General relativity is one of the most successful theories mankind has ever developed, but one of its predictions—the existence of gravity waves—has never been confirmed. Gravity waves are analogous to the more familiar electromagnetic waves: they are disturbances in space time that travel at the speed of light and result from accelerating masses. Scientists are confident these waves exist, but experimental measurements of gravity waves are mind bogglingly complicated because they must detect changes on the order of of 10-22. This translates to detecting displacements 10,000 times smaller than an atomic nucleus over kilometer-scale distances.
So far, the gravity wave detectors have come up dry in their primary mission. But, in doing so, they've put significant constraints on where the gravity waves can exist, which has some significant consequences for cosmology.
Despite the incredible accuracy needed, several gravity wave observatories have been built, including the Laser Interferometer Gravitational-wave Observatory in the United States (3 detectors at two locations), the Virgo interferometer in Italy, and Germany's GEO. These observatories all use laser interferometry to accurately measure changes in the distance between two points.
Laser interferometry involves splitting a single laser beam so that it travels down two paths, bounces off a mirror at the end of each path, and returns to a detector where the beams are recombined to form an interference pattern. Very small changes in the distance traveled by either beam will appear as relatively large changes in the interference pattern.
A passing gravity wave will distort spacetime along each beam's path, resulting in different path lengths, and therefore interference pattern changes. By comparing data between observatories that are several thousand kilometers apart, a gravity wave could conceivably be tracked as it traveled through the earth. To date, however, no gravity waves have been confirmed by any of these detectors.
Despite the lack of gravity waves, all is not lost. In this week's Nature, the LIGO and Virgo scientific collaborations describe how their data establishes a new upper limit to the gravitational wave background, which has important ramifications for cosmology. Similar to the cosmic microwave background, cosmological theories predict "noise" in spacetime that is left over from the first moments of the big bang. While the cosmic microwave background has been mapped for large portions of the observable universe, the cosmic gravitational wave background has never been directly detected because these disturbances are far too small for the sensitivity in current experiments.
However, by comparing signal-to-noise ratios collected over two years at the various observatories, scientists have determined the maximum possible energy of the cosmic gravitational wave background must be less than 7 x 10-6 of the Universe's total energy. This is more than one order of magnitude lower than any previous measurement (prior measurements were obtained using entirely different data such as light from pulsars and models of nuclear formation following the big bang).
All of this is incredibly important because the cosmic gravitational wave background gives us a much better glimpse into the early Universe than any other observable phenomenon. For instance, accurately describing the cosmic microwave background has been vital for cosmology, but the universe was completely opaque to electromagnetic radiation for its first 379,000 years. Therefore, CMB data is useless for describing the universe in its infancy.
Gravity waves, on the other hand, have been freely propagating since 10-38 s after the big bang. In other words, gravity waves and the cosmic gravity wave background provide us with the earliest possible information about the Universe.
The upper limit determined here is not a direct observation of the background, but it puts significant constraints on cosmological theories. A perspective on the work by Marc Kamionkowski points out that this data does not overturn any of the most popular cosmological theories, but the study's authors think that these measurements rule out cosmological theories with high equation of state parameters, as well as some of the most popular superstring theories (those that rely on low string tension parameters).
Until we are able to detect gravity waves, there is little hope of directly measuring the cosmic gravity wave background. Constant sensitivity improvements in existing experiments, like LIGO, leave some hope, but new experiments will most likely be needed for direct detection. Just as mapping the cosmic microwave background changed our view of the universe and its origins, direct detection of the gravity wave background promises to revolutionize our view of the early universe.
Article: Nature DOI:10.1038/nature08278