Researchers in RIT’s Center for Computational Relativity and Gravitation have helped launch the era of gravitational wave astronomy and introduced a new branch of physics with their colleagues in the LIGO Scientific Collaboration. Their astrophysical simulations, data analyses, and big-data searches in support of the National Science Foundation-funded LIGO experiment validated the existence of gravitational waves and the black holes that produced them.
News of the landmark discovery, made public on Feb. 11, confirmed rumors that LIGO had detected the first gravitational waveform predicted by Albert Einstein’s 100-year-old general theory of relativity.
Following a five-year hiatus and a major upgrade, Advanced LIGO detected the first signal on Sept. 14, just before the start of the scheduled scientific run. The twin observatories in Livingston, La., and Hanford, Wash., detected tiny disturbances to space and time the moment gravitational waves passed through the Earth.
The L-shaped observatories, operated by Caltech and Massachusetts Institute of Technology, measure 2.5 miles long and contain precisely positioned mirrors at the end of the tunnels. Laser light travels back and forth down the length of the arms and measures the distance between the mirrors. Scientists estimate that the signal LIGO detected resulted from the impact of black holes weighing 29 and 36 times the mass of the sun.
Publication of the collaboration’s findings in Physical Review Letters, a journal of the American Physical Society, coincided with the international announcement, and included six RIT co-authors. They are James Healy, post-doctoral research fellow; Jacob Lange, graduate student in RIT’s astrophysical sciences and technology program; Carlos Lousto, professor in the School of Mathematical Sciences and an American Physical Society Fellow; Richard O’Shaughnessy, assistant professor in the School of Mathematical Sciences; John Whelan, associate professor in the School of Mathematical Sciences and principal investigator of RIT’s group in the LIGO Scientific Collaboration; and Yuanhao Zhang, graduate student in RIT’s astrophysical sciences and technology program. Center director Manuela Campanelli, professor in the School of Mathematical Sciences and an American Physical Society Fellow, Hans-Peter Bischof, professor of computer science, and RIT students Jackson Henry, Ryan Hesse, Marc McClure, Monica Rizzo, and Jam Sadiq, are also members of the RIT LIGO group.
LIGO’s landmark paper prominently cites Campanelli’s team as one of three groups that advanced modeling of black- hole mergers on supercomputers and accurately predicted gravitational waveforms. Campanelli’s 2005 breakthrough method, known as the moving puncture approach, played an integral role in enabling and interpreting the LIGO discovery. Lousto, a member of the original team, and Healy used the method to inde— pendently calculate the gravitational waves observed by Advanced LIGO by modeling the merger of simulated black holes. The observed waveform matched their prediction.
“The inclusion of our results made the paper a much stronger case for relativity because it’s a double confirmation,” Lousto said. “It is not that we have only detected gravitational waves and statistically it makes sense, but they happened to be exactly what we predicted for the collision of black holes. It ties it together very well.”
Campanelli describes the detection of the black-hole merger as “an amazing confirmation of our theoretical calculations.”
“To me, the fascination is you have these mathematical equations that describe natural phenomena—such as how binary black holes coalescence should happen—and finally you see it in nature,” she said. “It was mind blowing.”
Predicted but never confirmed until now, black holes are massive stars that have collapsed into compact objects with gravity too strong for light to escape. Einstein’s general theory of relativity predicted that massive bodies undergoing the cataclysmic event of merging, spinning, or exploding could produce gravitational waves. As the sensitivity of Advanced LIGO increases during subsequent scientific runs, scientists expect to see more black-hole mergers and other sources of gravitational waves at different frequencies, such as black hole-neutron stars or binary neutron star collisions, highly spinning neutron stars—known as pulsars—supernovas, and the Big Bang.
“At RIT, we’re working on a wide range of gravitational wave astrophysics,” Campanelli said. “We’re one of a handful of groups worldwide developing the tools and performing the simulations needed to interpret phenomena dominated by strong-field physics in Einstein’s theory of gravity.”
Research in complementary areas at the center focuses on analyzing and interpreting gravitational waveforms in the LIGO data and creating scientific visualizations. Several members of the RIT LIGO team were listed on the 12 companion papers that followed the initial discovery, including “Astrophysical Implications of the Binary Black-hole Merger GW150914,” published in The Astrophysical Journal Letters.
O’Shaughnessy and Whelan specialize in developing methods for detecting and interpreting gravitational wave signals. O’Shaughnessy’s research connects the gravitational-wave signatures observed by LIGO to the astrophysical sources that produced them. He estimates both the nature of these sources—in this case, a binary black hole—and how they formed. LIGO’s discovery is consistent with the specific method O’Shaughnessy and his collaborators use to predict how massive stars evolve into black holes and form merging pairs.
“The discovery of a black-hole merger is only the tip of the iceberg,” O’Shaughnessy said. “We’re on the cusp of a revolution in our understanding of how massive stars evolve.”
Monica Rizzo, a second-year student in the School of Physics and Astronomy, is part of the LIGO Scientific Collaboration. She works with O’Shaughnessy on models that simulate gravitational wave signals for colliding neutron stars. These stellar remnants have collapsed under their own weight but lack the mass to form into black holes. Although Rizzo did not contribute directly to the initial discovery, her research has helped advance techniques for interpreting future data.
“My work over the past few semesters has led to the development of new techniques we can use to analyze gravitational waves from binary neutron stars and has helped me become more knowledgeable as a researcher,” Rizzo said.
The scope of gravitational wave astronomy will widen as the international network of detectors becomes fully operational. Scientific runs at increasing levels of sensitivity are planned for the U.S.-based LIGO detectors and the Italian counterpart, Advanced Virgo, Whelan noted.
“We’re looking not just for binary mergers, but for a range of signals from unexplained bursts to a background ‘hum’ from many weak signals from the distant universe or even the Big Bang,” said Whelan, graduate program coordinator of RIT’s astrophysical sciences and technology program. “Closer by, our own galaxy is also full of potential strong sources such as rapidly spinning neutron stars.”
Merging pairs of neutron stars produce a fainter signal than binary black holes, but are expected to be more common in nearby galaxies. Whelan predicts that these mergers will be detected as the network’s sensitivity improves.
“One of the strengths of the center is that we now play a major role in both the simulation of gravitational waves and the scientific analysis of the LIGO data itself,” Whelan said.
The recent gravitational wave discovery is only the beginning. With financial support through RIT’s strategic research initiative, the center will build a wide-spanning and integrated program around the new science of gravitational wave astronomy and multimessenger astrophysics. CCRG researchers also plan to play a leadership role in the science of the upcoming “third-generation” gravitational-wave detectors (the current LIGO detectors are second-generation instruments), and future space-base detectors such as the Evolved Laser Interferometer Space Antenna (eLISA). Sources such as coalescing compact binary systems, neutron stars in low-mass X-ray binaries, stellar collapses and pulsars, massive black hole binaries are all possible candidates for future observatories. With these future observations, researchers will be able to map the geometry of the universe, and in the long run improve our understanding of cosmology.
Campanelli says CCRG is already contributing to many aspects of research involving supermassive binary black holes.
“On the technology side, there will be opportunities to include advancements developed for LIGO into other domains,” said Campanelli. “These include highly stable optics, squeezed light, extremely sophisticated suspension systems, and new materials. We foresee many opportunities for synergistic collaborations between theorists and engineers at RIT.”