The Laser Interferometer Gravitational-wave Observatory (LIGO) has made a third detection of gravitational waves, ripples in space and time, demonstrating a new window in astronomy has been firmly opened.

As in the first two detections, the waves were generated when two black holes collided to form a larger black hole.
          
The newly discovered black hole has a mass about 49 times that of our Sun. This falls in a gap between the masses of the two merged black holes detected previously by LIGO, 62 and 21 solar masses respectively.

“We have further confirmation of the existence of stellar-mass black holes larger than 20 solar masses. These are objects we didn’t know existed before LIGO detected them,” says the Massachusetts Institute of Technology’s (MIT) David Shoemaker, the newly elected spokesman for the LIGO Scientific Collaboration (LSC), a body of more than 1,000 international scientists who perform LIGO research together with the European-based Virgo Collaboration.

“It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distance from us. The entire LIGO and Virgo scientific collaborations worked to put all these pieces together.”

The new detection occurred during LIGO’s current observing run, which began Nov. 30, and will continue through the summer. LIGO is an international collaboration. Its observations are carried out by twin detectors — one in Hanford, Washington, and the other in Livingston, Louisiana — operated by the California Institute of Technology and MIT with funding from the National Science Foundation (NSF).

LIGO made the initial direct observation of gravitational waves in September 2015 during its first observing run since undergoing major upgrades in a program called Advanced LIGO. The second detection was made in December 2015. The third detection, called GW170104 and made on Jan. 4, is described in a new paper published in the journal Physical Review Letters.

In all three cases, each of the twin detectors of LIGO observed gravitational waves from the tremendously energetic mergers of black hole pairs. These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time. The recent detection appears to be the farthest yet, with the black holes located about 3 billion light-years away. The black holes in the first and second detections are located 1.3 and 1.4 billion light-years away, respectively.

The newest observation 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 — what astronomers refer to as aligned spins — and sometimes they spin in the opposite direction of the orbital motion. What’s more, black holes also can be tilted away from the orbital plane. Essentially, black holes can spin in any direction.

The new LIGO 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 with the overall orbital motion. More observations with LIGO are needed to definitively determine the spins of binary black holes, but these early data offer clues about how these pairs may form.

The progenitor stars may form together and stay together after supernova explosions turn the stars into black holes. Alternatively, the progenitors and black holes may form separately in crowded stellar clusters where they may come together later in life. Non-aligned spins favor the latter scenario.

“We’re starting to gather real statistics on binary black-hole systems,” said Keita Kawabe of Caltech, an editor of the paper, who is based at the LIGO Hanford Observatory. “That’s interesting because some models of black-hole binary formation are somewhat favored over the others even now and, in the future, we can further narrow this down.”

The study once again puts Albert Einstein’s theories to the test. For example, researchers looked for an effect called dispersion, which occurs 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. 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.

“This event was twice as far away as our first one,” said Benjamin Owen, a professor in the Department of Physics and Astronomy at Texas Tech University who was involved in the work on testing relativity. “So we have more room to test Einstein’s predictions about how waves propagate, and this lets us state with more confidence than before that Einstein’s general theory of relativity gets it right.”

The LIGO-Virgo team is continuing to search the latest LIGO data for signs of space-time ripples from the far reaches of the cosmos. They are also working on technical upgrades for LIGO’s next run, scheduled to begin in late 2018, during which the detectors’ sensitivity will be improved. Virgo, the European detector, is expected to begin taking data by this summer.

Alessandra Corsi, assistant professor of Physics and Astronomy at Texas Tech, started her gravitational waves career at Virgo.

“Having Virgo back on line will help us better determine the directions to the events,” she said. “That will help us hunt for any associated electromagnetic signals with radio and optical telescopes, for instance.”

“With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe,” says David Reitze of Caltech, executive director of the LIGO Laboratory. “While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars.”

“Black holes are best for testing relativity,” said Owen, “but neutron stars one day will tell us about nuclear physics and extreme condensed matter physics.”

Corsi added, “Especially in combination with electromagnetic observations, which we are pursuing at Texas Tech.”

(Press release from Texas Tech University)