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New Data on 2 Doomsday Ideas, Big Rip vs. Big Crunch
February 21, 2004
By JAMES GLANZ
MARINA DEL REY, Calif., Feb. 20 - A dark unseen energy is
steadily pushing the universe apart, just as Einstein
predicted, suggesting the universe may have a more peaceful
end than recent theories envision, according to striking
new measurements of distant exploding stars by the orbiting
Hubble Space Telescope.
The energy, whose source remains unknown, was named the
cosmological constant by Einstein. In a prediction in 1917
that he later called "my greatest blunder," Einstein
posited a kind of antigravity force that was pushing
galaxies apart with a strength that did not change over
billions of years of cosmic history.
Theorists seeking to explain the mysterious force have
suggested that it could, in fact, become stronger or weaker
over time - either finally tearing the universe apart in a
violent event called "the big rip" or shutting down in the
distant future - tens of billions of years from now.
If the force somehow shut down, gravity would again
predominate in the cosmos, and the universe would collapse
on itself. That version of oblivion is sometimes called
"the big crunch."
On Friday, Dr. Adam Riess of the Space Telescope Science
Institute in Baltimore presented the first broad set of
observational figures that gauge the strength of the
antigravity force over time. The information, he said,
suggests that the cosmos will gradually expand, cool and
darken, more akin to a slide into senescence rather than a
violent apocalypse.
Dr. Riess and his team, which included Drs. Louis Strolger
of the science institute and Alexei V. Filippenko of the
University of California at Berkeley, used the Hubble to
search for exploding stars, or supernovae, that are swept
up in the cosmic push of the dark energy. They discovered
42 new supernovas in their survey area, including six of
the seven most distant known.
Rather than seeing the changes in the push that many
theorists had predicted, Einstein's unchanging cosmological
constant fits the data better than any of the alternatives.
"What we've found is that it looks like a semipermanent
kind of dark energy," Dr. Riess said. "It appears like it's
been with us for a long time. If it is changing, it's doing
so slowly. Einstein's theory is looking a lot better than
before this data."
A cosmologist not involved in the work, Dr. Michael S.
Turner of the University of Chicago, said: "This is the
biggest mystery in all of science, whether or not dark
energy varies with time. It's a big, big clue, and this is
the first information we have."
Although the new results favor Einstein's nearly
century-old prediction, Dr. Turner said, they still do not
rule out some alternative theories. The information
specifically leaves open the chance that the antigravity
force will eventually strengthen and tear apart planets,
stars and even atoms in a big rip, among other exotic
possibilities.
Dr. Turner said that future measurements were quite likely
to turn up smaller changes in the force over time and that
those subtleties could help unravel the mystery.
The results do suggest that any ultimate cataclysm could
not occur for, perhaps, 30 billion years, Dr. Riess said.
Several physicists said that those estimates were highly
uncertain, but that the findings could lead to a
wide-ranging reassessment of models that predict strange
variable energy densities in space.
"Models which predict wild dark energy densities which
change a lot with time don't look so good," Dr. Yun Wang, a
cosmologist at the University of Oklahoma, said. "Everybody
will go back to the drawing board."
Dr. Filippenko presented some of the results at a
conference here on Sources and Detection of Dark Matter and
Dark Energy in the Universe.
The measurements raise new questions about the NASA
decision, which is under review, to let the Hubble die a
slow death in space rather than try another service mission
with a space shuttle. The administrator of National
Aeronautics and Space Administration, Sean O'Keefe, has
said a service mission would be too risky after the
disaster of the Columbia space shuttle.
Dr. Riess said he disagreed with the decision to stop the
Hubble, which would halt the research for years to come. He
pointed out that the remarkable clarity of the work
depended on the Advanced Camera for Surveys, which
spacewalking astronauts installed in Hubble two years ago.
The organizer of the conference here, Dr. David B. Cline
of the University of California, Los Angeles, said that in
view of the team's results, he was inclined to agree with
Dr. Riess's assessment.
"They really shocked everybody by showing they could do
this," Dr. Cline said. "You have to say obviously it's
shame that the Hubble can't continue its life."
All the scientists agreed that because the problem had
become so knotty, a complete solution might have to wait
for a proposed satellite, tentatively called the
Supernova/Acceleration Probe, or SNAP, which is planned to
observe thousands of exploding stars from space.
"It's so tantalizing and so beautiful to see this kind of
data set," said Dr. Saul Perlmutter, a physicist at
Lawrence Berkeley National Laboratory who is leading the
SNAP team. "This kind of result is so exciting for those of
us who are eager to get to that next step."
Einstein invented the cosmological constant 1917 to explain
why the universe, filled with countless stars and galaxies
that are attracted to one another by gravity, did not
collapse on itself. He added a constant term, called
lambda, to his equations of general relativity, which
describe the workings of gravity and the curvature of space
on large scales. Lambda provided a repulsive force to
counteract gravity.
In 1929, the astronomer Edwin Hubble, for whom the space
telescope is named, discovered that the universe was
expanding - from the primeval explosion called the Big
Bang, scientists would learn. In that picture, gravity
would slow the expansion and, possibly, reverse it at a
far-off time. Einstein abandoned lambda as unnecessary and
called it a blunder.
Six years ago, however, two supernova groups - one led by
Dr. Perlmutter and the other by several scientists,
including Drs. Filippenko and Riess - found that cosmic
expansion was speeding up rather than slowing. The evidence
was that distant supernovas, swept up in some sort of
cosmic repulsion, were farther away than they would
otherwise be and therefore appeared dimmer.
Lambda was reborn, but astronomers still had little
information on whether the antigravity force was truly
constant. The latest results suggest, within the accuracy
of the measurements, at least, that something like
Einstein's cosmological constant is indeed at work.
"His greatest blunders are our greatest ideas," Dr. Sean M.
Carroll, a cosmologist at the University of Chicago, said.
"It is a triumph of general relativity."
Einstein's theory says nothing about why the energy of the
cosmological constant, or anything else, should be filling
space in the first place, Dr. Carroll said. Theorists have
turned to explanations as bizarre as parallel universes
that exert an influence on our own or extra unseen
dimensions in the fabric of space.
Dr. Carroll said the supernova findings might raise the
stock of certain ideas that have emerged from string
theory, which assumes that fundamental physics is a
manifestation of minuscule strings that vibrate in an
11-dimensional space. String theorists have calculated what
they call cosmic "landscapes," in which empty space
contains energy densities that resemble a cosmological
constant.
Because dark energy has never been directly detected and
all these explanations are so bizarre, a few scientists say
the ultimate explanation might be something else entirely.
Others conceded that there was a bit of a letdown that
nearly a century after Einstein invented the cosmological
constant nary a crack in his theory had turned up.
"This gives me a sinking feeling," Dr. Max Tegmark of the
University of Pennsylvania said. "My nightmare is that
we're going to be forever stuck with this puzzle."
http://www.nytimes.com/2004/02/21/science/21DARK.html?ex=1078892295&ei=1&en=b7ffdc2e06f211f9
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Received on Fri Feb 27 2004 - 09:32:52 PST