By
dropping atoms just above an aluminum sphere, physicists have tested a
hypothesis about what's stretching the universe on the largest scales.
dropping atoms just above an aluminum sphere, physicists have tested a
hypothesis about what's stretching the universe on the largest scales.
Tiny fountain of atoms sparks big insights into dark energy
Tiny fountain of atoms sparks big insights into dark energy
It's a study in contrasts. For more than a decade, physicists have
been puzzling over dark energy, the mysterious stuff that’s blowing
space apart and has been detected only by studying the universe on the
largest scales. Now, researchers have probed its properties using about
the smallest tools available—atoms falling freely in a vacuum chamber.
The experiment, reported today in Science, doesn't reveal what dark energy is, but it helps nail down what it isn't.
In particular, it narrows the prospects for one popular idea: that dark
energy resides in hypothesized "chameleon particles" hiding in plain
sight.
"I find it exciting to be able to use laboratory-scale experiments to
test such ideas," says Amol Upadhye, a theoretical physicist at the
University of Wisconsin, Madison, who was not involved in the work. The
test doesn’t entirely rule out chameleons, he says, but future
improvements might put the idea to the ultimate test.
The discovery of dark energy rocked physics and cosmology. Scientists
thought the expansion of the universe was slowing, as the galaxies
tugged on one another with their gravity and counteracted the expansion
that began with the big bang. However, in 1998, two teams of cosmologists showed that in fact the expansion is accelerating
by studying stellar explosions called supernovae. The result has been
bolstered by analyses of galaxy clusters, the afterglow of the big bang
(the cosmic microwave background), and other cosmological phenomena.
Physicists attribute the acceleration to some sort of space-stretching
dark energy.
But what is dark energy? There are two possibilities. It could be
energy hidden in the vacuum of empty space itself—a cosmological
constant, as Albert Einstein hypothesized in 1917. Or it could be a
quantum field that fills space and blows it up like a balloon. Both
alternatives have problems. Given the standard model of particle
physics, theorists can calculate what the cosmological constant should
be, and they get a value vastly too big to explain the relatively modest
acceleration—suggesting some unknown physics just zeroes it out. On the
other hand, the presence of a quantum field would affect things like
the orbits of the planets in the solar system—but dark energy doesn’t
seem to.
