Scientists are hoping that the discovery of a second set of gravitational waves, almost simultaneously detected on Christmas Day in Livingston Parish and Hanford, Washington, will provide the start for mapping out black holes throughout the universe.
On Wednesday, the second discovery of gravitational waves, or ripples in the fabric of space-time, was announced during a news conference where the scientists discussed the detection of a second black hole merger, which occurred about 1.4 billion years ago.
The detection was made at 3:38 a.m. UTC on Dec. 26 (or 9:38 p.m. CST on Christmas Day) by the Laser Interferometer Gravitational-Wave Observatory’s twin detectors in Louisiana and Washington.
LIGO scientists determined that this set of waves was produced during the final moments of the merger of two black holes — 14 and 8 times the mass of the sun — to produce a single, spinning black hole that is 21 times the mass of the sun.
“It is very significant that these black holes were much less massive than those observed in the first detection,” said Gabriela Gonzalez, spokeswoman for the LIGO Scientific Collaboration and a professor of physics and astronomy at LSU. “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 population of black holes in our universe.”
Gravitational waves are caused by cosmic events like colliding black holes or neutron stars, explosive supernovas and even the birth of the universe. The waves travel across the universe at the speed of light, carrying with them information about their origins and about the nature of gravity that cannot be otherwise obtained.
In February, LIGO announced the world’s first detection of gravitational waves had been made on Sept. 14, just days before the detectors were scheduled to begin a four-month observing period looking for such events. That detection was of the final fifth of a second of a 1.3 billion-year-old merger of two black holes, measuring 36 and 29 solar masses apiece, and giving off three solar masses worth of gravitational waves when they merged.
The Dec. 26 gravitational wave signal represents about one solar mass of energy.
Both discoveries were made possible by the enhanced capabilities of Advanced LIGO — a five-year, $205 million upgrade that LIGO officials have said ultimately will increase the sensitivity of the instruments about tenfold compared with the first generation LIGO detectors, enabling an exponential increase in the volume of the universe they can probe.
“We are starting to get a glimpse of the kind of new astrophysical information that can only come from gravitational wave detectors,” said MIT’s David Shoemaker, who led the Advanced LIGO detector construction program.
“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 France A. Cordova, director of the National Science Foundation, which funds the LIGO observatories operated by Caltech and MIT. “NSF’s 40-year investment in this foundational research is already yielding new information about the nature of the dark universe.”
The observatories also recorded a possible gravitational waves detection Oct. 12, but that signal was not statistically significant enough for it to be considered a detection.
While the Sept. 14 signal was the strongest — leaping out from the background data with a signal-to-noise ratio of 24, compared with 13 for the December signal and 9.7 for the October signal — the December signal was the longest, giving the scientists a look at the black holes’ last 27 orbits around each other before their merger.
Within that signal, scientists found evidence that at least one of the pair of black holes was spinning before the merger and that the final combined black hole also spins, Gonzalez said. What they cannot yet decipher from the data is how the black holes formed in the first place.
“There are several scenarios, and we cannot yet distinguish those,” Gonzalez said.
David Reitze, LIGO’s executive director, said more detections will help scientists better understand what might cause these systems to form.
“With two detections, we’re not quite there yet,” Reitze said.
Joseph Giaime, head of the Livingston observatory and an LSU professor of physics and astronomy, said the December detection was “a wonderful capstone” to Advanced LIGO’s first observational run.
“The months of round-the-clock work by the LIGO staff and collaborators during the run allowed us to see this new pair of black holes and begin to understand the variety nature offers,” Giaime said.
As the scientists continue to analyze the data from the first run for evidence of signals from nonblack hole sources — pulsars or neutron stars, for example — the team also is preparing to begin a second observing run this fall.
The scientists are working to improve the detectors’ sensitivity, including increasing the power of the laser from 25 watts to 50 watts, Reitze said. The equipment is designed to eventually reach 125 watts.
Increasing sensitivity will allow the scientists to probe even farther into space, increasing the likelihood they will detect objects and phenomena less massive than black holes.
The two black hole mergers whose gravitational waves shook LIGO’s instruments late last year were each more than a billion light years away, but a less massive binary neutron star system would be detectable only within about 500 million light years, Reitze said.
Virgo, the European interferometer near Pisa, Italy, is expected to join in the latter half of the fall observing period.
Based on the arrival time of the December signal — with the Livingston detector measuring the waves 1.1 milliseconds before the Hanford detector — scientists can roughly determine where in the sky the waves originated.
When Virgo comes online, the scientists will be able to more accurately pinpoint the source of any gravitational waves detected and improve their contributions to multimessenger astronomy, said Fulvio Ricci, spokesman for the Virgo Collaboration.
The gravitational wave detections confirm a major prediction of Albert Einstein’s 1915 general theory of relativity and mark the beginning of the new field of gravitational-wave astronomy.
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.”
LSU’s investment in gravitational-wave detection spans more than four decades and is among the longest of the more than 90 universities and research institutes across 15 countries contributing to the LIGO Scientific Collaboration.
The LIGO Livingston observatory is located on LSU property, and LSU faculty, students and research staff are major contributors to the collaboration, which develops detector technology and analyzes data for the network of LIGO interferometers and the GEO600 detector in Germany.
LSU Assistant Professor Thomas Corbitt, whose research focuses on advanced quantum metrology techniques for a future detector, said scientists “have to find ways to make detectors even more sensitive, so that we can observe other types of events that are not as loud as black holes colliding. The discrete, or quantum, nature of the light used in the measurement limits the sensitivity, but we are developing methods to manipulate this noise source in order to improve future detectors.”
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