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Posted on 10/12/2003 10:23:49 AM PDT by UnklGene
A 'Cosmic Jerk' That Reversed the Universe
By DENNIS OVERBYE
Published: October 11, 2003
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."
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