From: LARRY KLAES (ljk4_at_msn.com)
Date: Sun Oct 12 2003 - 18:05:27 PDT
A 'Cosmic Jerk' That Reversed the Universe
October 11, 2003
By DENNIS OVERBYE
CLEVELAND, Oct. 10 - Astronomers said on Friday that they
had determined the time in cosmic history when a mysterious
force, "dark energy," began to wrench the universe apart.
Five billion years ago, said Dr. Adam Riess, an astronomer
at the Space Telescope Science Institute in Baltimore, the
universe experienced a "cosmic jerk." Before then, Dr.
Riess said, the combined gravity of the galaxies and
everything else in the cosmos was resisting the expansion,
slowing it down. Since the jerk, though, the universe has
been speeding up.
The results were based on observations by a multinational
team of astronomers who used the Hubble Space Telescope to
search exploding stars known as Type 1a supernovas,
reaching back in time three-quarters of the way to the Big
Bang, in which the universe was born. The results should
help quell remaining doubts that the expansion of the
universe is really accelerating, a strange-sounding notion
that has become a pillar of a new and widely accepted model
of the universe as being full of mysterious dark matter and
even more mysterious dark energy.
"This gives great confidence that we've been on the right
track," said Dr. Riess, who announced his results at a
meeting here on the Future of Cosmology sponsored by the
Center for Education and Research in Cosmology and
Astrophysics at Case Western Reserve University and the
Kavli Institute.
Dr. Lawrence M. Krauss, an astrophysicist at Case Western,
called the turnaround from slowing down to speeding up
important confirmation.
"The big surprise," Dr. Krauss said, "would have been if it
hadn't happened."
Dr. Joseph Lykken, a physicist at the Fermi National
Accelerator Laboratory, known as Fermilab, in Batavia,
Ill., said, "I could go home now and be happy."
Knowing how and when the jerk occurred, astronomers said,
was an important step in figuring out just what the dark
energy is.
"He gave us information about when the universe hit the gas
pedal," said Dr. Michael S. Turner, a cosmologist at the
University of Chicago who is director of mathematics and
physics at the National Science Foundation. Different
theories of dark energy, Dr. Turner said, predict different
times for the transition.
"The supernovae have come through," Dr. Wendy L. Freedman,
director of the Carnegie Observatories in Pasadena, Calif.,
said. "The whole result could have disappeared," referring
to the dark energy acceleration.
A result was also a vindication for Dr. Riess, who was a
pivotal member of one of two competing groups, the one led
by Dr. Brian P. Schmidt of the Mount Stromlo and Siding
Spring Observatories in Australia. The other team was
headed by Dr. Saul Perlmutter of the Lawrence Berkeley
Laboratory of the University of California, who discovered
the cosmic acceleration five years ago. The groups used
supernovas to chart the expansion of the universe at
different times in the past.
The goal was to measure how much the universe was being
slowed by the collective gravity of the cosmos and
determine whether the universe would go on forever or
recollapse in a "Big Crunch" on one distant day.
The groups found, though, that nearby supernovas looked
dimmer than they should, implying that the universe was
growing faster than expected, speeding up, under the
influence of some form of antigravity - perhaps embedded in
the fabric of spacetime itself.
The results were buttressed by studies of radiation left
over from the Big Bang that suggested that two-thirds of
the mass-energy of the universe resided in this dark
energy.
"But there was always a nagging doubt," Dr. Riess told his
colleagues today, that dust or some other astrophysical
effect was dimming the supernovae, mimicking the effects of
acceleration. If that were the case, supernovae even
farther away than the ones already observed should be even
dimmer.
On the other hand, if it was really an antigravity energy
in space, then as space expanded, the push from this dark
energy would grow along with it. In the early years of the
universe, the dark energy would have been too small to
counteract the gravity of the matter in the universe, and
the expansion would have initially slowed. After the
universe grew big enough, though, the dark energy would
dominate, and the universe would start to expand.
Dr. Riess described the difference between the matter, most
of which is dark, and dark energy as, "One pulls, the other
pushes."
To test which idea was true, Dr. Riess and his colleagues
had to find supernovas farther in the past than previous
surveys, seven billion light-years ago. In 2001, Dr. Riess
and his collaborators found Hubble observations of a
supernova 10 billion years in the past. It proved to be
anomalously bright, lending credence to the idea that a
dark energy had taken over some time in between.
"But a single object is just not robust enough," he said.
For the last year, he and his colleagues have used the
Hubble in collaboration with a large galaxy survey known as
Goods to find distant supernovas.
"We found lots of weapons of mass destruction," he said,
showing Hubble pictures of some exploding with the
brilliance of small galaxies 8 billion to 10 billion
light-years away.
More important, they were brighter than expected. When he
plotted their velocities against distance, or time in the
past, Dr. Riess found that the universe had to have changed
direction, from slowing to speeding up, over a period of
time five billion years ago, the so-called cosmic jerk,
using the technical term for a change in acceleration.
"It's great to see it," Dr. Riess said.
In Dr. Lykken's words, and as borne out by discussions at the
meeting here, "theorists don't have a clue" about the identity
of the dark energy that is so important.
A leading candidate is something invented by Einstein
called the cosmological constant. But physicists despair of
understanding Einstein's constant. Efforts to calculate it
wind up missing by a factor of 1060, which has been called
the worst mismatch in the history of science.
Another candidate that has lately won some following among
theorists is that Einstein was wrong, that the acceleration
is sort of an illusion that could be dispelled by modifying
his general theory of relativity, the sourcebook of cosmic
gravity.
The stakes for the universe are not small. If the
cosmological constant is the culprit, the universe will
expand faster and faster as time goes on.
"It will be cold and dark in a few billion years," Dr.
Frank Wilczek, the Herman Feshbach professor of physics at
the Massachusetts Institute of Technology, said. "That
would be very sad."
http://www.nytimes.com/2003/10/11/science/11COSM.html?ex=1067006725&ei=1&en=dd1d6e8deaa3526b
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