In the first moment following the Big Bang, scientists believe the universe got very big, very quickly.

On Monday, US astronomers said they had peered further back in time than ever before and detected compelling evidence of this dramatic and rapid expansion, a theory known as ''cosmic inflation''.

''It is somewhat of a mad theory, which was introduced in the 1980s to solve a lot of issues with the way the universe looked,'' said University of Melbourne cosmologist Alan Duffy.

''This discovery is really the first confirmation that a trillionth of a trillionth of a second after the Big Bang the universe grew enormously,'' he said. ''We're all pretty ecstatic about this result.''

The announcement is also significant because it provides proof for Einstein's final prediction, the theory of general relativity, which predicts a violent event such as the rapid expansion of the universe would create ripples in space-time called gravitational waves.

After using a radio telescope at the South Pole, a team from the Harvard-Smithsonian Centre for Astrophysics said it had detected very specific patterns of light, known as B-modes, which were almost certainly caused by very early gravitational waves as a result of cosmic inflation.

''We're very excited to present our results because they seem to match the prediction of the theory so closely,'' said John Kovac, the leader of the BICEP project.

While strong circumstantial evidence existed for gravitational waves, no one had observed them directly, which was why the detection of light patterns left by primordial gravitational waves had astronomers so excited.

If the result was verified it promised to be one of the biggest advances in cosmology in 20 years, comparable to Australian Nobel prize winner Brian Schmidt discovering the expansion of the universe was accelerating. It also had implications for scientist's understanding of quantum mechanics.

University of Sydney astrophysicist Bryan Gaensler said normally the effect of gravitational waves was so small it was extremely difficult to detect.

But the rapid and violent expansion caused by inflation would have set the whole universe ringing with them, he said.

And although these early waves had long disappeared, the theory of inflation predicted they would leave behind a very specific and subtle imprint in the form of B-modes.

Professor Gaensler said scientists had been looking for B-modes for years, but the radio signal they emitted was very faint and travelled from the edge of the universe.

''It's like looking through a really dirty window at something that's thousands of kilometres away,'' he said.

University of Melbourne cosmologist Katie Mack said while the detection of primordial gravitational waves in the early universe was a ''really big deal'' and was consistent with the theory of inflation, their presence alone was not direct evidence of the theory.

''We can't say for sure that those gravitational waves were produced by inflation,'' she said.

But she said their detection was evidence that gravity had quantum mechanical properties, like particles, something physicists suspected but had not seen evidence of.