Wednesday, September 27, 2017

Latest LIGO/Virgo Detection Marks Dawn of a New Era: Gravitational Wave Astronomy

Speaking today at a press conference in western Italy, scientists from the LIGO and Virgo collaborations reported new results detailing the observation of yet another gravitational wave signal, this one emanating from a source at least a billion light years away.
If you've been keeping up with astrophysics news recently, it might seem that gravitational wave detections are now becoming so frequent that this colossally energetic merging of astronomical bodies could be described as "humdrum". However, this latest detection offers an additional, exciting dimension—directionality.

Based on the shape of the gravitational wave signal in prior detections, scientists had been able to generate estimates of how massive the merging black holes must be. Knowing how big the collision was, in turn, let us use the signal's strength to figure out how far away it must have been. Pointing out precisely where it came from, however, was still beyond us.

But the fact that the signals were picked up by both of LIGO's gravitational wave antennae gave us some clues. If both detectors picked up the signal simultaneously, it would tell us that it originated somewhere along the plane halfway between the two. Since one detector picked up the signal before the other, it tells us which side of the sky the signal came from. Assuming that gravitational waves travel at the speed of light, (as general relativity predicts and evidence suggests) measuring the time elapsed between the signal detections helps us further narrow down the likely source location.

A thin band of sky, the region of space that the first-ever confirmed gravitational wave detection is likely to have come from, is outlined in black.
Image Credit: LIGO collaboration
This approximation is about as good as geometry allows with two detectors, but this latest signal was picked up not only by LIGO's two receivers, but a third gravitational wave antenna as well—the newly operational Virgo interferometer in Italy. Two detectors is enough to narrow things down to a plane, so performing that same plane-finding operation between three pairs of points lets us further restrict the possible source location down to a much smaller range, about 60 square degrees. For comparison, make a fist with your arm outstretched—the breadth of your fist occupies about ten degrees of your field of view, so sixty square degrees would be roughly a fist by half a fist's worth of sky.

A figure from the forthcoming work by the LIGO and Virgo scientific collaborations, recently accepted for publication by the American Physical Society's journal Physical Review Letters, showing the localization of the signal. The yellow band shows the localization provided by the LIGO antennae, with the additional data from Virgo shown in green.
Image Credit: B.P. Abbott, et al.
While the implications of this may not be immediately apparent, they're pretty huge. Knowing that incomprehensibly large masses are colliding somewhere (relatively) nearby is neat, but knowing where this collision is happening is infinitely more useful—it lets us turn our eyes toward that point, collecting more than just gravitational wave data. Mergers like this seem to be happening all the time, but space is huge, dark, and mostly empty; the odds are slim-to-none that a telescope like Hubble or the upcoming JWST would happen to catch a process like this occurring without some forewarning.

There's speculation in the physics community that this very technique is already in use—in mid-August of this year, several telescopes announced a sudden swivel to focus on a small region of sky in the direction of the constellation Hydra. What they're looking for is still unknown, but rumors suggest that this activity is in response to a gravitational wave signal from LIGO and Virgo, possibly indicating a merger of two neutron stars.


A NASA video describing some of the physics around the collision of two neutron stars.

Will we get our first look at a neutron star collision, courtesy of our gravitational wave detectors? Only time will tell—stay tuned!

—Stephen Skolnick

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