigger gamma-ray bursts, among the brightest and most energetic explosions known in the universe.
Yet while the influence of these cosmic troublemakers is sometimes visible with traditional telescopes, the objects themselves are not. Gravity waves offer a direct probe, and as a bonus they don’t get impeded by gas, dust and other cosmic absorbers as light does. “It opens up a new window into astronomy that we never had,” says John Mather, a Nobel-winning astrophysicist in attendance at the news conference. Before this discovery, scientists had never observed a pair of black holes orbiting each other. A big next step, scientists say, is to observe a nearby supernova or the collision of neutron stars via both gravity waves and light.
Gravitational wave astronomy has begun with the Advanced LIGO detection, but there’s lots more to come. LIGO scientists still have three months of data to sort through from their first round of observing, and the analysis of the signal suggests similar events should occur multiple times a year. At the same time, the researchers are upgrading the detectors so that they can spot neutron star and black hole collisions even farther away. The observatory should be back up and running by late summer, says LIGO chief detector scientist Peter Fritschel.
Later this year, European partners of the LIGO collaboration plan to restart their revamped gravity wave observatory, Advanced Virgo, near Pisa, Italy, providing a crucial third ultrasensitive detector for pinpointing gravity wave sources. Similar detectors are in the works for Japan and India.
Researchers designed LIGO to spot waves in the sweet spot for converging black holes and neutron stars, with a frequency ranging from tens of hertz to several thousand. But just as scientists use radio and gamma-ray telescopes to probe different frequencies of light, physicists are building detectors sensitive to a range of gravity wave frequencies. The eLISA mission, a space observatory consisting of three miniature satellites, will hunt for waves with frequencies under 1 hertz when it launches in the 2030s. The satellite trio should be able to resolve black holes from the early universe as well as hefty ones millions of times the mass of the sun. On January 22, a satellite designed to test eLISA technology settled into orbit around the sun about 1.5 million kilometers away. “We have detection techniques at various frequencies that are all becoming viable at roughly the same time,” Northwestern’s Larson says.
The LIGO result is not relevant to the 2014 claim of a gravity wave sighting, since rescinded, by scientists with the BICEP2 telescope near the South Pole (SN: 2/21/15, p. 13). BICEP2 and similar telescopes hunt for gravity waves with a much lower frequency, signaling reverberations from a split-second span just after the Big Bang called inflation, when space itself stretched rapidly. Though not detectable directly, these inflation-era gravity waves should be encoded in the universe’s earliest light, the cosmic microwave background.
Scientists may well detect those flavors of gravitational waves very soon. But for now, they can bask in a discovery 100 years in the making. “This was truly a scientific moonshot,” Reitze said. “We did it. We landed on the moon.”