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Dark energy, the unidentified force
that's pushing the universe to expand at ever faster rates, was
already at work as early as nine billion years ago, scientists
reported last week. New Hubble Space Telescope sightings of distant
supernova explosions support the explanation of dark energy as
energy of the vacuum whose density has stayed constant throughout
the universe's history, the scientists said.
 At left is an illustration of the possible fates of the universe. Unstable dark energy could cause a "big rip" (the universe expands violently, then the stars, planets and atoms come unglued) or a "big crunch" (the universe implodes or compresses). Image courtesy of NASA/STScI/Ann Feild. Click on image for larger view. Image. NASA
This cosmic
acceleration was first revealed in 1998 by two separate teams of
astrophysicists. By measuring the brightness of supernova explosions
from up to seven billion light years ago, the scientists discovered
an unexpected discrepancy. The supernovae appeared dimmer, and thus
farther, than expected from their measured red shifts. Put another
way, supernovae at a given distance were less redshifted than
expected. Because red shift measures how much light waves stretch as
the universe expands, the lower red shift meant that, early on, the
light from these distant supernovae had traveled in a universe that
was expanding at a slower rate than the current universe (whose rate
of expansion is known by other means). The then-widely accepted
model of cosmology required instead that the universe be slowing
down in its expansion, owing to the mutual gravitational tug of all
of the matter and energy contained in it.
Using the Hubble, a team led by Adam Riess, an astrophysicist at the
Space Telescope Science Institute and at Johns Hopkins University
has now observed 23 new supernovae dating back to 8 to 10 billion
years ago, he said in a Nov. 16 NASA press conference. That was an
era of intense star formation, when galaxies were three times as
bright as they are today. Until now, astronomers had only seen seven
supernovae from that period, Riess said, too few to measure the
properties of dark energy. The data show that the repulsive action
of dark energy was already active at that time, and are consistent
with a constant energy density -- in other words, with an energy of
the vacuum that does not dilute itself as the universe expands,
eventually fueling an exponential growth of the universe.
More
complicated models with non-constant energy density -- including a
class known as quintessence models -- are not completely ruled out,
Riess said during the press conference: the new data still allows
for variations of up to 45 percent from constant density. "It's
still pretty crude," Riess said. For more recent ages, dark energy
is known to have been constant up to a 10 percent variation. Mario
Livio, another STScI astronomer who also was at hand at the press
conference, said, "The results only rule out certain variants of
quintessence models," but not all of them.
Lawrence Berkeley Lab astrophysicists Saul Perlmutter, who leads
another supernova search, says that this is a step in the right
direction, but that only a new, dedicated space telescope will be
able to constrain the variation enough to convince scientists that
dark energy is constant. "We expect that the differences will be
much more subtle between the various models of dark energy," he
says. Perlmutter says his team is also looking at supernovae from
the distant past, focusing on ones from dust-free regions of the
universe, in order to estimate the statistical and systematic
uncertainties of the measurements.
The new data also confirm the reliability of supernovae as signposts
of the universe's expansion, Riess said. The particular kind of
supernova used for this kind of measurement, called type Ia, takes
place when a white dwarf star becomes heavier by accreting matter
from a companion star, until -- at a critical mass of about 1.4
times the mass of our sun -- it undergoes a thermonuclear explosion.
Virtually all type Ia supernovae have very standard characteristics
-- they all follow the same cycle, have roughly the same brightness
and relative abundances of elements, as seen from their spectra.
This makes astrophysicists believe that type Ia's have a predictable
intrinsic brightness, making their distances easy to estimate. It
now appears that the same is true for the oldest supernovae, even
though the elemental composition of the universe as a whole was
different back then.
Source: AIP
Related Links:
To appear in Astrophysical Journal, 10 February 2007
Paper available at http://arxiv.org/abs/astro-ph/0611572
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