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.

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 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.

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, which appears in the August 20 issue of the journal 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.

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 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, a 2 km and a 4 km detector in Hanford, Washington, and a 4 km instrument in Livingston, Louisiana. 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.

According to the general theory of relativity, one interferometer arm is slightly stretched while the other is slightly compressed when a gravitational wave passes by.

The interferometer is constructed in such a way that it 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.

Because of this extraordinary sensitivity, the instruments can now test some models of  the evolution of the early universe that are expected to produce the stochastic background.

"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.

"We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old," Mandic adds. "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 says, "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 says. Once it goes online 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.

"Advanced LIGO will go a long way in probing early universe models, cosmic-string models, and other models of the stochastic background. We can think of the current result as a hint of what is to come," he adds.

"With Advanced LIGO, a major upgrade to our instruments, we will be sensitive to sources of extragalactic gravitational waves in a volume of the universe 1,000 times larger than we can see at the present time. This will mean that our sensitivity to gravitational waves from the Big Bang will be improved by orders of magnitude," says Jay Marx of the California Institute of Technology, LIGO's executive director.

"Gravitational waves are the only way to directly probe the universe at the moment of its birth; they're absolutely unique in that regard. 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," says David Reitze, a professor of physics at the University of Florida and spokesperson for the LIGO Scientific Collaboration.

"The scientists of the LIGO Scientific Collaboration and the Virgo Collaboration have joined their efforts to make the best use of their instruments. Combining simultaneous data from the LIGO and Virgo interferometers gives information on gravitational-wave sources not accessible by other means. It is very suggestive that the first result of this alliance makes use of the unique feature of gravitational waves being able to probe the very early universe. This is very promising for the future," says Francesco Fidecaro, a professor of physics with the University of Pisa and the Istituto Nazionale di Fisica Nucleare, and spokesperson for the Virgo Collaboration.

Maria Alessandra Papa, senior scientist at the Max Planck Institute for Gravitational Physics and the head of the LSC overall data analysis effort adds, "Hundreds of scientists work very hard to produce fundamental results like this one: the instrument scientists who design, commission and operate the detectors, the teams who prepare the data for the astrophysical searches and the data analysts who develop and implement sensitive techniques to look for these very weak and elusive signals in the data."

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 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 (STFC).

The Virgo Collaboration designed and constructed the 3 km long Virgo interferometer located in Cascina, Italy, funded by the Centre National de la Recherche Scientifique (France) and by the Istituto Nazionale di Fisica Nucleare (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.

The next major milestone for LIGO is the Advanced LIGO Project, slated to begin operation in 2014. Advanced LIGO will incorporate advanced designs and technologies that have been developed by the LIGO Scientific Collaboration. It is supported by the NSF, with additional contributions from the U.K.'s STFC and Germany's Max Planck Society.

The paper is entitled "An Upper Limit on the Amplitude of Stochastic Gravitational-Wave Background of Cosmological Origin."

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FIGURE 1 –This image of the Himiko object is a composite and in false color. The thick horizontal bar at the lower right corner presents a size of 10 thousand light year. (Credit: This image is created by M. Ouchi et al., which is the reproduction of Figure 2 in the article of The Astrophysical Journal May 2009 - 10 v696 issue.)

The researchers are puzzled by the object. Even with superb data from the world’s best telescopes, they are not sure what it is. Because it is one of the most distant objects ever found, its faintness does not allow the researchers to understand its physical origins. It could be ionized gas powered by a super-massive black hole; a primordial galaxy with large gas accretion; a collision of two large young galaxies; super wind from intensive star formation; or a single giant galaxy with a large mass of about 40 billion Suns. Because this mysterious and remarkable object was discovered early in the history of the universe in a Japanese Subaru field, the researchers named the object after the legendary mysterious queen in ancient Japan.

“The farther out we look into space, the farther we go back in time, “ explained lead author Masami Ouchi, a fellow at the Observatories of the Carnegie Institution who led an international team of astronomers from the U.S., Japan, and the United Kingdom. “I am very surprised by this discovery. I have never imagined that such a large object could exist at this early stage of the universe’s history. According to the concordance model of Big Bang cosmology, small objects form first and then merge to produce larger systems. This blob had a size of typical present-day galaxies when the age of the universe was about 800 million years old, only 6% of the age of today’s universe!”

Extended blobs discovered thus far have mostly been seen at a distance when the universe was 2 to 3 billion years old. No extended blobs have previously been found when the universe was younger. Himiko is located at a transition point in the evolution of the universe called the reionization epoch—it’s as far back as we can see to date. And at 55 thousand light years, Himiko is a big blob for that time.

This reionizing chapter in the universe was at the cosmic dawn, the epoch between about 200 million and one billion years after the Big Bang. During this period, neutral hydrogen began to form quasars, stars, and the first galaxies. Astronomers probe this era by searching for characteristic hydrogen signatures from the scattering of photons created by ionized gas clouds.

The team initially identified Himiko among 207 distant galaxy candidates seen at optical wavelengths using the Subaru telescope from the Subaru/XMM-Newton Deep Survey Field located in the constellation of Cetus. They then made spectroscopic observations to measure the distance with the Keck/DEIMOS and Carnegie’s Magellan/IMACS instrumentation. Himiko was an extraordinarily bright and large candidate for a distant galaxy. “We hesitated to spend our precious telescope time by taking spectra of this weird candidate. We never believed that this bright and large source was a real distant object. We thought it was a foreground interloper contaminating our galaxy sample,” continued Ouchi. “But we tried anyway. Then, the spectra exhibited a characteristic hydrogen signature clearly indicating a remarkably large distance—12.9 billion light years!”

“Using infrared data from NASA’s Spitzer Space Telescope and the United Kingdom Infrared Telescope, radio data from the VLA, and X-ray imaging from the XMM-Newton satellite, we were able to estimate the star-formation rate and stellar mass of this galaxy and to investigate whether it contains an active nucleus powered by a super-massive black hole,” remarked James Dunlop a team member at Edinburgh. “We found that the stellar mass of Himiko is an order of magnitude larger than other objects known at a similar epoch, but we cannot as yet tell if the center houses an active and growing black hole.”

“One of the puzzling things about Himiko is that it is so exceptional,” said Carnegie’s Alan Dressler, a member of the team. “If this was the discovery of a class of objects that are ancestors of today’s galaxies, there should be many more smaller ones already found—a continuous distribution. Because this object is, to this point, one-of-a-kind, it makes it very hard to fit it into the prevailing model of how normal galaxies were assembled. On the other hand, that’s what makes it interesting!”

The research is published in the May 10, 2009, issue of The Astrophysical Journal. The work was funded by the NASA through an award issued by JPL/Caltech, the Department of Energy, and the Carnegie Institution. The research is based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA); the Spitzer Telescope, managed by JPL for NASA; the Magellan telescopes operated by a consortium consisting of the Carnegie Institution, Harvard University, MIT, the University of Michigan, and the University of Arizona; and the United Kingdom Infrared Telescope, which is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK.