From: Tony Spadafora (ALSpadafora@lbl.gov)
Date: Tue Jun 03 2003 - 10:47:01 PDT
From Distant Galaxies, News of a 'Stop-and-Go Universe'
June 3, 2003
By JOHN NOBLE WILFORD
NASHVILLE, May 30 - New observations of exploding stars far
deeper in space, astronomers say, have produced strong
evidence that the proportions of the mysterious forces
dominating the universe have undergone radical change over
cosmic history.
The findings, reported here at a meeting of the American
Astronomical Society, which ended Thursday, supported the
idea that once the universe was expanding at a decelerating
rate but then began accelerating within the last seven
billion years, scientists concluded.
"We are now seeing hints that way back then the universe
was slowing down," said Dr. John Tonry, an astronomer at
the University of Hawaii who is a member of one team
studying exploding stars, or supernovas, for signs of
cosmic expansion rates.
The new research by Dr. Tonry's group and another, led by
Dr. Saul Perlmutter of Lawrence Berkeley National
Laboratory in California, confirmed the earlier surprising
discovery that the universe is indeed expanding at an
accelerating rate and has been for at least the last 1.2
billion years. But four supernovas, almost 7 billion
light-years away, appeared to exist at a time the universe
was slowing down, Dr. Tonry said.
"A stop-and-go universe" is the way Dr. Robert P. Kirshner
of the Harvard-Smithsonian Center for Astrophysics
characterized the phenomenon. Well, the expansion never
really stopped, he conceded, but it has certainly revved
up.
"Right now, the universe is speeding up, with galaxies
zooming away from each other like Indy 500 racers hitting
the gas when the green flag drops," said Dr. Kirshner, a
member of the Tonry team. "But we suspect that it wasn't
always this way."
The changing pace of cosmic expansion, combined with
recently announced measurements of the cosmic microwave
background, revealing conditions soon after the Big Bang,
encourages theorists in thinking that a tug-of-war has been
going on between dark forces of matter and energy no one
yet understands.
The combined gravitational pull from all matter in the
universe, most of which is beyond detection, has acted as a
brake on cosmic expansion. The so-called dark matter
apparently had the advantage when the universe was younger,
smaller and denser. Now the ever-increasing pace of
expansion suggests that something else even more mysterious
is at work. Theorists are not sure what the antigravity
force is, but they call it dark energy. It has apparently
gained the upper hand.
This is the latest turn of events in the unfolding story of
cosmic history. Once scientists believed the universe was
everlastingly static. Along came Edwin P. Hubble, who
discovered seven decades ago that the galaxies of stars are
rushing away from one another in all directions. The
universe, Hubble announced, is expanding.
Five years ago, astronomers were in for a surprise. They
had assumed that after an initial burst of rapid expansion
from the originating Big Bang the gravity of matter was
gradually slowing things down. Then the two supernova
survey teams found that the universe was accelerating
instead. This pointed to the existence of some kind of dark
energy permeating all of space.
For the current research, astronomers observe what are
called Type Ia supernovas, stellar explosions that at their
peak are brighter than a billion stars like the Sun. They
are thus visible across billions of light-years of space,
and a close examination of their light reveals the
distances, motions and other evidence of conditions. As the
light travels to Earth, the wavelengths are stretched by an
amount that reflects the universe's expansion when the star
exploded.
Dr. Kirshner said the four extremely distant supernovas
indicated that the universe seven billion years ago was "in
fact winning this sort of cosmic tug-of-war," but now dark
energy is more dominant.
Scientists said they assumed that with the stretching out
of space the proportion of dark energy to dark matter had
been reversed. In the earlier and denser universe, matter
of all kinds, the invisible dark matter and the visible
ordinary matter of stars and planets, predominated.
The team of Dr. Tonry and Dr. Kirshner estimates that about
60 percent of the universe is filled with dark energy and
30 percent of the mass is dark matter. The remaining 10
percent consists of ordinary matter, only 1 percent of
which is visible in the galaxies. Theorists offer roughly
the same estimates and surmise that the changeover from
dark matter to dark energy domination probably occurred
before 6.3 billion years ago.
Dr. Perlmutter said that much more research would be
necessary to determine whether the changing density of the
expanding universe was the only reason dark energy came to
rule cosmic dynamics. Or have the physical properties of
dark energy, whatever it is, changed?
Dr. Perlmutter said that in the words of Dr. Edward Witten,
a theoretical astrophysicist at the Institute for Advanced
Study at Princeton, the true nature of dark energy "would
be No. 1 on my list of things to figure out."
The research teams are planning new observations of more
distant supernovas to determine when cosmic acceleration
began and to gather clues about the properties of dark
energy. Some observations will be conducted with
ground-based telescopes, others with the Hubble Space
Telescope. Dr. Perlmutter's group has proposed putting a
spacecraft in orbit with telescopes especially designed for
supernova hunting and pinning down the nature of dark
energy.
In "The Extravagant Universe," published last fall by
Princeton University Press, Dr. Kirshner wrote: "We are not
made of the type of particles that make up most of the
matter in the universe, and we have no idea yet how to
sense directly the dark energy that determines the fate of
the universe. If Copernicus taught us the lesson that we
are not at the center of things, our present picture of the
universe rubs it in."
http://www.nytimes.com/2003/06/03/science/space/03ASTR.html?ex=1055661327&ei=1&en=288944faef555d08
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