Long after the big bang, the expansion of the universe has again begun to
accelerate, as shown in this diagram.
NASA/WMAP Science Team
Is dark energy an illusion?
By Adrian ChoApr. 3, 2017 , 2:00 PM
For the past 20 years, physicists have known that the expansion of the universe is
accelerating, as if some bizarre “dark energy” is blowing up space like a balloon. In
fact, cosmologists’ well-tested standard model assumes that 69% of the content of
the universe is dark energy. However, there may be no need for the mysterious stuff,
a team of theorists claims. Instead, the researchers argue, the universe’s
acceleration could be driven by variations, or inhomogeneities, in its density. If so,
then one of the biggest mysteries in physics could be explained away with nothing
other than Albert Einstein’s familiar general theory of relativity. Other researchers are
skeptical, however.
“If it’s right, somebody is going to have to take back Nobel prizes” awarded in 2011
for the discovery of the accelerating expansion of the universe, says Nick
Kaiser, a cosmologist at the University of Hawaii in Honolulu. Tom Giblin, a
computational cosmologist at Kenyon College in Gambier, Ohio, who has worked on
a similar analysis, says, “I would love if inhomogeneities explained dark energy.”
However, he says, “I don’t see any evidence from our simulations to expect it to be
as big an effect as they see here.”
At issue is the way cosmologists calculate how the universe evolved over the past
13.8 billion years. Roughly speaking, they rely on two equations. One describes how
matter coalesces into galaxies and clusters of galaxies. The other, known as the
Friedmann–Lemaître–Robertson–Walker (FLRW) metric, comes out of Einstein’s
theory of gravity, or general relativity, and scientists use it to calculate how much the
universe has expanded at any time. At each step in time in a simulation, the
cosmologists’ program uses the FLRW metric to calculate the “scale factor,” which
specifies how much the universe has grown. The program then uses the scale factor
as an input to calculate how the formation of galaxies and clusters advances in that
step.
Strictly speaking, however, the FLRW equation applies to a smooth and
homogeneous universe. So to calculate the scale factor at each step, cosmologists
typically assume the universe is smooth and use its average density—determined
from the simulation—as the FLRW metric’s input. That’s a bit dicey (risqué), because
general relativity says that mass and energy warp spacetime. As a result, space
should expand faster in emptier regions and slower in crowded ones, where the
galaxies’ gravity pulls against the expansion. Thus, in principle, inhomogeneities in
the universe can feed back (réaction) through the dynamics and affect the universe’s
expansion.
Gábor Rácz and László Dobos, astrophysicists at Eötvös Loránd University in
Budapest, and their colleagues set out to capture that “backreaction.” They simulated
a cube of space measuring 480 million light-years along each side. Instead of using
the FLRW metric to calculate at each time step a single scale factor for the entire
cube, they broke the cube into 1 million miniuniverses and then used the equation to
calculate the scale factor in each of them. “We assume that every region of the
universe determines its expansion rate itself,” Dobos says. The researchers then
calculated the average of the many scale factors, which can differ from the scale
factor calculated from the average density.
The team’s virtual universe evolved much as the real one has, with its expansion
accelerating over the past few billion years. That happened even without adding
space-stretching dark energy to the simulation, the researchers report in a paper in
press at the Monthly Notices of the Royal Astronomical Society. The results suggest
that it may be possible to explain away dark energy as an illusion, Dobos says.
Others are cautious. Giblin notes that the simulation he and his colleagues performed
differs from the new one. The new work tracks the evolution of the universe to finer
spatial scales, but involves certain assumptions and approximations, he says. In spite
of the differences, Giblin says, his work suggests that backreaction would change the
expansion rate of the universe by less than a percent, whereas the new simulation
suggests an effect in excess of 20%.
Kaiser also expects the effects of inhomogeneity to be small. He notes that the best
evidence for the accelerated expansion of the universe comes from measuring the
distances and ages of stellar explosions known as type 1a supernovae in the
relatively nearby universe. However, in the local universe, plain Newtonian gravity
should work well enough. That suggests the difference in how the scale factor is
determined in a relativistic theory shouldn’t exert a big effect. “If they’re right, there’s
something very funny going on,” he says.
Still, experts say it’s reasonable to investigate backreaction. “I would say that this is
now part of the mainstream in that people want to calculate the size of this effect,”
says Thomas Buchert, a cosmologist at the University of Lyon in France, who
pioneered the topic in the 1990s. Giblin notes “mainstream cosmology has done such
a bad job of solving the dark energy problem that it will likely be some
nonmainstream idea like this that does.” But, he adds, “I don’t know if this is the one.”
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DOI: 10.1126/science.aal0994