An international team of scientists that includes UMD physicists has opened an unprecedented new window on the universe with the first observation of ripples in the fabric of space-time. These ripples, known as gravitational waves, were generated by the colliding of two massive black holes a billion light-years away from Earth. Though such black hole collisions have long been predicted, they had never before been observed.
This finding confirms Albert Einstein's prediction of gravitational waves in his 1915 general theory of relativity. And it is built, in part, on more than 50 years of work by UMD physicists, starting in the early1960s when the late UMD Physics Professor Joseph Weber, who was also a pioneer of lasers, built the world's first gravitational wave detectors on the university's College Park, Md. campus, inspiring a new field of gravitational waves research.
"Sharing the detection of the binary black hole merger event GW150914 is a sheer joy for all of us who have been working in this field," said Peter Shawhan, an associate professor of physics at UMD and a principal investigator in the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration that announced these findings on February 11. It's a dream finally realized, but it is just the beginning of the science that we can do with LIGO and other gravitational wave detectors."
Gravitational waves carry information about their dramatic origins and about the nature of gravity and of the universe that cannot be obtained any other way. This first detection of gravitational waves occurred on September 14, 2015 at 5:51 a.m. Eastern Daylight Time (9:51 UTC) by both of the twin LIGO detectors, located in Livingston, Louisiana, and Hanford, Washington.
The LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT. The discovery, published in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.
Shawhan helped to validate the analysis software that identified the black-hole merger signal a few minutes after the LIGO detectors recorded it. He also acted as a liaison with astronomers before and during the LIGO observing run.
Alessandra Buonanno, a UMD professor of physics who also has an appointment as Director at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, together with many students and postdoctoral researchers at both institutions, have developed highly accurate models of gravitational waves that black holes would generate in the final process of orbiting and colliding with each other.
"We spent years modeling the gravitational-wave emission from one of the most extreme events in the universe: pairs of massive black holes orbiting each other and then merging. And that's exactly the kind of signal we detected!" said Buonanno, who is also an LSC principal investigator.
Buonanno also co-led an effort to determine whether the signal detected by LIGO matches exactly the predictions of Einstein's theory of general relativity. So far, all tests find the signal to be consistent with this theory.
"It is overwhelming to see how exactly Einstein's theory of relativity describes reality," said Buonanno. "GW150914 gives us a remarkable opportunity to see how gravity operates under some of the most extreme conditions possible."
Early UMD work rings through LIGO finding
With this new LIGO discovery, UMD physicists continue the university's more than 50 year tradition of gravitational wave research. Weber's early detectors used "resonant bars", which were designed to ring when a gravitational wave passed through them. UMD Physics Professors Emeriti Ho Jung Paik and Jean-Paul Richard improved on Weber's technique to develop more sensitive resonant detectors. Later technology improvements enabled the more-sensitive laser interferometer technique used by LIGO, which was originally proposed as a means of detecting these gravitational waves in the 1980s by Rainer Weiss, professor of physics, emeritus, from MIT; Kip Thorne, Caltech's Richard P. Feynman Professor of Theoretical Physics, emeritus; and Ronald Drever, professor of physics, emeritus, also from Caltech.
"LIGO has been a half century quest," said Thorne, during a February 11 press conference announcing the detection of gravitational waves. "It arose in part in the 1960s from pioneering work by Joseph Weber at the University of Maryland."
Over the years, the gravity theory research group at UMD has also made many key contributions to the theory of black hole dynamics, gravitational wave emission and possible alternative theories of gravity, through the work of UMD Physics Professors Emeriti Dieter Brill and Charles Misner and UMD Physics Professor Ted Jacobson.
LIGO research is carried out by the LSC, a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration.
UMD students and alumni who contributed to the new LIGO research include UMD physics graduate student Min-A Cho, who developed software to communicate the properties of promising signals to astronomers for follow-up observations with their telescopes and other instruments. Cregg Yancey, also a UMD physics graduate student, helped to check that the detectors operated properly when the signal was detected. The black-hole merger signal stood up to all scrutiny during months of painstaking analysis and cross checks and was ultimately named GW150914, indicating the date of its arrival at Earth.
UMD alumnus Andrea Taracchini (Ph.D. '14, physics), who is now a postdoctoral researcher in Buonanno's division at the Max Planck Institute in Germany, Buonanno and Yi Pan, a former assistant research scientist in physics at UMD, developed waveform models that were employed in the search that observed the black-hole merger with high-enough significance to be confident in its detection. Researchers then used the waveform models to infer the actual astrophysical parameters of the source — the masses and spins of the two black holes, the binary black hole's orientation and distance from Earth, and the mass and spin of the enormous black hole that the merger produced.