Artist's impression of gravitational waves from two orbiting black holes.
Credit: K. Thorne (Caltech) and T. Carnahan (NASA GSFC)
This time, scientists are excited to find nothing.
In results announced today, a huge physics experiment built to detect gravitational waves has yet to find any.
Rather than be disappointed by the null findings, physicists say the results were expected, and in fact help them narrow down possibilities for what the universe was like just after it was born.
The Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration (LIGO) is a set of instruments in Louisiana and Washington built to search for evidence of gravitational waves, which are theoretical ripples in space-time thought to be caused by the acceleration of mass. No one has yet directly detected these waves, though they are predicted by Einstein's theory of General Relativity, and are widely thought to permeate our universe.
In theory, every time mass accelerates - even when you rise up out of your chair - the curvature of space-time changes, and ripples are produced. However, the gravitational waves produced by one person are so small as to be negligible. The waves produced by large masses, though, such as the collision of two black holes or a large supernova explosion, could be large enough to be detected.
LIGO has only been running for a few years - the new results are based on measurements taken between 2005 to 2007 - and it is not yet at its highest level of sensitivity. The fact that this first period of observations did not detect gravitational waves allows researchers to rule out the possibility of waves above a certain amplitude threshold. Simply put, if there were any waves big enough for LIGO to have detected them, it would have. Since it didn't, they aren't likely to exist.
"I wouldn't say it's surprising that we're ruling them out," said Vuk Mandic, a physicist at the University of Minnesota who led the new analysis. Most physicists think the models that would have produced gravitational waves above the threshold that could have been seen so far are unlikely, he said.
The next phase of the project, called Advanced LIGO, will improve the experiment's sensitivity, allowing scientists to probe a volume of space about 1,000 times larger than the current project's range.
"If Advanced LIGO doesn't see gravitational waves I think people will be very surprised," Mandic told SPACE.com. "It is likely such a situation would require revision of General Relativity."
In particular, scientists are hoping to eventually find evidence for gravitational waves created by the Big Bang, the explosion thought to have begun the universe. According to theory, the Big Bang would have caused a flood of gravitational waves whose aftermath could still be seen today. This aftermath of many waves of different sizes and directions superimposed on top of each other, much like the chaotic surface of a pond after rain has fallen on it, is called the "stochastic background."
LIGO's null result has limited the possible strength of the stochastic background. Now that researchers know this background can't be strong enough to have been detected so far, they can put new constraints on the details of how the universe looked in its earliest moments.
"The thing that makes this exciting is that this is really the only way to probe the early universe," said David Reitze, a physicist at the University of Florida and the spokesperson for the LIGO Scientific Collaboration. "It's starting to limit some of the exotic models of cosmology."
For example, some models predict the existence of cosmic strings, which are loops in space-time that may have formed in the early universe and gotten stretched to large scales along with the expansion of the universe. These objects are thought to produce bursts of gravitational waves as they oscillate. Since no large-amplitude gravitational waves were found, cosmic strings, if they exist at all, must be smaller than some models predict.
The results are detailed in the Aug. 20 issue of the journal Nature.
Laser beam detectors
LIGO is composed of a series of L-shaped detectors. At each detector, a laser beam is split in half, with each half routed through one of the arms of the L, and then the two are reunited. If the two beams have travelled the same distance in their separate arms, when they are merged again they will overlap perfectly.
Gravitational waves are thought to stretch and compress dimensions of space-time perpendicular to the plane of the wave.
If a strong enough gravitational wave passes through the area of the detectors, it will stretch one of the arms of the L and shrink the other, causing the first laser beam to be offset from the second. Researchers would notice this as a slight lessening of power in the resulting laser beam.
If the same signal is found at the detectors in both Louisiana and Washington, scientists can rule out a local fluke for the results.
"The experiments are challenging," Caltech physicist Marc Kamionkowski, who was not involved in the LIGO project, wrote in an accompanying essay in Nature. "They require detection of minute changes."
Kamionkowski called LIGO "an experimental tour de force."
If LIGO does eventually detect gravitational waves, the particularities of the waves, including their frequencies, will tell researchers whether they are part of the stochastic background, or caused by something more recent, like a nearby supernova.
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