Interestingly, this most recent event, called GW151226, included black holes that were much smaller, the pair "weighed in" at only 14 and 8 times the mass of the sun -- the September event, called GW150914, consisted of two merging black holes of 29 and 36 times the mass of the sun. During both events, as the black holes pairs rapidly spiraled in, they warped spacetime, generating gravitational waves. During the collisions, "new", more massive black holes emerged from the collision and information of these mergers were encoded in their gravitational wave signals that LIGO could detect and decipher.
The December event spawned a new black hole of 21 solar masses, but during the collision, one whole solar mass was converted from matter into energy, blasting powerful gravitational waves across the intergalactic expanse.
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"It is very significant that these black holes were much less massive than those observed in the first detection," said Gabriela Gonzalez, LIGO Scientific Collaboration (LSC) spokesperson in a statement. "Because of their lighter masses compared to the first detection, they spent more time -- about one second -- in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe."
LIGO consists of two "L"-shaped detectors located in Louisiana and Washington, a little under 2,000 miles apart. Each building consists of two perpendicular 2.5 mile-long tunnels that guide extremely sensitive laser interferometers. The project, funded by the National Science Foundation and conceived and managed by Clatech and MIT physicists, underwent a sensitivity upgrade last year, allowing "Advanced LIGO" to detect the very slight spacetime wiggle as the gravitational waves washed over us.
And "slight" is an understatement. Advanced LIGO can detect the slightest warping, to a precision of 10,000 times smaller than the width of a proton.
The second discovery "has truly put the 'O' for Observatory in LIGO," said Caltech's Albert Lazzarini, deputy director of the LIGO Laboratory. "With detections of two strong events in the four months of our first observing run, we can begin to make predictions about how often we might be hearing gravitational waves in the future. LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe."
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For the first time, humanity has been given a glimpse of the "dark" universe, a domain that has, until now, remained invisible to us. The collision of two black holes wouldn't necessarily generate any emissions in the electomagnetic spectrum (i.e. light), so traditional astronomy, for the most part, cannot witness these events. But LIGO is "feeling" the motion of spacetime, waiting for when these violent collisions do occur.
"With the advent of Advanced LIGO, we anticipated researchers would eventually succeed at detecting unexpected phenomena, but these two detections thus far have surpassed our expectations," said NSF Director France A. Córdova.
But we are scraping the surface of gravitational wave detections. Only this time last year, gravitational waves were a confounding phenomenon that existed in theory but had no direct observational evidence. Now Advanced LIGO has become sensitive enough to detect the most powerful gravitational events in the universe. As it becomes even more sensitive, and as even more gravitational wave detectors around the globe go online, it's hard to predict what else we'll find going "bump" in the gravitational night.