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Public Affairs Department
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusets

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468
daguilar{at}cfa.harvard.edu

Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
clafon{at}cfa.harvard.edu

For Release: 9:30 a.m. CDT, May 26, 2003

Release No.: 03-12

Do We Live In A "Stop And Go" Universe?

Anyone who drives is familiar with the frustration of being caught in 
"stop and go" traffic, a phenomenon found in urban areas all over the 
world. Astronomers have found that stop-and-go traffic is even more 
widespread than that, affecting galaxies throughout the universe.
Today at the 202nd meeting of the American Astronomical Society,
Robert Kirshner (Harvard-Smithsonian Center for Astrophysics), on
behalf of the international High-z Supernova Search Team led by Brian
Schmidt (Mount Stromlo Observatory), presented evidence that the
expanding universe slowed for billions of years before galaxies began
accelerating, like cars that get past a bottleneck.

"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 and the pace car gets out of their way. But we suspect
that it wasn't always this way," said Kirshner.

John Tonry (University of Hawaii), principal investigator of the team 
for the new and collected previous observations reported on today, 
agreed. "We've been hoping to see this effect of slowing in the
distant past. We saw evidence 5 years ago that the expansion of the
universe currently is accelerating, but we didn't know for sure what
it was doing 7 billion years ago. We are now seeing hints that, way
back then, the universe was slowing down."

Astronomers discovered seven decades ago that the universe is
expanding, with galaxies rushing away from each other in all
directions. Physics suggested that the expansion, which began with
the Big Bang, should slow down over time due to the combined
gravitational pull from all matter in the cosmos.

Two groups-the High-z Supernova Search Team and the Supernova
Cosmology Project-sought to study the universe's expansion by
observing distant exploding stars called Type Ia supernovae. At their
peak, these explosions are brighter than a billion stars like the
Sun, enabling astronomers to see and study them across billions of
light-years of space. 

Five years ago, both teams announced that their studies of Type Ia 
supernovae showed the expansion of the universe is speeding up. The 
accelerating expansion pointed to the existence of an unexplained
"dark energy" that permeates all of space.

Those initial findings were based on a few dozen supernovae. Now, the 
High-z Supernova Search Team has expanded that work to 79 distant and 
140 nearby supernovae, some newly observed and some previously
studied by observers worldwide. The additional data show with higher
precision that the discovery of five years ago was correct and the
universe currently is accelerating.

More importantly, Kirshner reported that Tonry and the High-z
Supernova Search Team snagged four supernovae so distant that their
light may well have left at a time when the universe was still
slowing down, before dark energy began to dominate the gravitational
pull of matter. 

Future plans include doubling the number of well-observed Type Ia 
supernovae through an ambitious program at the National Science 
Foundation's Cerro-Tololo Inter-American Observatory. The ESSENCE 
project (standing for "Equation of State: SupErNovae trace Cosmic 
Expansion") seeks to make an accurate measurement of the cosmic 
parameter w, which provides clues about the nature of the dark
energy.  The parameter w is defined as p/rho, the ratio of the dark
energy's pressure to its energy density.

"A better measurement of w will help answer the question: Is the dark 
energy Einstein's cosmological constant, or is it something else such
as the so-called 'quintessence'?" said Chris Stubbs (University of
Washington), one of the leaders of the ESSENCE project. "This is an 
important question considering that about 70 percent of the energy in 
the universe is dark energy, while only 30 percent is due to matter. 
Whatever dark energy is, it's the dominant stuff of the cosmos. We
can't lose: No matter what we find, this will be interesting."

Currently, the value of w is known only to within a factor of 2. The 
ESSENCE project will do 10 times better, reducing the level of 
uncertainty to plus or minus 10 percent.

Adam Riess (Space Telescope Science Institute), as principal 
investigator for the Higher-z Supernova Search Team, is cooperating
with the Great Observatories Origins Deep Survey (GOODS) to look
for higher-redshift supernovae using the Hubble Space Telescope's
Advanced Camera for Surveys (ACS). That program uses the ACS to find
Type Ia supernovae at very large redshifts (and hence large
distances), in order to look back even farther in time. The Higher-z
project will have the best chance to determine whether the universe
really was slowing down before cosmic acceleration kicked in.

The paper describing the results reported in this press release is
online at
     http://arxiv.org/abs/astro-ph/0305008
and has been accepted for publication in The Astrophysical Journal.

NOTE TO EDITORS: A high-resolution artwork image is available at
     http://cfa-www.harvard.edu/press/pr0312image.html

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian 
Center for Astrophysics (CfA) is a joint collaboration between the 
Smithsonian Astrophysical Observatory and the Harvard College 
Observatory. CfA scientists organized into six research divisions
study the origin, evolution, and ultimate fate of the universe.

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