Ref: http://www.fnal.gov/pub/presspass/press_releases/darkenergy_3-16-05.html
05-08
PRESS RELEASE
March 16, 2005
Italian, US cosmologists present alternate explanation for accelerating
expansion of the universe: Was Einstein right when he said he was
wrong?
Why is the universe expanding at an accelerating rate, spreading its
contents over ever greater dimensions of space? 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 McGill University (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."
INFN (Istituto Nazionale di Fisica Nucleare), Italy's national nuclear
physics institute, supports, coordinates and carries out scientific
research in subnuclear, nuclear and astroparticle physics and is
involved in developing relevant technologies.
Fermilab, in Batavia, Illinois, USA, is operated by Universities
Research Association, Inc. for the Department of Energy's Office of
Science, which funds advanced research in particle physics and
cosmology.
For further information:
Antonio Riotto, Infn of Padova
phone: +39 049 827 7256 (office), + 39 041 24 11 208 (home), mob. +39 320 486 2153
e-mail:
Sabino Matarrese, University of Padova
phone: +39 049 827 7120 (office), +39 0444 92 36 48 (home)
e-mail:
Edward Kolb, Fermilab
+630 651 4695
email:
Barbara Gallavotti, Head of the Infn Comunication Office
phone: + 39 06 6868162; mob. + 39 335 6606075
e-mail:
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