Black holes everywhere! Ten years of LIGO and gravitational waves
On September 14, 2015, a signal arrived on Earth, carrying information about a pair of remote black holes that had spiraled together and merged. The signal had traveled about 1.3 billion years to reach us at the speed of light — but it was not made of light. It was a different kind of signal: a quivering of space-time called gravitational waves first predicted by Albert Einstein 100 years prior. On that day 10 years ago, the twin detectors of the U.S. National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO) made the first-ever direct detection of gravitational waves, whispers in the cosmos that had gone unheard until that moment.
Now, a decade later, researchers on this collaboration have “heard” the clearest evidence for a theorem proposed by Stephen Hawking in 1971 that merging black holes must grow their collective surface areas, despite energy loss and other competing factors. The team, which includes scientists at Penn State, published the finding today (Sept. 10) in Physical Review Letters.
It’s only the most recent finding resulting directly from the initial discovery of gravitational waves.
“I have seen first-hand the transformation of knowledge brought on by gravitational wave discoveries,” said Chad Hanna, professor of physics and of astronomy and astrophysics in the Eberly College of Science at Penn State, a co-hire of the Penn State Institute for Computational and Data Sciences (ICDS) and a leader of the Penn State LIGO group. “Reflecting over the last 10 years I am reminded of a job interview which occurred years before the initial gravitational wave discovery. After delivering a lecture on searching for binary black hole mergers with a null result, one faculty member in the audience was quick to point out the flaw in my research — binary black holes detectable by LIGO don’t exist! Of course, we now know that binary black hole mergers do exist, are quite common in the universe, and are regularly detected by the LIGO-Virgo-KAGRA (LVK) collaboration, but before the gravitational wave discovery 10 years ago many scientists were skeptical that LIGO would be successful.”
The first historic discovery meant that researchers could sense the universe through three different means. Light waves, such as X-rays, optical, radio and other wavelengths of light as well as high-energy particles called cosmic rays and neutrinos had been captured before, but this was the first time anyone had witnessed a cosmic event through its gravitational warping of space-time.
“LIGO finally captured the unmistakable signature of a black hole,” said B.S. Sathyaprakash, Bert Elsbach Professor of Physics and professor of astronomy and astrophysics in the Eberly College of Science at Penn State and a leader of the Penn State LIGO group. “Black holes are believed to be ubiquitous, but distinguishing them from other exotic objects that might mimic their behavior has been a longstanding challenge. The decisive evidence lies in the unique spectrum of gravitational waves that only black holes can produce. For the first time, we confirmed, with high statistical confidence, that the observed signals indeed bear this unmistakable signature. After decades of pursuit, it was deeply gratifying to see LIGO, with its unmatched technology, finally capture the definitive proof of black holes.”
