A team of scientists using the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo Collaboration have put new constraints on how the Universe looked in its earliest moments.

Analysis of data taken over a two year period between 2005 and 2007 has set the most stringent limits yet on the amount of gravitational waves that could have been created in the big bang. The results are reported in this week's edition of the journal Nature.

"Our results are a major step toward the detection of primordial gravitational waves – ripples in the fabric of space and time – that were created as the Universe expanded in its earliest moments," says Lee Samuel Finn of Penn State University. Detecting gravitational waves would provide vital clues to understanding how the structure of the Universe evolved and answer questions like why is our Universe clumped into galaxies?

The existence of gravitational waves was first predicted by Albert Einstein in 1916 and LIGO has been actively searching for them since 2002, with Virgo joining in the search in 2007. The team reporting the latest results say that these elusive waves would likely be observed as a "stochastic background," analogous to a superposition of different size and shaped ripples created on the surface of a pond. The amplitude of this background is directly related to the parameters that govern the behaviour of the Universe during the first minute after the big bang.

"Space-time is the living stage upon which the drama of the Universe plays out," says Finn. "The primordial stochastic gravitational waves are the warps, twists, and bends in space-time that were laid down as the Universe expanded from its earliest moments to the present. The observations we report in this paper are the closest direct examination of the framework of the living, breathing Universe."

Although the stochastic background of gravitational waves has not yet been discovered, earlier measurements of the cosmic microwave background (CMB), which was also produced by the big bang, provided the upper limits of the state of the Universe when it was 380,000 years old. The new results probe the gravity wave background at much shorter timescales than studies of the CMB have allowed.

"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," says 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."

The research also constrains models of so-called 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. Cosmologists say that these strings can form loops that produce gravitational waves as they oscillate, decay, and eventually disappear.

Each of the L-shaped LIGO interferometers has a laser split into two beams that travel back and forth down long interferometer arms, which monitor the difference between the two arm lengths of the interferometer. According to the general theory of relativity, when a gravitational wave passes by, one interferometer arm is slightly stretched while the other is slightly compressed. The interferometers have a sensitivity such that they can detect a change of less than a thousandth the diameter of an atomic nucleus in the lengths of the arms relative to each other.

"Gravitational waves are the only way to directly probe the Universe at the moment of its birth; they're absolutely unique in that regard," says David Reitze, 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."

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