Editor's Note (Feb. 11): Gravitational waves predicted by Albert Einstein's general theory of relativity have been detected directly at last. Einstein was right. You can see our full discovery story here; a video on the find here; and our complete coverage of the historic scientific discovery here.
Gravitational waves are the "smoking gun" of the Big Bang.
Predicted by Albert Einstein's theory of general relativity in 1916, a massive object like Earth distorts space-time around it like a bowling ball dropped on a trampoline. The larger the object, the more space-time is distorted by it. If a marble were circling around the bowling ball on the dimpled trampoline, it would fall inward, toward the bowling ball, like a rock in space circling a planet. Gravitational waves are ripples in space-time that travel outward from a source.
Scientists think that powerful gravitational waves are created when two extremely dense objects — like a pair of neutron stars or a black hole and a neutron star — orbit one another in binary pairs. The interaction of those two objects swirl space-time, creating ripples that theoretically can be measured using powerful instrumentation. [See images of gravitational waves]
In 2014, the Harvard-Smithsonian Center for Astrophysics found a faint signal in the cosmic microwave background radiation (CMB) that signifies the first direct evidence of gravitational waves ever discovered. Gravitational waves were the last untested part of Einstein's general theory of relativity.
The Harvard-Smithsonian study spotted gravitational waves as ripples in space-time possible left over from the rapid expansion of the universe (called inflation) right after the Big Bang nearly 13.8 billion years ago.
Scientists working on the study found a distinct curling pattern in the CMB — the comic fog that fills the universe and represents the earliest detectable radiation — that further supports the idea that the universe went through a huge period of inflation a fraction of a second after the Big Bang.
"This work offers new insights into some of our most basic questions: Why do we exist? How did the universe begin?," astrophysicist Avi Loeb, who wasn't a member of the study team said in a statement about the Harvard-Smithsonian research. "These results are not only a smoking gun for inflation, they also tell us when inflation took place and how powerful the process was."
CMB radiation came into existence about 380,000 years after the Big Bang. Scientists have mapped the CMB across the sky and found that it is a uniform temperature, evidence that bolsters cosmic inflation theory.
"Why the cosmic microwave background temperature is the same at different spots in the sky would be a mystery if it was not for inflation saying, well, our whole sky came from this tiny region," Chuck Bennett, principal investigator of NASA's Wilkinson Microwave Anisotropy Probe (WMAP) mission, told Space.com in 2013. "So the idea of inflation helps answer some of these mysteries, and it explains where these fluctuations came from." ['Smoking Gun' of Universe's Inflation: Gravitational Waves (Infographic)]
Experiments like Advanced LIGO (Laser Interferometer Gravitational Wave Observatory) and Advanced Virgo, could potentially detect those binary-created ripples in space-time, although neither has found a clear signature yet.
"The advanced LIGO detectors that are now being installed will see out through a substantial part of the universe," California Institute of Technology emeritus professor of physics Kip Thorne, a leading proponent of LIGO, said in 2012. "We expect to see black holes colliding at a rate of perhaps somewhere between once an hour and once a year."
Gravitational waves are different from gravity waves, which are ripples created in the atmospheres of planets by the interactions of winds whipping over geological features on the planet's surface.