The first few moments of the Big Bang — according to most theories of how it actually worked — should have produced gravitational waves in the fabric of space-time that are still rippling through all of space today. But by now they should be extremely weak, and no detector has had the capability to detect them at any plausible predicted level.
Now that's changing. Scientists working with LIGO, the Laser Interferometer Gravitational-wave Observatory, are about to release two years of data that set a meaningful upper limit on certain frequencies of these waves from the start of everything. The limits are good enough to rule out some versions of inflation theory, the current basis for how and why the Big Bang happened.
Gravitational waves, according to Einstein's general relativity, are produced by any large masses violently changing speed. A "gravitational-wave backgound" is expected from the violent early moments of the Big Bang, rather like the cosmic microwave background that fills the sky with radio waves from the early universe. In recent years the microwave background has yielded the universe's age, density, composition, and much else to high precision.
But while the microwave background originated about 380,000 years after the Big Bang, the gravitational-wave background should come directly from events in the first one minute. The waves' strength, spectrum of frequencies, and other details should tell about the behavior of the universe during that brief, critical time.
The researchers' results, published in Nature for August 20th, set an upper limit on the gravitational-wave background by combining data from LIGO and a similar detector in Europe named Virgo. The limit also puts constraints on the existence of "cosmic strings": immensely long, line-like flaws in space-time that, theorists have proposed, might also be left over from the universe's very beginning.
"Since we have not observed the stochastic [random] background, some of these early-universe models that predict a relatively large stochastic background have been ruled out," says Vuk Mandi, assistant professor at the University of Minnesota, in a Caltech press release about the results. "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, Mandic continues, "because such strings could also be so-called fundamental strings, appearing in string-theory models" (inflated from microscopic size by the early expansion of space). "So our measurement also offers a way of probing string-theory models, which is very rare today."
LIGO is still ramping up. Due to come online in 2014 is Advanced LIGO, which will have upgraded lasers, detectors, and test-mass isolation systems. These should increase the sensitivity by at least a factor of 10, vastly enlarging the volume of space in which LIGO should be able to sense such things as chaotic supernova cores imploding and neutron-star pairs spiraling together and merging. No such events have yet been observed, but the upgrade is expected to bring them within reach.