Post by Admin on Feb 18, 2016 1:12:26 GMT
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.
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 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.
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.
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.