Violence detected: Sensors hit by second set of gravitational waves

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Artist's impression of the ripples in spacetime created in the final 27 orbits of two black holes, eight and 14 times the mass of our sun prior to their collision(Credit: LIGO/T.Pyle)

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Scientists making use of the twin Laser Interferometer Gravitational Wave Observatory (LIGO) instruments have announced the second confirmed detection of gravitational waves resulting from a collision between two black holes. Gravitational waves are ripples in the fabric of spacetime created by exceptionally violent cosmic events. Analysis of the waves, which contain unique information unattainable by conventional telescopes, could allow scientists to gain a rare insight into some of the most energetic events occurring in our Universe.

Gravitational waves were first predicted as an element of Albert Einstein's theory of general relativity in 1916. Of course, the great man lacked the means to detect the waves himself. However, the announcement of the eventual discovery, which happened to coincide with the 100th anniversary of the prediction of the waves in September 2015, has opened up an entirely new window through which to unravel the secrets of the universe.

The new discovery was made by the twin LIGO detectors located in Livingstone, Louisiana, and Hanford, Washington in December 2015, and was just announced today.

The detectors work by reflecting laser beams down 4 km (2.5 mile)-long L-shaped arms onto mirrors equipped with cutting-edge sensors. By observing the characteristics of the light as it exits the machines, scientists are able to measure any changes in the length of the arms caused by the ripples in spacetime. LIGO can detect disturbances in the fabric of spacetime down to a fraction of the width of a proton.

Illustration displaying the discovery dates of the two confirmed gravitational wave discoveries, displayed with solid lines, and the detection of a third, weaker candidate signal that could not be unambiguously confirmed(Credit: LIGO)

A recent upgrade to the LIGO project has further improved the sensitivity of the detectors, opening up a greater volume of the universe for analysis by the 1,000-plus scientists involved in the running of the cutting-edge instruments.

LIGO first struck gold on Sep. 14, 2015 with the detection of gravitational waves created 1.3 billion years ago by the merging of two black holes — one 29 and the other 36 times the mass of our Sun — into a single colossal black hole. The discovery opened up an entirely new field of astronomy that allows scientists to peer beyond the scope of conventional telescopes.

Hot on the heels of its first groundbreaking discovery, the LIGO Scientific Collaboration has announced its second confirmed detection of gravitational waves. It took the super-computer-driven processing system analyzing the data from the twin LIGO facilities a mere 70 seconds to isolate the waves from the background noise, and inform its human counterparts of the discovery. Further analysis of the signal led LIGO scientists to conclude that, as with the initial discovery, the second set of waves originated from a pair of merging black holes, this time weighing in at 8 and 14 times the mass of our parent star.

A three-dimensional representation of the Milky Way displaying the probable locations of the two confirmed, and one unconfirmed (dotted) gravitational wave events discovered in LIGO's first observation period(Credit:  LIGO (Leo Singer) /Milky Way image (Axel Mellinger))

The December collision, which is believed to have occurred roughly 1.4 billion years ago, created a black hole with a mass the equivalent of 21 Suns. Compared to the first gravitational wave event, this one was comparatively lightweight, so finding them is a testament to the enhanced sensitivity of LIGO. These waves were thought to be ripples in spacetime formed during the final 27 orbits of the black holes prior to their merger.

"It is very significant that these black holes were much less massive than those observed in the first detection," states Gabriela González, LIGO Scientific Collaboration (LSC) spokesperson and professor of physics and astronomy at Louisiana State University. "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."

The waves were detected at the Livingstone LIGO detector 1.1 milliseconds before its counterpart located in Hanford. This time delay will allow astronomers to gain a vague sense of where the black-hole merger took place, and aid in the search for a visible light source created by the cataclysmic collision.

A chart displaying the equivalent solar masses of the constituent and fully merged black holes believed to have created the two confirmed instances of gravitational waves – a third set of black holes represented in dotted lines displays an unconfirmed candidate signal(Credit: LIGO)

Following the initial discovery of gravitational waves in September 2015, astronomers spent three weeks combing the sky for a trace of a visible light flash to mark the exact location of the rare event. Unfortunately, the search came up empty.

The second detection will allow astronomers to estimate the frequency by which gravitational wave events can be expected to occur. LIGO's second data-taking run is set to commence this fall, with scientists optimistic of making further detections thanks to additional modifications to the instruments, which will allow them to analyze two times as much of the volume of the universe currently accessible to the detectors.

For the latter half of the campaign, LIGO will be joined by the European Virgo detector. The addition of the third L-shaped interferometer, which is administered by over 250 physicists and engineers, will allow for a more accurate determination of the source of future signals, which could lead to superior follow-up observations, including the detection of a visible-light component to a gravitational-wave event.

This MIT video has more information about how the waves were produced and detected.

Source: MIT

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