Three billion years ago in a distant galaxy, two massive black holes slammed together, merged into one and sent space–time vibrations, known as gravitational waves, shooting out into the universe.
The waves passed through Earth and were detected early this year by an international team of scientists, including WSU physicists Sukanta Bose, Bernard Hall and Nairwita Mazumder.
The newfound black hole, first reported in the journal Physical Review Letters in June, has a mass about 49 times that of the sun. The collision that produced it released more power in an instant than is radiated by all the stars and galaxies in the universe at any moment.
Findings from this and two previous discoveries of black hole mergers are providing the WSU scientists and colleagues at the Laser Interferometer Gravitational-Wave Observatory (LIGO) an unprecedented glimpse into the early universe and shedding new light on how binary black holes form. The researchers are using data from the three detections to subject Einstein’s theory of general relativity to increasingly stringent observational tests. So far, however, there is no indication that these events deviate from Einstein’s predictions.
“The recent detection appears to be the farthest yet, with the black holes located about three billion light-years away,” said Bose, professor and researcher in the Department of Physics and Astronomy. “Data from the discoveries are helping us explore the history of our universe in ways that were not possible before.”
How do binary black holes form?
LIGO’s latest findings provide clues about the directions in which the black holes were spinning before they collided. As pairs of black holes spiral together, heading towards a collision, they also spin on their own axes-like a pair of figure skaters spinning individually while also circling around each other.
The LIGO team’s analysis suggests these spins were misaligned, indicating that the pair of black holes might not always have been together in a tight binary system but, rather, randomly came together over time.
“While our findings are not conclusive, they suggest that binary black holes aren’t always born together,” Bose said. “In a fraction of cases, they may be born from stars in very different parts of a galaxy or star cluster then come together later in life.”
Scientists at WSU collaborated with members of the LIGO Scientific Collaboration to distinguish gravitational wave signals from noise artifacts. They contributed to the work of canceling out the other myriad noises picked up by the twin LIGO detectors in Hanford, Wash., and Livingston, La., that weren’t gravitational waves.
LIGO’s detectors are designed to register the slightest of vibrations — 1/10,000th the diameter of a proton — caused by signals from space. But the devices also detect other disturbances triggered by such earthly events as trucks on a highway, earthquakes, explosions, lightning strikes and even waves crashing on the shore hundreds of miles away.
Bose, Mazumder and Hall worked with other LIGO scientists to identify the frequencies of these disturbances. Akin to a giant set of noise-canceling headphones, their work helped researchers home in on deep space signals while blocking out everything else.
“Since 2015, we have worked hard with our colleagues to improve the sensitivity of the LIGO Detectors. A false alarm, where LIGO mistakes a signal from a non- astrophysical source like an earthquake or lightning strike as a gravitational wave, is only likely to occur once every 70,000 years,” Bose said. “LIGO’s increased sensitivity is helping to extend the depth to which we can search for these cosmic events.”
Boosting location capabilities
Finding the exact location of the source of gravitational waves deep in space is challenging work. LIGO scientists were able to identify the wide patch of sky where the black hole merger took place but were unable to pinpoint its exact location.
Bose is helping to develop a third LIGO detector in India that will provide the triangulation necessary to more precisely locate gravitational wave-producing objects in space. The new facility is expected to be operational by 2024.
The first two detections of gravitational waves generated by the collision of two black holes were reported last year. They marked the end of a decades-long, multimillion dollar quest to find them and confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity.
The LIGO Scientific Collaboration (LSC) is funded by the National Science Foundation and led by the California and Massachusetts Institutes of Technology. The LSC and the Virgo Collaboration in Europe made this discovery, and together consist of more than 1,000 scientists from around the world.
— By Will Ferguson, College of Arts and Sciences