Press Release

Do We Live In A “Stop And Go” Universe?

By SpaceRef Editor
May 27, 2003
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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
and has been accepted for publication in The Astrophysical Journal.

NOTE TO EDITORS: A high-resolution artwork image is available at

SpaceRef staff editor.