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University of Minnesota
October 5, 2009
University physicist Vuk Mandic is part of the LIGO experiment, which includes this installation in Louisiana.
Photo: LIGO Laboratory
Home page photos: Kenny Hoang
Physicist Vuk Mandic is on a hunt for a prize quarry: gravitational waves generated as far back as the Big Bang
By Deane Morrison
Gravity may not seem weak when you're falling flat on your face, but take it from Einstein—it's puny.
In his theory of general relativity, Einstein described gravity as a curvature of space, like the sag in a mattress beneath a bowling ball. He predicted that cataclysmic events like the merging of two black holes would send ripples called gravitational waves coursing through the fabric of space at the speed of light. But they would be too feeble to detect.
That was 1916. Now, University of Minnesota physicist Vuk Mandic and 700 colleagues around the world are engaged in an ingenious, National Science Foundation-funded search called LIGO (Laser Interferometer Gravitational-wave Observatory) that will ultimately either detect the waves or spark a rethinking of Einstein's theory.
The LIGO physicists are looking beyond violent but recurring events like black hole mergers or supernova explosions. Gravitational waves were likely generated in the first unimaginably tiny fraction of a second after our Universe's birth in the Big Bang 13.7 billion years ago; thus, they hold clues to how everything came to be.
Mandic, an assistant professor of physics, is lead author of a paper in an August issue of Nature that analyzed two years' worth of LIGO data. No gravitational waves were detected, but just as listening for a sound and not hearing it can reveal something about its maximum possible loudness, so the analysis by Mandic and his colleagues placed limits on the energy content of gravitational waves. The finding is already helping to shape ideas about how the Universe formed.
Just a little perturbed
Gravitational waves perturb space in the plane perpendicular to their direction of movement, alternately stretching it in one dimension while shrinking it in the other and vice versa.
In LIGO, two perfectly synchronized laser beams travel down two 2.5-mile "arms” set at right angles to each other. Reflected by mirrors, the beams return and hit a detector, still in perfect synch.
If a gravitational wave were to stretch and shrink the arms, "the difference in the arm lengths would be about one ten-thousandth the size of the smallest atom,” says Mandic. Still, the shifting travel distances would put the beams out of synch when they hit the detector. (But no wonder Einstein doubted gravitational waves could be observed.)
LIGO operates three detectors, two at a site in Washington state and one in Louisiana. Comparing signals at widely separated sites allows scientists to discard any from local sources like seismic activity or rumbling trucks.
In 2014 an upgraded version of the experiment, Advanced LIGO, will come on line. Ten times as sensitive as the current LIGO, it will be able to explore a thousand-fold greater volume of space and so will stand a much better chance of finding gravitational waves.
"It would be surprising if Advanced LIGO didn't see anything,” says Mandic. "If it doesn't, this result would likely trigger revisions of general relativity.”
A gravity telescope
Gravitational waves are weak because gravity hardly interacts with matter at all, says Mandic. This sets gravity apart—far apart—from the other three fundamental forces of nature: electromagnetism; the strong force, which holds the nuclei of atoms together; and the weak force, which causes radioactivity.
In his role as co-chair of the Stochastic Working Group of the LIGO Scientific Collaboration, Vuk Mandic led an analysis of data from 2005 to 2007. The search revealed that over the frequency range monitored, the energy locked up in gravitational waves from any source must be less than 0.0007 percent of the Universe's total energy. Watch Mandic on a webcast by the National Science Foundation.
The four forces are thought to have been united when the Big Bang began. Because gravity interacts so weakly with matter, it's expected to have been the first to uncouple and become its own force. When and how that happened holds tremendous interest to physicists and is currently not understood.
It's clear, however, that gravitational waves will play a central role in reconstructing the Big Bang and its aftermath—especially a period known as inflation, which occurred sometime in the first trillionth (millionth of a millionth) of a second after the Big Bang.
During that instant the extremely hot, dense Universe expanded enormously. The scale was such that a pea-sized object would have grown to many times bigger than our current Universe.
As it continued to expand, the Universe cooled and gradually took the shape we observe today. One minute after the Big Bang, the first atoms formed. Another milestone was passed 300,000 years later, when the first photons of light decoupled from the stew of matter and energy and began to travel through the Universe, rendering it visible. (Of course, no one was around to see it.)
Since no visible light exists from before the 300,000-year mark, ordinary light-gathering telescopes can't probe the Universe at earlier ages. However, physicists can recreate and study the conditions that existed between one minute and 300,000 years after the Big Bang.
"Unfortunately, the exceedingly hot conditions that existed in the earliest moments after the Big Bang cannot be recreated and studied in laboratories,” says Mandic. But gravitational waves from those times, even from the period of inflation, may still ripple the fabric of space and time.
"This leaves gravitational waves as potentially the only way of looking back at the very beginning of the Universe and of studying the physical laws that apply at correspondingly high temperatures,” Mandic says.
LIGO is thus a gravity-based telescope turned on the first moments of creation, seeking revelations no other signal can carry. When detected, the ancient gravitational waves will be mixed with more recent ones from black holes, supernova explosions, etc. in a cacophony whose amplitude holds clues to the behavior of the Universe all the way back to the Big Bang.
"We're really looking for the ‘white noise' of the Universe,” muses Mandic.
It's a noise he can't wait to hear.