Nov. 16, 2006

Erica Hupp/Dwayne Brown
Headquarters, Washington
1-202-358-1237 / 1-202-358-1726
erica.hupp-1@nasa.gov / dwayne.c.brown@nasa.gov

Ray Villard
Space Telescope Science Institute, Baltimore
1-410-338-4514
villard@stsci.edu

RELEASE: 06-353

NASA'S HUBBLE FINDS EVIDENCE FOR DARK ENERGY IN THE YOUNG UNIVERSE

Scientists using NASA's Hubble Space Telescope have discovered that
dark energy is not a new constituent of space, but rather has been
present for most of the universe's history. Dark energy is a
mysterious repulsive force that causes the universe to expand at an
increasing rate.

Investigators used Hubble to find that dark energy was already
boosting the expansion rate of the universe as long as nine billion
years ago. This picture of dark energy is consistent with Albert
Einstein's prediction of nearly a century ago that a repulsive form
of gravity emanates from empty space.

Data from Hubble provide supporting evidence that help astrophysicists
to understand the nature of dark energy. This will allow scientists
to begin ruling out some competing explanations that predict that the
strength of dark energy changes over time.

Researchers also have found that the class of ancient exploding stars,
or supernovae, used to measure the expansion of space today look
remarkably similar to those that exploded nine billion years ago and
are just now being seen by Hubble. This important finding gives
additional credibility to the use of these supernovae for tracking
the cosmic expansion over most of the universe's lifetime.

"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 Adam Riess of the Space Telescope Science Institute
and Johns Hopkins University in Baltimore. Reiss led one of the first
studies to reveal the presence of dark energy in 1998 and is the
leader of the current Hubble study. "Our latest clue is that the
stuff we call dark energy was relatively weak, but starting to make
its presence felt nine billion years ago."

To study the behavior of dark energy of long ago, Hubble had to peer
far across the universe and back into time to detect supernovae.
Supernovae can be used to trace the universe's expansion. This is
analogous to seeing fireflies on a summer night. Fireflies glow with
about the same brightness, so you can judge how they are distributed
in the backyard by their comparative faintness or brightness,
depending on their distance from you. Only Hubble can measure these
ancient supernovae because they are too distant, and therefore too
faint, to be studied by the largest ground-based telescopes.

Einstein first conceived of the notion of a repulsive force in space
in his attempt to balance the universe against the inward pull of its
own gravity, which he thought would ultimately cause the universe to
implode.

His "cosmological constant" 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 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: there really
was a repulsive form of gravity in space that was soon after 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. These 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."

Previous Hubble observations of the most distant supernovae known
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 five to six
billion years ago. 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, astronomer and Hubble team member at Western Kentucky
University, Bowling Green, Ky.

For images and additional information on the web about dark energy,
visit:

http://www.nasa.gov/hubble