Dark energy is the name given to an unexplained force that is
drawing galaxies away from each other, against the pull of gravity, at an
accelerated pace.
Dark energy is a bit like anti-gravity. Where gravity pulls
things together at the more local level, dark energy tugs them apart on the
grander scale.
Its existence isn't proven, but dark energy is many
scientists' best guess to explain the confusing observation that the universe's
expansion is speeding up. Experts still don't know what's
driving this force, but the quest to learn more about dark energy is one of
cosmologists' top priorities.
Confounding expectations
The story of how dark
energy was discovered is a classic case of science confounding
expectations.
In the mid-1990s, astronomers set out to measure how fast
the universe was expanding. Because gravity draws mass together, most experts
expected to find that gravity had slowed down the universe's rate of
ballooning, or perhaps that the rate was staying about the same.
Instead, it appeared that the expansion was doing neither:
It was speeding up.
"The data wasn't behaving as we thought it would. There
was a lot of nervous laughter," said Brian Schmidt of the Australian
National University in Canberra, who led a team along with Johns Hopkins
University astrophysicist Adam Riess that helped discover dark energy in 1998.
The evidence was based on measurements of bright exploding
stars, called supernovae, that astronomers were using as lampposts to track
distance. By looking farther away, scientists are able to peer
back in time, since the light from distant objects has taken billions of
years to reach us.
The scientists observed many supernovae at different
distances to determine how fast they are speeding away from us. (They measured
the objects' red-shift, or how much their light had been changed due to the
Doppler effect, which is the compression or expansion of waves that occurs when
an object is moving toward or away from you. An analogy is the siren of an
ambulance that changes pitch as it moves toward you, then passes you and heads
the other way — its waves are first compressed, then stretched.) These
measurements gave astronomers a picture of how fast the universe was expanding
at different points in its history.
Shocking results
The researchers also found that the universe is expanding
faster today than at any time in the past.
"At first we were reluctant to believe our result,"
said Saul Perlmutter, an astrophysicist at the Lawrence Berkeley Laboratory of
the University of California, Berkeley, who led a competing team that found the
same results as Schmidt and Riess. "But the more we analyzed it, the more
it wouldn't go away."
To explain these puzzling findings, some scientists have
revived an old idea of Einstein's that had been discarded as false: that the
vacuum of space has energy in it that acts repulsively and accelerates the
expansion of the universe. Einstein called this idea the cosmological constant,
and referred to it as his "biggest
blunder."
Now the cosmological constant is one of the leading theories
of why the universe is blowing up like a balloon at ever-increasing speed.
Dark matter
Dark energy is sometimes confused with the similarly
mysterious dark matter, though the two are separate entities.
Dark matter is a hypothesized form of matter that doesn't
interact with light, so it is invisible. Astronomers deduced its presence by
noting its gravitational pull on stars in galaxies.
Taken together, dark matter and dark energy seem to make up
most of the mass of the universe (matter and energy are considered to be two
forms of the same thing, thanks to Einstein's famous equation E=Mc^2). Dark
energy is thought to account for 74 percent of the universe, while dark matter
adds about 22 percent, and normal, visible matter contributes a puny 4 percent.
As if the discovery of dark energy weren't bizarre enough,
it has stirred up a whole host other issues. For example, dark energy adds fuel
to the fire of believers in multiple
universes, or the idea that our own existence is just one of countless
worlds in which the constants and conditions are different. There might be
other universes in which dark energy doesn't exist, and the universe does slow
in its expansion, cosmologists say. Maybe that's why our universe is so
peculiar.
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