An original solution to this puzzle, certainly the most fascinating question in modern cosmology, was put forward by four theoretical physicists, Edward W. Kolb of the U.S. Department of Energy's Fermi National Accelerator Laboratory, Chicago (USA): Sabino Matarrese of the University of Padova; Alessio Notari from the University of Montreal (Canada); and Antonio Riotto of INFN (Istituto Nazionale di Fisica Nucleare) of Padova (Italy).

Their study was submitted yesterday to the journal Physical Review Letters. Over the last hundred years, the expansion of the universe has been a subject of passionate discussion, engaging the most brilliant minds of the century.

Like his contemporaries, Albert Einstein initially thought that the universe was static: that it neither expanded nor shrank. When his own Theory of General Relativity clearly showed that the universe should expand or contract, Einstein chose to introduce a new ingredient into his theory.

His "cosmological constant" represented a mass density of empty space that drove the universe to expand at an ever-increasing rate.

When in 1929 Edwin Hubble proved that the universe is in fact expanding, Einstein repudiated his cosmological constant, calling it "the greatest blunder of my life."

Then, almost a century later, physicists resurrected the cosmological constant in a variant called dark energy. In 1998, observations of very distant supernovae demonstrated that the universe is expanding at an accelerating rate.

This accelerating expansion seemed to be explicable only by the presence of a new component of the universe, a "dark energy," representing some 70 percent of the total mass of the universe.

Of the rest, about 25 percent appears to be in the form of another mysterious component, dark matter; while only about 5 percent comprises ordinary matter, those quarks, protons, neutrons and electrons that we and the galaxies are made of.

"The hypothesis of dark energy is extremely fascinating," explains Padova's Antonio Riotto, "but on the other hand it represents a serious problem. No theoretical model, not even the most modern, such as supersymmetry or string theory, is able to explain the presence of this mysterious dark energy in the amount that our observations require.

"If dark energy were the size that theories predict, the universe would have expanded with such a fantastic velocity that it would have prevented the existence of everything we know in our cosmos."

The requisite amount of dark energy is so difficult to reconcile with the known laws of nature that physicists have proposed all manner of exotic explanations, including new forces, new dimensions of spacetime, and new ultralight elementary particles.

However, the new report proposes no new ingredient for the universe, only a realization that the present acceleration of the universe is a consequence of the standard cosmological model for the early universe: inflation.

"Our solution to the paradox posed by the accelerating universe," Riotto says, "relies on the so-called inflationary theory, born in 1981. According to this theory, within a tiny fraction of a second after the Big Bang, the universe experienced an incredibly rapid expansion.

This explains why our universe seems to be very homogeneous. Recently, the Boomerang and WMAP experiments, which measured the small fluctuations in the background radiation originating with the Big Bang, confirmed inflationary theory.

It is widely believed that during the inflationary expansion early in the history of the universe, very tiny ripples in spacetime were generated, as predicted by Einstein's theory of General Relativity.

These ripples were stretched by the expansion of the universe and extend today far beyond our cosmic horizon, that is over a region much bigger than the observable universe, a distance of about 15 billion light years.

In their current paper, the authors propose that it is the evolution of these cosmic ripples that increases the observed expansion of the universe and accounts for its acceleration.

"We realized that you simply need to add this new key ingredient, the ripples of spacetime generated during the epoch of inflation, to Einstein's General Relativity to explain why the universe is accelerating today," Riotto says.

"It seems that the solution to the puzzle of acceleration involves the universe beyond our cosmic horizon. No mysterious dark energy is required."

Fermilab's Kolb called the authors' proposal the most conservative explanation for the accelerating universe. "It requires only a proper accounting of the physical effects of the ripples beyond our cosmic horizon," he said.

Data from upcoming experiments will allow cosmologists to test the proposal. "Whether Einstein was right when he first introduced the cosmological constant, or whether he was right when he later refuted the idea will soon be tested by a new round of precision cosmological observations," Kolb said.

"New data will soon allow us to distinguish between our explanation for the accelerated expansion of the universe and the dark energy solution."

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