Using NASA's
Hubble Space Telescope, researchers have discovered
that dark energy, a mysterious repulsive force that makes
the universe expand at an ever-faster rate, is not new but
rather has been present for most of the universe's
13-billion-year history.
A team led by Adam Riess, a professor in the
Henry A. Rowland
Department of Physics and Astronomy at Johns Hopkins
and a Space Telescope
Institute researcher, found that dark energy was
already accelerating the expansion of the universe at least
as long as 9 billion years ago. This picture of dark energy
would be consistent with Albert Einstein's prediction,
nearly a century ago, that a repulsive form of gravity
emanates from empty space.
The team announced these findings in a media
teleconference at NASA headquarters in Washington on
Thursday, Nov. 6, and the findings will be published in the
Feb. 10 issue of Astrophysical Journal.
"Although dark energy accounts for more than 70
percent of the energy of the universe, we know very little
about it, so each clue is precious," said Riess, who in
1998 led one of the first studies to reveal the presence of
dark energy. "Our latest clue is that the stuff we call
dark energy was present as long as 9 billion years ago,
when it was starting to make its presence felt."
Hubble's new evidence is important because it will
help astrophysicists start ruling out competing
explanations that predict that the strength of dark energy
changes over time, Riess said.
In addition, the researchers found that the exploding
stars, or supernovae, used as markers to measure the
expansion of space today look remarkably similar to those
that exploded 9 billion years ago and are just now seen by
Hubble. This is an important finding, researchers say,
because it gives added credibility to the use of these
supernovae as tools for tracking the cosmic expansion over
most of the universe's lifetime.
To study the behavior of dark energy long ago, Hubble
had to peer far across the universe and back into time to
detect ancient supernovae, which can be used to trace the
universe's expansion and determine its expansion rate at
various times. The method, Riess said, is analogous to
watching fireflies on a summer night. Because all fireflies
glow with about the same brightness, you can judge how they
are distributed throughout the backyard by their
comparative apparent faintness or brightness, which depends
on their distance from you.
Only Hubble can measure these supernovae because they
are too distant, and therefore too faint, to be studied by
the largest ground-based telescopes.
Albert Einstein first conceived of the notion of a
repulsive force in space in his attempt to explain the
balance of the universe against the inward pull of its own
gravity. If such an opposing force did not exist, he
reasoned, gravity would ultimately cause the universe to
implode.
But Einstein eventually rejected his own so-called
"cosmological constant" idea, and it remained a curious
hypothesis until 1998, when Riess and the members of the
High-Z Supernova Team and the Supernova Cosmology Project
used ground-based telescopes and Hubble to first detect the
acceleration of the expansion of space from observations of
distant supernovae. Astrophysicists came to the realization
that Einstein may have been right after all, that there
really was a repulsive form of gravity in space. It soon
after was dubbed "dark energy."
Over the past eight years, astrophysicists have been
trying to uncover two of dark energy's most fundamental
properties: its strength and its permanence. The new
observations reveal that dark energy was present and
obstructing the gravitational pull of the matter in the
universe even before it began to win this cosmic "tug of
war."
Hubble observations of the most distant supernovae
known, reported in 2004 by Riess and colleagues, revealed
that the early universe was dominated by matter whose
gravity was slowing down the universe's expansion rate,
like a ball rolling up a slight incline. The observations
also confirmed that the expansion rate of the cosmos began
speeding up about 5 billion to 6 billion years ago, like a
roller coaster zooming down a track. That is when
astronomers believe that dark energy's repulsive force
overtook gravity's attractive grip.
The latest results are based on an analysis of the 24
most distant supernovae known, most found within the last
two years.
By measuring the universe's relative size over time,
astrophysicists have tracked the universe's growth spurts,
much as a parent may witness the growth spurts of a child
by tracking changes in height on a doorframe. Distant
supernovae provide the doorframe markings read by
Hubble.
"After we subtract the gravity from the known matter
in the universe, we can see the dark energy pushing to get
out," said Lou Strolger of the University of Western
Kentucky, a supernova hunter on the Riess team.
Further observations presently under way with Hubble
by Riess and his team should continue to offer new clues to
the nature of dark energy.