PULLMAN Einstein’s general theory of relativity predicts them. And tiny movements in large objects confirm them. But scientists have yet to see gravitational waves.
“I want to see them in the flesh,” said Sukanta Bose, a faculty member in WSU’s physics and astronomy department. “Until we detect them, they are a belief, not yet turned into truth.”
Gravitational waves are invisible ripples in “space-time.” They fill the universe and carry information about the Big Bang, the event that started our universe.
Bose has been part of a large, international team that has been researching gravitational waves for years. Results of the work have been published in the Aug. 20 issue of the journal Nature.
The article reports new constraints on the details of how the universe looked in its earliest moments, including information that touches on the scientific ideas of Stephen Hawking and other eminent physicists. Two of Bose’s graduate students, Szymon Steplewki and Dipongkar Talukder, contributed to the work reported in the article.
Gravitational waves can be detected by enormous devices that have been built around the world. One is at Hanford, Wash., and is called LIGO (Laser Interferometer Gravitational-Wave Observatory).
LIGO has two giant arms laid out at right angles on the surface of the Earth. One is 4 kilometers long (about 2.4 miles) and the other is half that size.
According to the general theory of relativity, one interferometer arm is slightly ever so slightly stretched while the other is slightly compressed as the gravitational wave passes through it. The difference in the length of the two great arms due to these changes is less than a thousandth’s of an atom’s nucleus, but it can be measured by interference in laser light bouncing back-and-forth along the arms.
Bose led the development of the signal simulation tools for an end-to-end testing and validation of LIGO’s data-acquisition and detection pipeline. It was critical to obtaining the most stringent limits yet on the amount of gravitational waves that could have come from the Big Bang.
Once gravitational waves are observed, Bose is sure, much more will be learned about them and their sources. That’s the goal that drives the work of faculty and students alike.
Much like it produced 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.
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 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.
The authors of the 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, and a 4-km instrument in Livingston, La. Because of their extraordinary sensitivity, the instruments can test some models of the evolution of the early universe that predict the existence of the stochastic background.
“Unlike any other known gravitational-wave signal, the detection of this cosmological background crucially depends on making coordinated observations with more than one detector around the world working in tandem,” said Bose. “This is because the random nature of the signal makes it impossible to predict what it will look like at a given place and time. However, given what one sees in one detector, one can predict what to expect of the signal in other detectors.”
The LIGO project, which is funded by the National Science Foundation, was designed and is operated by Caltech and the Massachusetts Institute of Technology for the purpose of detecting gravitational waves and for 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. Until recently WSU was the only university in the state and much of the Pacific and Inland Northwest to be represented in this collaboration owing to the participation of Bose, who is also a member of the collaboration’s council.
The Nature paper is entitled, “An Upper Limit on the Amplitude of Stochastic Gravitational-Wave Background of Cosmological Origin.” It can be found online here.
