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Mandic Group Plays Role in Double Discovery

Vuk Mandic
Vuk Mandic
Alex Schumann

Scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) confirmed that they have observed gravitational waves for the first time. These ripples in the fabric of spacetime caused by a cataclysmic event in the distant universe, were predicted by Einstein a hundred years ago, but never before observed. Physicists have concluded that the observed waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

"The observed event is not only the first direct detection of gravitational waves, confirming Einstein’s Theory of General Relativity. It also confirms that stellar-mass black holes exist, that they can live in pairs (binaries), and that the binaries lead a dynamic life potentially merging during the lifetime of the Universe," School of Physics and Astronomy Professor Vuk Mandic said. Mandic has been a part of the LIGO collaboration since he joined the School in 2007.

Part of the importance of the discovery of gravitational waves, lies in their usefulness as an observational tool. Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Mandic is one of the leads of a companion paper describing the implications of the observed gravitational waves event.

Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. By looking at the time of arrival of the signals—the detector in Livingston recorded the event seven milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second in the observed event, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, emitting gravitational waves of energy equivalent to three times the mass of the sun. The peak power output was estimated to be about 50 times that of the entire visible universe.

Researchers say this may be only the beginning. Gravitational waves are expected to open a new window onto the universe, providing information about many phenomena including the Big Bang itself.

“For every observed binary black hole merger, there are many more that are too distant and too faint to be directly observed,” postdoctoral researcher Gwynne Crowder said.

These mergers add up to make a ‘noise’ of gravitational waves, similar to the noisy surface of water in a pond. This noise is also potentially detectable by researchers. Such a detection would probe a very distant part of the population of black hole binaries and it would inform us about the evolution of these binaries and of the observed structure in the universe.

"The wealth of information extracted from the observed binary black hole merger is indicative of the scientific potential of gravitational wave observations, and it motivates us to look forward to future gravitational wave detectors," Mandic said.

Developing the next generation of gravitational-wave detectors with improved sensitivity requires addressing a number of technical obstacles, including the noise limitations induced by seismic motion of the ground near the detectors.

To this end, the Minnesota group is leading an interdisciplinary NSF-funded project with a group of geophysicists, aiming to study the behavior of seismic waves underground. The group has developed a unique three-dimensional array of seismometers at the Homestake mine, SD, which is now collecting data.

"The Homestake data will allow us to study the composition and the origin of seismic waves at different depths, hence informing the design of potential future underground gravitational-wave detectors," Mandic said.

With the NSF funding support, the Minnesota group has led a series of data analysis projects searching for gravitational waves in LIGO data, as well as detector characterization studies aimed at understanding and improving the performance of LIGO detectors and of their response to gravitational waves. The group’s work led to multiple publications in Nature, Physical Review Letters, and other leading physics journals. Other current members of Mandic’s group are graduate students Tanner Prestegard and Patrick Meyers.

About LIGO

The gravitational waves were detected on September 14, 2015 at 5:51 a.m. Eastern Daylight Time (9:51 a.m. UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA.

LIGO research is carried out by the LIGO Scientific Collaboration (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. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom, and the University of the Balearic Islands in Spain.

LIGO 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.

Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups: 6 from Centre National de la Recherche Scientifique (CNRS) in France; 8 from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; 2 in The Netherlands with Nikhef; the Wigner RCP in Hungary; the POLGRAW group in Poland and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy.

The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed—and the discovery of gravitational waves during its first observation run. The US National Science Foundation leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, and the University of Wisconsin-Milwaukee. Several universities designed, built, and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University in the City of New York, and Louisiana State University.