David Aguilar
(617) 495-7462

Christine Pulliam
(617) 495-7463


CfA Press Release
 Release No.: 03-12
For Release: 9:30 a.m. CDT, May 26, 2003

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

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

"Right now, the universe is speeding up, with galaxies zooming away fromeach other like Indy 500 racers hitting the gas when the green flag dropsand the pace car gets out of their way. But we suspect that it wasn't alwaysthis way," said Kirshner.

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

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

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

Five years ago, both teams announced that their studies of Type Iasupernovae showed the expansion of the universe is speeding up. Theaccelerating expansion pointed to the existence of an unexplained "darkenergy" that permeates all of space.

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

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

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

"A better measurement of w will help answer the question: Is the dark energyEinstein's cosmological constant, or is it something else such as theso-called 'quintessence'?" said Chris Stubbs (University of Washington), oneof the leaders of the ESSENCE project. "This is an important questionconsidering that about 70 percent of the energy in the universe is darkenergy, 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 wefind, this will be interesting."

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

Adam Riess (Space Telescope Science Institute), as principal investigatorfor the Higher-z Supernova Search Team, is cooperating with the GreatObservatories Origins Deep Survey (GOODS) to look for higher-redshiftsupernovae using the Hubble Space Telescope's Advanced Camera for Surveys(ACS). That program uses the ACS to find Type Ia supernovae at very largeredshifts (and hence large distances), in order to look back even farther intime. The Higher-z project will have the best chance to determine whetherthe 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: An animation of the stop-and-go universe and a high-resolution artwork image are available at: http://www.cfa.harvard.edu/news/archive/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.

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468

Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016

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