"The implications for this detection stagger the mind," said project leader Jamie Bock. "We are measuring a signal that comes from the dawn of time."
The universe burst into existence 13.8 billion years ago. Fractions of a second later, space and time were created, expanding exponentially in an episode known as "inflation". It was believed that inflation should also produce gravitational waves - ripples in space-time that spread throughout the universe.
The signal was found using a specialised telescope called Bicep (Background Imaging of Cosmic Extragalactic Polarisation) in Antarctica, the clearest place on Earth for stargazing.
It scans the sky at microwave frequencies, where it picks up light energy from slightly after the Big Bang - around 380,000 years later.
The scientists have found tiny ripples in this light which show it is being slightly stretched by gravitational waves. These cause a distinct twisting pattern known as a "curl" or "b-mode" in the cosmic microwave background radiation.
Joanna Dunkley, a lecturer in astrophysics at Oxford University, said the finding would give the most direct view possible of the Big Bang and tell scientists what was happening right at the beginning of the universe. Meanwhile, Chris Lintott, also an astrophysicist at the University of Oxford, said the finding was the "most significant cosmological discovery in nearly two decades". "It's like all our Christmases at once - I doubt many cosmologists will get much sleep tonight," he said.
The importance of this finding, announced at a press conference at Harvard University, cannot be overestimated; one leading physicist has gone so far as to describe it as "one of the most important scientific discoveries of all time". The researchers, headed by Professor John Kovac of the Harvard-Smithsonian Centre for Astrophysics, appear to have found the very echoes of creation.
If the discovery is confirmed, it will be the final experimental vindication of Einstein's theory of general relativity. But the implications are more profound even than that. The existence of gravity waves is the "smoking gun" for the controversial theory of cosmic inflation, the idea that right at the start of the universe everything underwent a colossally fast period of expansion - the "B of the Bang", if you like.
If cosmic inflation, which we need in order to explain several weird facts about our universe, is correct, then this provides strong support for the notion of the "multiverse"; the idea that what we see when we look up at the night sky is but a gnat on the back of the elephant that is the true totality of creation.
The existence of gravity waves is evidence that "our" universe may not only exist alongside an infinite number of parallel worlds, but may itself be infinite in extent, containing endless copies of our galaxy located trillions of light years apart.
Signal from dawn of time
* The universe burst into existence 13.8 billion years ago, scientists say. Space and time were created, expanding exponentially through what is known as "inflation".
* It is believed this inflation created "gravitational waves" or ripples in space-time spreading throughout the universe.
* A specialised telescope in Antarctica can pick up light that originated after the Big Bang.
* The telescope has been used to pick up ripples in the light, which show it has been stretched by gravitational waves.
* The discovery is the first direct evidence of gravitational waves, which were predicted in Einstein's theory of relativity in 1916.
