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Gravitational waves detected 100 years after Einstein's prediction

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LIGO opens new window on the universe with observation of gravitational waves from colliding black holes; University of Michigan researchers contribute to discovery

ANN ARBOR—For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at Earth from a cataclysmic event in the distant universe.

The discovery confirms a major prediction of Albert Einstein's 1915 general theory of relativity and opens an unprecedented new window onto the cosmos.

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational 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 gravitational waves were detected Sept. 14, 2015, at 5:51 a.m. Eastern Daylight Time (9:51 UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, La., and Hanford, Wash.

The LIGO Observatories are funded by the National Science Foundation and were conceived, built and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO600 Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

University of Michigan researchers have been involved in the collaboration from the start.

"This first observation of gravitational waves opens up a new field of astronomy, one in which we 'listen' to the vibrations of space itself, using instruments of unprecedented sensitivity," said Keith Riles, professor of physics at the U-M College of Literature, Science, and the Arts, and a member of the LIGO collaboration's executive committee.

The collaboration includes more than 1,000 scientists from universities around the world. U-M researchers joined the hunt for gravitational waves in 1997 as charter members of the collaboration.

Each LIGO observatory consists of about five miles of vacuum tubes arranged in an L-shape. The arms of the L are each about 2.5-miles long and capped off with mirrors. Interferometers measure interference patterns between two sources of light. In each LIGO detector, one laser beam gets split into two, and each offshoot travels down one of the arms and is reflected back by the mirror at the end. The system is set up so that if both arms' beams arrive back at the start together, they cancel one another, making no signal. But if one beam gets back after the other, a signal results.

Why would one beam get back later? Gravitational waves stretch spacetime—and everything in it—in a characteristic pattern. They can stretch one arm of LIGO while shrinking the other. That's what happened this fall. In one arm at both observatory locations, the scientists measured a change of less than a millionth of a billionth of an inch.

Richard Gustafson, senior research scientist in physics at U-M, is stationed at LIGO Hanford Observatory in Washington. He works with others to improve the interferometers' sensitivity by eliminating outside sources of environmental and instrumental noise.

Gustafson arrived at work that morning shortly after an online detection program had reported the exciting signal candidate. That prompted an immediate freeze of operations and a comprehensive assessment of the states of the interferometers at both Hanford and Livingston.

"I was planning to run some measurements in the optics lab that day, but that plan went out the window," Gustafson said, chuckling.

The team was surprised to observe such massive black holes so soon in the data run, especially at the relatively close distance of about a billion light-years away. They had expected such objects to be more rare and even more distant.

Before the team could celebrate, they had to do some verification. Riles also serves on LIGO's Detection Committee, a team of experienced scientists who spent more than four months scrutinizing this recent discovery to confirm that the signal came from the sky and not an Earth-bound source or an instrument glitch.

"Our first priority was making sure we weren't fooling ourselves," he said. "We looked at every possible non-astrophysical explanation and systematically ruled each one out. Only then did we turn our attention to the astrophysical implications."

A major upgrade to LIGO over the last five years dramatically reduced noise at low frequencies, which was critical to detecting these signals from massive black holes, Riles said. He believes this first signal is just the beginning.

"As LIGO sensitivity improves and more data is collected within the next few years, we should be detecting mergers of binary black hole systems routinely, giving us a unique probe of not only the most exotic objects in the universe, but also testing our fundamental understanding of gravity," he said.

Aside from helping improve LIGO instrumental performance, the Michigan group focuses on searching the data for signals even tinier than the one detected Sept. 14—but longer-lasting. They look for continuous-wave signals that are between 1,000 and 10,000 times weaker. Riles co-leads a group of about 40 physicists and astronomers from the U.S., Europe, Australia and Asia who search for these waves, which could be emitted by fast-spinning neutron stars in our galaxy.

Riles will speak at two upcoming Saturday Morning Physics lectures. On Feb. 13, he will explain what gravitational waves are and where they come from. On Feb. 20, he'll describe how these minute waves can be detected and what the Sept. 14 signal tells us about the fundamental laws of the universe and black holes. These free lectures are at 10:30 a.m. in 170 Weiser Hall on U-M's central campus.

LIGO research is carried out by the LIGO Scientific Collaboration, a group of more than 1,000 scientists from universities around the U.S. 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) and Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University and other universities in the United Kingdom, funded by the Science and Technology Facilities Council. Significant computer resources have been contributed by the AEI Atlas cluster, the LIGO Laboratory, Syracuse University and the University of Wisconsin-Milwaukee.

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 Scientific Collaboration, a group of more than 250 physicists and engineers belonging to 18 different European laboratories, six of Centre National de la Recherche Scientifique in France, eight of Istituto Nazionale di Fisica Nucleare in Italy, Nikhef in the Netherlands, the Wigner Institute in Hungary, the POLGRAW group in Poland and the European Gravitational Observatory, the laboratory hosting the Virgo interferometer.

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 U.S. 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) 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.

Other members of the Michigan group include graduate students Ansel Neunzert and Orion Sauter, research assistant Weigang Liu, and undergraduates Eilam Morag, Pranav Rao and Stephen Trembath-Reichert. Eight alumni are also involved.


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