Frame from a three-dimensional simulation of gravitational waves produced by merging black holes.
Credit: NASA
SYDNEY: A massive international effort to find evidence of ripples in space-time, known as gravitational waves, has yielded early results, narrowing the possible physical environments in which the universe was born.
As reported today in the British journal Nature, a huge international team of scientists, comprised of the LIGO Scientific Collaboration and the Virgo Collaboration, report on the most precise search yet made for these waves - although they haven't actually detected them yet.
"One of the goals of the research is the direct detection of black holes - to listen to their births and the ring-tones produced by their vibrations. Another goal is to listen to the birth of the universe itself, called the stochastic background, which should sound a bit like wind in the treetops," said Jesper Munch of the University of Adelaide in South Australia.
Munch heads up a consortium of researchers at six Australian universities who are involved in the project.
Einstein's ripples
Gravitational waves were predicted by Einstein's general theory of relativity. Physicists believe they are generated when objects accelerate and, since they are unaffected by matter, should still be bouncing around the universe since the Big Bang. If they can be found, they could carry information about the high-energy environment in which they were created.
Although gravity waves have not been directly detected, the new measurements by LIGO directly probes the gravitational wave background in the first minute of its existence, at time scales shorter than is possible by analysing the cosmic microwave background, the background radiation leftover by the Big Bang. This new insight helps to constrain models of the possible physical nature of the universe at that time.
The results were made with three interferometers, a 2-km and a 4-km detector in Hanford, Washington, and a 4-km detector in Livingston, Louisiana. The interferometers detect minuscule differences in the time of arrival of lasers sent down the arms of the L-shaped instruments. General relativity theory predicts one arm will be stretched and the other compressed as gravitational waves pass by.
Stiff energy
The result can be used to constrain the so-called "equation of state parameter" in the early universe (when the universe was younger than one minute), physicist Vuk Mandic from the University of Minnesota, in Twin Cities, U.S., told Cosmos Online.
Mandic is part of the LIGO Scientific Collaboration, and one of the paper's several hundred authors.
"This parameter cannot be very large, which rules out the "stiff" energy components in the early universe," for which a small energy change would correspond to a large pressure change, Mandic said.
The results also shed new light on cosmic superstrings; objects that some cosmological theories predict were generated in the Big Bang and then stretched across the cosmos. Loops in the cosmic strings would then create gravitational waves as they oscillate and decay, the theories predict.
Correction
I'd like to correct an incorrect statement in the above article. The article reads (quote): "Although they haven't detected the actual waves, the researchers have for the first time directly measured the amplitude of the "stochastic background" of gravitational waves, the random criss-cross of radiation in the universe, something like the intersecting ripples on the surface of a pond." The previous sentence
contradicts itself and makes the erroneous statement that researchers have for the first time directly measured the amplitude of the gravitational stochastic background. This is not correct: gravitational waves have not been directly detected to date, as the published article in Nature clearly states.
Marco Cavaglia
LIGO Scientific Collaboration