David Aguilar
(617) 495-7462

Christine Pulliam
(617) 495-7463

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CfA Press Release
 

Embargoed until:
12:40 pm, January 8, 1998

HUBBLE PINPOINTS DISTANT SUPERNOVAE TO MEASURE DECELERATION OF UNIVERSE

WASHINGTON, DC--Peering halfway across the universe toanalyze light from exploded stars which died longbefore our Sun even existed, NASA's Hubble SpaceTelescope has allowed astronomers to determine that theexpansion of the cosmos has not slowed since theinitial impetus of the Big Bang and, thus, shouldcontinue to balloon outward indefinitely.

Reporting their preliminary observations today at thewinter meeting of the American Astronomical Society,the large international team of scientists representedby Peter Garnavich of the Harvard- Smithsonian Centerfor Astrophysics (CfA) in Cambridge, MA, concludesthere is insufficient matter in the cosmos to providethe gravity necessary to halt its infinite expansion.

If these early conclusions are supported by additionalobservations, the lack of any significant decelerationsince the initial conditions also means the universecould be as much as 15 billion years old. This wouldclearly establish the universe as truly older than theoldest stars, thus resolving the potential paradoxcaused by earlier estimates favoring a youngeruniverse.

These results are based on unprecedented distancemeasurements to supernovae so far away they allowastronomers to determine if the universe was expandingat a faster rate long ago. The most distant supernovaseen is approximately halfway back to the Big Bang, andthus existed about 7.7 billion years ago. The twoothers studied each exploded approximately 5 billionyears ago, or just before our own solar system formed.

"We cannot make much of a conclusion from the singlefarthest supernova we've seen," says Garnavich. "But,when we average it with several others, we find, to a95 percent level of confidence, that the density ofmatter is insufficient to halt the expansion of theuniverse."

"In other words, we'd bet $100 against your $5 that theuniverse isn't bound by matter--dark matter, brightmatter, matter that clusters, or matter that's spreadout," says team member Robert Kirshner, also of CfA.

Because supernovae are the brightest events in theuniverse, they are ideal candidates to use asyardsticks for measuring vast cosmic distances. Thesupernovae studied by Hubble, belonging to a classcalled Type Ia, are considered reliable distanceindicators because there is a direct link between theirintrinsic brightness and rate of dimming followingtheir explosions. Although this class of supernovaehas been sought since the 1950s, astronomers had towait for Hubble's sharp vision to identify objects farenough away to provide evidence for the deceleration ofthe universe.

Because Hubble observations had to be scheduled long inadvance, a ground-based search for candidate supernovaewas made by the Canada-France-Hawaii Telescope (CFHT)on Mauna Kea, Hawaii. Spectroscopic observations weremade at the Keck Observatory, also on Mauna Kea, and atthe joint Smithsonian Institution-University of ArizonaMultiple Mirror Telescope Observatory (MMTO) on Mt.Hopkins, Arizona, to measure the supernovae'sredshifts, which indicate relative distances fromEarth.

Hubble was then used for follow-up observations usingdiscretionary time provided by Space Telescope ScienceInstitute (STScI) director, Robert Williams. Hubblemade five observations of each of the three supernovaeinitially targeted. The observations were separated byabout a week, to allow time for the supernovae to dimso astronomers could plot the required light curves.

The search for the "deceleration parameter"--which isfundamental to estimating the age of the universe andits ultimate fate--has been pursued by cosmologists fornearly a half-century. The researchers caution thattheir findings are preliminary and that a sample ofmany more supernovae is needed either to yield a valuefor the density of matter in space to within tenpercent or to refine estimates of the universe'sdeceleration. They plan to use Hubble's new infraredcapabilities to study supernovae at even greaterredshifts.

The results of the study will be published in theFebruary 1, 1998, edition of The Astrophysical JournalLetters. The full HST team, in addition to Garnavichand Kirshner, includes, P. Challis (CfA), J. Tonry(UHawaii), R.L. Gilliland (STScI), R.C. Smith (UMich),A. Clocchiati (CTIO), A. Diercks (UWash), A.V.Filippenko (UCB), M. Hamuy (UAriz), C.J. Hogan (UWash),B. Leibundgut (ESO), M.M. Phillips (CTIO), D. Reiss(UWash), A.G. Riess (UCB), B.P. Schmidt (MSSSO), J.Spyromilio (ESO), C. Stubbs (UWash), N.B. Suntzeff(CTIO), and L. Wells (UAriz).

The Hubble Space Telescope is available at http://oposite.stsci.edu/...

For more information, contact:
Ray Villard, STScI, villard@stsci.edu, 410/338-4514
James Cornell, CfA, jcornell@cfa.harvard.edu, 617/495-7462
Peter Garnavich, CfA, pgarnavich@cfa.harvard.edu, 617/495-9245
Robert Kirshner, CfA, rkirshner@cfa.harvard.edu, 626-395-8488, (if no response, then call 617/495-7519)

 
 
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