With their discovery of the most distant supernovas (exploding stars)
ever observed, an international scientific team led by researchers with
the Ernest Orlando Lawrence Berkeley National Laboratory may be on the
verge of learning the ultimate fate of our universe. The work is
being presented today at the American Astronomical Society meeting in
San Antonio, TX.
In all, 18 supernovas were discovered by the brilliant light of their
self-destruction: a supernova explosion can be brighter than the entire
galaxy of billions of stars in which it is born. Eleven of them, including
several of the most distant known stars, were found at the end of November
within one 48 hour period. This record number of discoveries at record
distances is dramatic proof of the technique developed at the Berkeley Lab to
make deep space supernova discoveries possible, and eventually even routine.
The supernovas were found in galaxies ranging from four to seven
billion light years away. (A light year is the distance light can
travel in a single year -- approximately 6 trillion miles.) This means
that the furthest of these explosions took place billions of years
before our own solar system was formed, halfway back to the beginning
of the universe. Only now is the light reaching Earth.
The discoveries were made as part of the "Supernova Cosmology Project"
which is headed by Saul Perlmutter, an astrophysicist in the Berkeley
Lab's Physics Division. The goal of the project is to measure the
universe's "deceleration" -- the rate at which the known continual
expansion of the universe is slowing down. At stake is an
understanding of the way the universe will end: having begun expanding
with the Big Bang, will it continue to expand forever, becoming ever
more cold and empty, or will it slow to a halt, turn around, and
eventually contract to a fiery dense finale -- a Big Crunch.
"Distant supernovas provide natural milemarkers useful in determining
these trends in the cosmic expansion," says Perlmutter. The redshift
of the distant supernovas -- a shift in the colors of their spectra's
features -- tells how much the universe has expanded during the
billions of years that the light has travelled from the supernova to
our telescopes here on Earth. If this total amount of expansion is
more than expected from the current expansion rate -- called the
"Hubble constant" -- then the universe must have been expanding faster
in the past and be slowing down. And if it is slowing enough, it will
eventually come to a halt and start to contract.
Conversely, the project could discover that the expansion is actually
accelerating. This scenario has recently been proposed by a number of
scientists, as a solution to discrepancies between the ages of the oldest
stars and the recent measurements of the Hubble constant.
Most of the distant supernovas are classified as "Type Ia," the
brightest type of supernova. "Type Ia supernovas can serve as a
measurement of distance because they can be calibrated as a standard
candle with a known brightness," says Perlmutter. "By measuring the
light reaching us from a distant Type Ia supernova and comparing this
to the known brightness of similar supernovas in nearby galaxies, we
can calculate the supernova's distance from earth."
Today's presentation at the AAS meeting, based on their first seven
supernova discoveries, already provides intriguing hints of
deceleration. Perlmutter and his colleagues on the Supernova Cosmology
Project are now tracking the brightness of the recently discovered
supernovas. Once these observations have been completed, the first
determination of the deceleration of the universe will follow shortly.
As the project continues to discover supernovas over the next few
years, the accuracy of the measurement will continue to improve.
The technique used to discover the 11 recent supernovas of 1995 is the
same that was used by the Supernova Cosmology Project team to discover
the seven supernovas of 1994 and 1993. An ultrasensitive electronic
camera attached to a telescope is used to photograph thousands of
deep-space galaxies at the time of a new moon. A second set of images
of those same galaxies is taken at the same telescope just before the
next new moon. Using a computer, the two sets of images are compared
and light from the older image is subtracted from new image light to
reveal the appearance of supernovas.
"A Type Ia supernova can shine almost as brightly as an entire galaxy,
but only for about a month before it becomes too faint for even the
largest telescopes to observe," says Gerson Goldhaber, a Berkeley Lab
and University of California, Berkeley professor and one of the key
members of the project. "For the purpose of measuring distances, it is
important that we are discovering Type Ia supernovas just before or at
their brightest moments."
The experiment has only recently become possible, since it depends on
the advances in light-detectors, computers, and the Internet, which
ties together the most important ingredients: astronomers using the
newest and largest telescopes around the world. All of the recent
supernovas were sighted on the Cerro Tololo Inter-American Observatory
4-meter (158-inch) telescope in Chile, the largest in the southern
hemisphere. Brightness measurements of the supernovas were collected
by Di Harmor, Daryl Willmark, and Dave Silva at the recently
commissioned WIYN telescope on Kitt Peak, AZ, by Heidi Newberg at the
recently commissioned ARC telescope, at Apache Point, NM, by Richard
McMahon, Mike Irwin, Ariel Goobar, and Dave Carter at the 4-meter and
2.5-meter telescopes on the Canary Islands (off Africa), and by Warrick
Couch and Richard Ellis at the Anglo-Australian 4-meter telescope in
Australia. It was the world's largest telescope, the Keck 10-meter
(400-inch) Telescope in Hawaii, that provided the observing power to
identify the supernova spectra -- observations made with University of
California, Berkeley astronomers Alex Filippenko, Aaron Barth, and
Berkeley Lab's Isobel Hook. The Keck Ten Meter Telescope was designed
at the Berkeley Lab specifically for the observation of faint distant
objects such as these supernovas.
The Central Bureau for Astronomical Telegrams sent an International
Astronomical Union telegram on December 6 announcing the recent
supernova discoveries to observers around the world. The participants
in the discoveries listed on the telegram were: S. Perlmutter, S.
Deustua, G. Goldhaber, D. Groom, I. Hook, A. Kim, M. Kim, J. Lee, J.
Melbourne, C. Pennypacker, and I. Small, Lawrence Berkeley Lab. and
the Center for Particle Astrophysics; A. Goobar, Univ. of Stockholm;
R. Pain, CNRS, Paris; R. Ellis and R. McMahon, Inst. of Astronomy,
Cambridge; and B. Boyle, P. Bunclark, D. Carter, and M. Irwin, Royal
Greenwich Obs.; with A. V. Filippenko and A. Barth (Univ. of
California, Berkeley) at the Keck telescope; W. Couch (Univ. of N.S.W.)
and M. Dopita and J. Mould (Mt. Stromlo and Siding Spring Obs.) at the
Siding Spring 2.3-m telescope; H. Newberg (Fermi National Accelerator
Lab.) and D. York (Univ. of Chicago); D. Harmor, D. Willmark, and D.
Silva at the WIYN telescope; and A. Walker, at CTIO.
The Supernova Cosmology Project is jointly sponsored by Berkeley Lab
and the National Science Foundation's Center for Particle Astrophysics
at the University of California, Berkeley. The project is also
affiliated with the new Institute for Nuclear and Particle Astrophysics
at the Berkeley Lab.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California.
For More Information: contact Dr. Saul Perlmutter (510) 486-5203
