Scientists Detect Gravitational Waves for Third Time

Scientists from the LIGO Scientific Collaboration and the Virgo Collaboration have made a third detection of gravitational waves, ripples in the fabric of space and time, first predicted by Albert Einstein more than a century ago. The research is published in the journal Physical Review Letters.

Artist’s conception shows two merging black holes similar to those detected by LIGO. The black holes are spinning in a non-aligned fashion, which means they have different orientations relative to the overall orbital motion of the pair. LIGO found hints that at least one black hole in the GW170104 system was non-aligned with its orbital motion before it merged with its partner. Image credit: LIGO / Caltech / MIT / Sonoma State / Aurore Simonnet.

The twin advanced detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) — located in Livingston, Louisiana, and Hanford, Washington — made the first-ever direct observation of gravitational waves, GW150914, on September 14, 2015.

The second detection, named GW151226, was made on December 25, 2015.

The third and latest detection, GW170104, was made on January 4, 2017, at 10:11:58.6 UTC.

In all three cases, each of the twin detectors of LIGO detected gravitational waves from the tremendously energetic mergers of black hole pairs.

The black holes in the first and second detections are located 1.3 and 1.4 billion light-years away, respectively.

The GW170104 detection points to merging black holes that are twice as far away from Earth as the two earlier pairs — approximately 3 billion light-years away.

The resulting black hole has a mass of about 50 solar masses (component black hole masses are 31 and 19 solar masses).

This fills in a gap between the masses of the two merged black holes detected previously, with solar masses of 62 (GW150914 event) and 21 (GW151226 event).

“Our handful of detections so far is revealing an intriguing black hole population we did not know existed until now,” said Northwestern University’s Dr. Vicky Kalogera, a senior astrophysicist with the LIGO Scientific Collaboration.

LIGO’s gravitational wave detections. Image credit: LIGO Scientific Collaboration / ARC Centre of Excellence for Gravitational Wave Discovery.

The GW170104 event also provides clues about the directions in which the black holes are spinning.

As pairs of black holes spiral around each other, they also spin on their own axes — like a pair of ice skaters spinning individually while also circling around each other.

Sometimes black holes spin in the same overall orbital direction as the pair is moving and sometimes they spin in the opposite direction of the orbital motion.

What’s more, black holes can also be tilted away from the orbital plane. Essentially, black holes can spin in any direction.

The new data cannot determine if the recently observed black holes were tilted but they imply that at least one of the black holes may have been non-aligned compared to the overall orbital motion.

“The discovery provided the first evidence that black holes in binary systems may not be aligned,” said Professor Susan Scott, from the Australian National University.

“This means that the two black holes could be spinning in opposite directions, which provides a tantalizing clue as to how the binary system may have formed.”

“It’s possible that this is a binary system of black holes formed in the early Universe that contributes significantly to the dark matter in the cosmos.”

The study also once again puts Albert Einstein’s theories to the test.

For example, the researchers looked for an effect called dispersion, which is known to occur when light waves in a physical medium such as glass travel at different speeds depending on their wavelength. This is how a prism creates a rainbow, for example.

Einstein’s general theory of relativity forbids dispersion from happening in gravitational waves as they propagate from their source to Earth.

LIGO did not find evidence for this effect in GW170104.

“It looks like Einstein was right — even for this new event, which is about two times farther away than our first detection,” said Georgia Tech researcher Dr. Laura Cadonati, Deputy Spokesperson of the LIGO Scientific Collaboration.

“We can see no deviation from the predictions of general relativity, and this greater distance helps us to make that statement with more confidence.”

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B.P. Abbott et al (LIGO Scientific and Virgo Collaboration). 2017. GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2. Phys. Rev. Lett 118 (22): 221101; doi: 10.1103/PhysRevLett.118.221101