U of M physicist is lead author on research about early evolution of the universe

Article in Nature is first major paper to result from international collaboration

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View video interview with University of Minnesota physicist Vuk Mandic about the LIGO Scientific Collaboration's research about the early universe.University of Minnesota Institute of Technology physicist Vuk Mandic is the lead author of new research that significantly advances scientific understanding of the early evolution of the universe. Mandic is co-chair of the Stochastic Working Group of the LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration (LSC), a National Science Foundation collaboration of about 700 scientists from around the world. The research findings are published in the Aug. 20 issue of Nature, an international weekly journal of science.

Founded in 1997, the LSC seeks to directly detect gravitational waves, use them to explore the fundamental physics of gravity, and develop the emerging field of gravitational wave science as a tool of astronomical discovery. Gravitational waves carry with them information about their violent origins and about the nature of gravity that cannot be obtained by conventional astronomical tools. This is the first major paper to result from LIGO research on the early universe.

Analysis of data taken over a two-year period, from 2005 to 2007, has set the most stringent limits yet on the amount of gravitational waves that could have come from the Big Bang in the gravitational wave frequency band where LIGO can observe. In doing so, the gravitational-wave scientists have put new constraints on the details of how the universe looked in its earliest moments.

"This paper is exciting," said Beverly Berger, NSF program manager for gravitational physics. "LIGO is making real astronomical measurements and looking at the universe in a completely new way."

Much like it produced the cosmic microwave background, the Big Bang is believed to have created a flood of gravitational waves--ripples in the fabric of space and time -- that still fill the universe and carry information about the universe as it was immediately after the Big Bang. These waves would be observed as what is called the "stochastic background," similar to a mixture of many waves of different sizes and directions on the surface of a pond. The amplitude of this background is directly related to the parameters that govern the behavior of the universe during the first minute after the Big Bang.

Earlier measurements of the cosmic microwave background have placed the most stringent upper limits of on the stochastic gravitational wave background at very large distance scales and low frequencies. The new measurements by LIGO directly probe the gravitational wave background in the first minute of its existence, at time scales much shorter than accessible by the cosmic microwave background.

The authors of the research report that the stochastic background of gravitational waves has not yet been discovered. But the nondiscovery of the background described in the Nature paper already offers its own brand of insight into the universe's earliest history.

The research also constrains models of cosmic strings, objects that are proposed to have been left over from the beginning of the universe and subsequently stretched to enormous lengths by the universe's expansion; the strings, some cosmologists say, can form loops that produce gravitational waves as they oscillate, decay and eventually disappear.

"We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old," Mandic said. "We also know that if cosmic strings or superstrings exist, their properties must conform with the measurements we made -- that is, their properties, such as string tension, are more constrained than before."

This is interesting, he added, "because such strings could also be so-called fundamental strings, appearing in string-theory models. So our measurement also offers a way of probing string-theory models, which is very rare today."

"This result was one of the long-lasting milestones that LIGO was designed to achieve," Mandic said. In 2014, Advanced LIGO, which will utilize the infrastructure of the LIGO observatories and be 10 times more sensitive than the current instrument, will allow scientists to detect cataclysmic events such as black-hole and neutron-star collisions at 10-times-greater distances.

Mandic said his research group at the University of Minnesota specifically conducted the analysis of the LIGO data that produced the result published in the Nature paper and studies of implications of this result for different theoretical models of the stochastic background. The University of Minnesota group is also performing other searches for gravitational waves leading some of the research and development efforts for future generations of gravitational wave detectors, and is contributing to the ongoing new science run of LIGO.

The LIGO project, which is funded by the National Science Foundation (NSF), was designed and is operated by Caltech and the Massachusetts Institute of Technology. Research is carried out by the LIGO Scientific Collaboration, a group of about 700 scientists from more than 60 institutions and 11 countries worldwide.

August 20, 2009