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Gravitational Wave Observatory listens for echoes of universe's birth

An investigation by a major scientific group headed by a University of Florida professor has advanced understanding of the early evolution of the universe.

An analysis of data from the Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration, or LIGO, and the Virgo Collaboration 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, scientists have put new constraints on the details of how the universe looked in its earliest moments.

"Gravitational waves are the only way to directly probe the universe at the moment of its birth; they're absolutely unique in that regard," said David Reitze, a UF professor of physics and the spokesperson for the LIGO Scientific Collaboration. "We simply can't get this information from any other type of astronomy. This is what makes this result in particular, and gravitational-wave astronomy in general, so exciting."

The research is set to appear in the Aug. 20 issue of the journal Nature. Seventeen UF faculty members, postdoctoral associates and graduate students join the paper's authors.

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 carry information about the universe as it was immediately after the Big Bang. These waves would be observed as the "stochastic background," analogous to a superposition 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 infant universe.

Earlier measurements of the cosmic microwave background have placed the most stringent upper limits of 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 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. These strings, some cosmologists say, can form loops that produce gravitational waves as they oscillate, decay and eventually disappear.

Gravitational waves carry with them information about their violent origins and about the nature of gravity that cannot be obtained by conventional astronomical tools. The existence of the waves was predicted by Albert Einstein in 1916 in his general theory of relativity. The LIGO and GEO instruments have been actively searching for the waves since 2002; the Virgo interferometer joined the search in 2007.

The UF LIGO research group built one of the most important and complex parts of the gravitational wave detector, the input optics, said David Tanner, a UF professor of physics. The input optics takes light from the laser, shapes the beam into an ideal form, and directs it to the interferometer at the heart of the gravitational wave detector. UF scientists are working to design and build a second version of the input optics for a major upgrade to LIGO scheduled to go on line in three to four years.

"UF also plays important role in analysis of LIGO data, including searches for sharp bursts of gravitational waves, and for the stochastic background of gravitational waves … the subject of the just published paper," Tanner wrote in an e-mail.

The authors of the new paper 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 analysis used data collected from the LIGO interferometers in Hanford, Wash., and Livingston, La. Each of the L-shaped interferometers uses a laser split into two beams that travel back and forth down long interferometer arms. The two beams are used to monitor the difference between the two interferometer arm lengths.

"Since we have not observed the stochastic background, some of these early-universe models that predict a relatively large stochastic background have been ruled out," said Vuk Mandic, assistant professor at the University of Minnesota and the head of the group that performed the analysis. "We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old."

Dave Reitze | EurekAlert!
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