What happened after the Big Bang is one of the biggest puzzles of all that has intrigued scientists and philosophers alike. Now, an investigation by the LIGO (which stands for Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration and the Virgo Collaboration has significantly advanced our understanding the early evolution of the universe.
The Aug. 20 issue of the journal Nature reports new measurements taken by LIGO that directly probe the gravitational wave background in the first minute of its existence, at time scales much shorter than accessible by earlier measurements of the cosmic microwave background.
Gravitational wave scientists who form the LIGO Scientific Collaboration and the Virgo Collaboration have set the most stringent limits yet on the amount of gravitational waves that could have emanated from the Big Bang in the gravitational wave frequency band observable to LIGO. The results reported in “An Upper Limit on the Amplitude of Stochastic Gravitational-Wave Background of Cosmological Origin,” by authors from the LIGO Scientific Collaboration and the Virgo Collaboration are based on two years’ worth of data collected beginning in 2005.
The paper’s authors include John Whelan, an associate professor in Rochester Institute of Technology’s School of Mathematical Sciences and a member of RIT’s Center for Computational Relativity and Gravitation. Whelan is the principal investigator of RIT’s LIGO Scientific Collaboration group, which includes fellow faculty Hans-Peter Bischof and Carlos Lousto, center director Manuela Campanelli, postdoctoral researcher Hiroyuki Nakano and several students. Whelan first presented preliminary results from this stochastic background search on behalf of the LIGO Scientific Collaboration at the American Astronomical Society meeting in January 2008.
“This is the latest in a series of LIGO results which break new ground and tell us things about the universe that complement what we know from conventional astronomical observations,” Whelan said. “Gravitational wave observatories are now doing astronomy alongside optical and radio telescopes, neutrino and cosmic ray detectors, and the like.”
The paper appearing in Nature 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.
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 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 universe during the first minute after the Big Bang.
The LIGO project, funded by the National Science Foundation, was designed and is operated by Caltech and the Massachusetts Institute of Technology for the purpose of detecting gravitational waves, and for the development of gravitational-wave observations as an astronomical tool.
Research is carried out by the LIGO Scientific Collaboration, a group of 700 scientists at universities around the United States and in 11 foreign countries. The LIGO Scientific Collaboration interferometer network includes the LIGO interferometers and the GEO600 interferometer, which is located near Hannover, Germany, and designed and operated by scientists from the Max Planck Institute for Gravitational Physics, along with partners in the United Kingdom funded by the Science and Technology Facilities Council.
The Virgo Collaboration designed and constructed the three-kilometer long Virgo interferometer located in Cascina, Italy, funded by the Centre National de la Recherche Scientifique in France and by the Istituto Nazionale di Fisica Nucleare in Italy. The Virgo Collaboration consists of 200 scientists from five Europe countries and operates the Virgo detector. Support for the operation comes from the Dutch–French–Italian European Gravitational Observatory Consortium. The LIGO Scientific Collaboration and Virgo work together to jointly analyze data from the LIGO, Virgo, and GEO interferometers.
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For more information, please see Caltech’s LIGO Listens for Gravitational Echoes of the Birth of the Universe, which reports on “An Upper Limit on the Amplitude of Stochastic Gravitational-Wave Background of Cosmological Origin,” by B. P. Abbott et al (The LIGO Scientific Collaboration & The Virgo Collaboration), Nature 460, 990-994 (2009).