In the beginning, the universe got very big very fast, transforming itself in a fraction of an instant from something almost infinitesimally small to something imponderably vast, a cosmos so huge that no one will ever be able to see it all.

This is the premise of an idea called cosmic inflation — a powerful twist on the big-bang theory — and Monday it received a major boost from an experiment at the South Pole called BICEP2. A team of astronomers led by John Kovac of the Harvard-Smithsonian Center for Astrophysics announced that it had detected ripples from gravitational waves created in a violent inflationary event at the dawn of time.

“We’re very excited to present our results because they seem to match the prediction of the theory so closely,” Kovac said in an interview. “But it’s the case that science can never actually prove a theory to be true. There could always be an alternative explanation that we haven’t been clever enough to think of.”

The reaction in the scientific community was cautiously exultant. The new result was hailed as potentially one of the biggest discoveries of the past two decades.

Cosmology, the study of the universe on the largest scales, has already been roiled by the 1998 discovery that the cosmos is not merely expanding but doing so at an accelerating rate, because of what has been called “dark energy.” Just as that discovery has implications for the ultimate fate of the universe, this new one provides a stunning look back at the moment the universe was born.

View Graphic Ripples from the Big Bang

“If real, it’s magnificent,” said Harvard astrophysicist Lisa Randall.

Lawrence Krauss, an Arizona State University theoretical physicist, said of the new result, “It gives us a new window on the universe that takes us back to almost the very beginning of time, allowing us to turn previously metaphysical questions about our origins into scientific ones.”

The measurement, however, is a difficult one. The astronomers chose the South Pole for BICEP2 and earlier experiments because the air is exceedingly dry, almost devoid of water vapor and ideal for observing subtle quirks in the ancient light pouring in from the night sky. They spent four years building the telescope, and then three years observing and analyzing the data. Kovac, 43, who has been to the South Pole 23 times, said of the conditions there, “It’s almost like being in space.”

The BICEP2 instrument sorts through the cosmic microwave background (CMB), looking for polarization of the light in a pattern that reveals the ripples of gravitational waves. The gravitational waves distort space itself, squishing and tugging the fabric of the universe. This is the first time that anyone has announced the detection of gravitational waves from the early universe.

There are other experiments by rival groups trying to detect these waves, and those efforts will continue in an attempt to confirm the results announced Monday.

“I would say it’s very likely to be correct that we are seeing a signal from inflation,” said Adrian Lee, a University of California at Berkeley cosmologist who is a leader of PolarBear, an experiment based on a mountaintop in Chile that is also searching for evidence of inflation. “But it’s such a hard measurement that we really would like to see it measured with different experiments, with different techniques, looking at different parts of the sky, to have confidence that this is really a signal from the beginning of the universe.”

The fact that the universe is dynamic at the grandest scale, and not static as it appears to be when we gaze at the “fixed stars” in the night sky, has been known since the late 1920s, when astronomer Edwin Hubble revealed that the light from galaxies showed that they were moving away from one another.

This led to the theory that the universe, once compact, is expanding. Scientists in recent years have been able to narrow down the age of the universe to about 13.8 billion years. Multiple lines of evidence, including the detection of the CMB exactly 50 years ago, have bolstered the consensus model of modern cosmology, which shows that the universe was initially infinitely hot and dense, literally dimensionless. There was no space, no time.

Then something happened. The universe began to expand and cool. This was the big bang.

Cosmic inflation throws gasoline on that fire. It makes the big bang even bangier right at the start. Instead of a linear expansion, the universe would have undergone an exponential growth.

In 1979, theorist Alan Guth, then at Stanford, seized on a potential explanation for some of the lingering mysteries of the universe, such as the remarkable homogeneity of the whole place — the way distantly removed parts of the universe had the same temperature and texture even though they had never been in contact with each other. Perhaps the universe did not merely expand in a stately manner but went through a much more dramatic, exponential expansion, essentially going from microscopic in scale to cosmically huge in a tiny fraction of a second.

It is unclear how long this inflationary epoch lasted. Kovac calculated that in that first fraction of a second the volume of the universe increased by a factor of 10 to the 26th power, going from subatomic to cosmic.

This is obviously difficult terrain for theorists, and the question of why there is something rather than nothing creeps into realms traditionally governed by theologians. But theoretical physicists say that empty space is not empty, that the vacuum crackles with energy and that quantum physics permits such mind-boggling events as a universe popping up seemingly out of nowhere.

“Inflation — the idea of a very big burst of inflation very early on — is the most important idea in cosmology since the big bang itself,” said Michael Turner, a University of Chicago cosmologist. “If correct, this burst is the dynamite behind our big bang.”

Princeton University astrophysicist David Spergel said after Monday’s announcement, “If true, this has revolutionary impacts for our understanding of the physics of the early universe and gives us insight into physics on really small scales.”

Spergel added, “We will soon know if this result is revolutionary or due to some poorly understood systematics.”

The inflationary model implies that our universe is exceedingly larger than what we currently observe, which is humbling already in its scale. Moreover, the vacuum energy that drove the inflationary process would presumably imply the existence of a larger cosmos, or “multiverse,” of which our universe is but a granular element.

“These ideas about the multiverse become interesting to me only when theories come up with testable predictions based on them,” Kovac said Monday. “The powerful thing about the basic inflationary paradigm is that it did offer us this clear, testable prediction: the existence of gravitational waves which are directly linked to the exponential expansion that’s intrinsic to the theory.”

The cosmological models favored by scientists do not permit us to have contact with other potential universes. The multiverse is, for now, conjectural, because it is not easily subject to experimental verification and is unobservable — from the South Pole or from anywhere else.