Researchers believe they have found the signal left in the sky by the super-rapid expansion of space that must have occurred just fractions of a second after everything came into being.
It takes the form of a distinctive twist in the oldest light detectable with telescopes.
The work will be scrutinised carefully, but already there is talk of a Nobel.
“This is spectacular,” commented Prof Marc Kamionkowski, from Johns Hopkins University.
“I’ve seen the research; the arguments are persuasive, and the scientists involved are among the most careful and conservative people I know,” he told BBC News.
The breakthrough was announced by an American team working on a project known as BICEP2.
This has been using a telescope at the South Pole to make detailed observations of a small patch of sky.
The aim has been to try to find a residual marker for “inflation” – the idea that the cosmos experienced an exponential growth spurt in its first trillionth, of a trillionth of a trillionth of a second.
Theory holds that this would have taken the infant Universe from something unimaginably small to something about the size of a marble. Space has continued to expand for the nearly 14 billion years since.
Inflation was first proposed in the early 1980s to explain some aspects of Big Bang Theory that appeared to not quite add up, such as why deep space looks broadly the same on all sides of the sky. The contention was that a very rapid expansion early on could have smoothed out any unevenness.
But inflation came with a very specific prediction – that it would be associated with waves of gravitational energy, and that these ripples in the fabric of space would leave an indelible mark on the oldest light in the sky – the famous Cosmic Microwave Background.
The BICEP2 team says it has now identified that signal. Scientists call it B-mode polarisation. It is a characteristic twist in the directional properties of the CMB. Only the gravitational waves moving through the Universe in its inflationary phase could have produced such a marker. It is a true “smoking gun”.
“Detecting this signal is one of the most important goals in cosmology today. A lot of work by a lot of people has led up to this point,” said Prof John Kovac of the Harvard-Smithsonian Center for Astrophysics and a leader of the BICEP2 collaboration.
The sensational nature of the discovery means the BICEP2 data will be subjected to intense peer review.
It is possible for the interaction of CMB light with dust in our galaxy to produce a similar effect, but the BICEP2 group says it has carefully checked its data over the past three years to rule such a possibility.
Other experiments will now race to try to replicate the findings. If they can, a Nobel Prize seems assured for this field of research.
Who this would go to is difficult to say, but leading figures on the BICEP2 project and the people who first formulated inflationary theory would be in the running.
“I can’t tell you how exciting this is,” said Dr Jo Dunkley, who has been searching through data from the European Planck space telescope for a B-mode signal.
“Inflation sounds like a crazy idea, but everything that is important, everything we see today – the galaxies, the stars, the planets – was imprinted at that moment, in less than a trillionth of a second. If this is confirmed, it’s huge.”