LIGO Detects Gravitational Waves for Third Time
Results confirm new population of black holes06/01/2017
BATON ROUGE – The Laser Interferometer Gravitational-wave Observatory, or LIGO, has made a third detection of gravitational waves, which are ripples in space and time, demonstrating that a new window in astronomy has been firmly opened. As was the case with the first two detections, the waves were generated when two black holes collided to form a larger black hole.
“We are very proud of the contributions of the LSU group to the detections of gravitational
waves with LIGO detectors - this is just the beginning of a new, exciting era,” said
Gabriela González, former LIGO Scientific Collaboration spokesperson and professor
of physics and astronomy at LSU.
The newfound black hole, formed by the merger, has a mass about 49 times that of our sun. This fills in a gap between the masses of the two merged black holes detected previously by LIGO, with solar masses of 62 from the first detection and 21 from the second detection.
“We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses—these are objects we didn’t know existed before LIGO detected them,” said David Shoemaker of MIT, the newly elected spokesperson for the 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 a billion years ago and a billion light-years distant 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, 2016, and will continue through the summer. LIGO is an international collaboration with members around the globe. Its observations are carried out by twin detectors—one in Hanford, Wash., and the other in Livingston, La.—operated by Caltech and MIT with funding from the National Science Foundation, or NSF.
As the only university within driving distance of LIGO instrumentation, LSU undergraduate and graduate students conduct research daily at the observatory and have been involved in the three detections thus far.
“When we detect a gravitational wave signal, I am part of the team that carefully analyzes the state of the instruments to make sure that the signal isn’t caused by a glitch in the detector,” said Marissa Walker, who recently received her Ph.D. from LSU. She will continue to work with LIGO as a postdoctoral researcher in the California State University-Fullerton’s gravitational waves group.
LIGO made the first-ever 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 made on Jan. 4, 2017, is described in a new paper accepted for publication in the journal Physical Review Letters.
In all three cases, each of the twin detectors of LIGO detected gravitational waves from the tremendously energetic mergers of black hole pairs—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 can also 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 compared to the overall orbital motion. More observations with LIGO are needed to say anything definitive about the spins of binary black holes, but these early data offer clues about how these pairs may form.
“This is the first time that we have evidence that the black holes may not be aligned, giving us just a tiny hint that binary black holes may form in dense stellar clusters,” said Bangalore Sathyaprakash of Penn State University and Cardiff University, one of the editors of the new paper, which is authored by the entire LSC and Virgo Collaborations.
There are two primary models to explain how binary pairs of black holes can be formed. The first model proposes that the black holes are born together: they form when each star in a pair of stars explodes, and then, because the original stars were spinning in alignment, the black holes likely remain aligned.
In the other model, the black holes come together later in life within crowded stellar clusters. The black holes pair up after they sink to the center of a star cluster. In this scenario, the black holes can spin in any direction relative to their orbital motion. Because LIGO sees some evidence that the GW170104 black holes are non-aligned, the data slightly favor this dense stellar cluster theory.
“We’re starting to gather real statistics on binary black hole systems,” said Keita Kawabe of Caltech, also 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 also once again puts Albert Einstein’s theories to the test. For example,
the 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.
“It looks like Einstein was right—even for this new event, which is two times farther away than our first detection,” said Laura Cadonati of Georgia Tech and the Deputy Spokesperson of the LSC. “We can see no deviation from the predictions of general relativity, and this greater distance allows us to make that statement with more confidence.”
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.
“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,” said 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.”
Faculty and students at LSU are working towards these future detections.
“My research is focused on how to improve the future sensitivity of LIGO,” said Jonathan Cripe, a graduate student who works with LSU Assistant Professor of Physics Thomas Corbitt. “Each of the following detections are still exciting because we continue to learn more about the binary black holes and the universe that produced them.”
LIGO is funded by the National Science Foundation (NSF), and operated by MIT and Caltech, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,000 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. LIGO partners with the Virgo Collaboration, a consortium including 280 additional scientists throughout Europe supported by the Centre National de la Recherche Scientifique (CNRS), the Istituto Nazionale di Fisica Nucleare (INFN), and Nikhef, as well as Virgo’s host institution, the European Gravitational Observatory. Additional partners are listed at: http://ligo.org/partners.php.
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