ASTRONOMY:
Exploding Stars Flash New Bulletins From Distant Universe

James Glanz

In just the past few months, astronomers have glimpsed an extraordinary new
picture of the universe in the glare of the cosmic flashbulbs called
supernovae. Everyone from theoretical physicists to philosophers of science
is struggling with the startling implication that emerged after observers
laboriously discovered and studied scores of these distant, exploding
stars: A mysterious repulsive force has been at work over billions of
years, counteracting gravity and speeding up the cosmic expansion rate
(Science, 27 February, p. 1298; 30 January, p. 651). Now the light of these
same supernovae is adding some intriguing new details to this picture.

After further analyzing their observations of how fast these beacons are
rushing away from Earth, the two teams that made the original discovery are
now ready to report a cascade of new findings about how the universe
behaves both in our own cosmic neighborhood and over the largest scales.
They have found evidence that we might live in a "Hubble bubble"--a region
that is expanding slightly faster than the universe as a whole. They have
also picked up clues to just what kind of energy might be filling space and
causing the acceleration and have offered a preliminary assessment of the
universe's total density of energy and matter. "We have a tool that can be
used to approach cosmology from another angle," says Saul Perlmutter of
Lawrence Berkeley National Laboratory and the University of California,
Berkeley, the leader of one of the teams. Perlmutter's team has squeezed
yet another finding from the supernova data: large-scale confirmation that
time itself runs slower when objects--in this case, the supernovae--are
traveling at a large fraction of the speed of light because of the
expansion of the universe.

Both the Perlmutter group and the other team, the High-z Supernova Search
Team led by Brian Schmidt of the Mount Stromlo and Siding Spring
Observatory in Australia, stress that the data are far from conclusive for
most of these claims. But the findings testify to the power of so-called
type Ia supernovae as cosmic probes. These exploding white dwarf stars all
blow up with nearly the same brightness, acting as "standard candles,"
whose apparent brightness as seen from Earth can be translated into
distances. The supernovae can be seen across most of the visible universe,
at distances corresponding to earlier times in cosmic history.

By plotting the distances of the supernovae against the speed at which
expansion is carrying them away from Earth--easily found from the redshift,
or stretching, of their light--astronomers can see how cosmic expansion has
changed over time. For nearby supernovae, that plot is nearly linear,
implying no change in the expansion rate, or Hubble constant. Farther away
the line subtly bends in a direction that shows the expansion has
accelerated since the light was emitted.

Last year, the Perlmutter team concluded from such "Hubble diagrams" that
the universe is expanding roughly uniformly around us on scales of billions
of light-years. But Idit Zehavi and Avishai Dekel of The Hebrew University
in Israel and Adam Riess of Berkeley and the High-z team noticed a slight
shift in the diagrams at a few hundred million light-years or so from
Earth. They say the shift may indicate that our region is expanding about
6% faster than the universe at large.

The location of the shift caught their eye: It corresponds to the distance
of several large agglomerations of galaxies, including the so-called Great
Wall, discovered in the 1980s by Margaret Geller and John Huchra of the
Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge,
Massachusetts. Zehavi, Riess, and Dekel, along with CfA's Robert Kirshner,
think the gravitational pull of the mass concentrated at the borders of our
cosmic neighborhood might help speed up cosmic expansion locally by tugging
galaxies outward toward the Great Wall, resulting in a more tenuous region
in which our own galaxy sits.

The group's paper, to be published in the Astrophysical Journal, should be
regarded as "a preliminary discovery paper," says Dekel. "It may still be a
statistical fluke." But if the 6% bubble doesn't burst, says Edwin Turner
of Princeton University, then techniques that measure the Hubble constant
using nearby objects, such as variable stars, "might be giving the right
value but not the true, global value." That might help explain what some
astronomers regard as a persistent, nagging discrepancy between those
techniques and ones that measure the constant from distant beacons like
supernovae. Wendy Freedman of the Carnegie Observatories in Pasadena,
California, who specializes in measuring the Hubble constant, isn't yet
persuaded that the Hubble bubble can explain the differences but is still
intrigued. "There are very few methods that have the kind of promise this
does for attacking the problem," she says.

Peter Garnavich of CfA and his colleagues on the High-z team are looking
much farther out along a Hubble diagram based on 16 supernovae recently
analyzed by Riess and others. The exact shape of the curve in the diagram
should reflect just what kind of energy is at work on large scales, giving
a boost to the expansion. Garnavich and his colleagues are comparing the
data to the curves expected from the cosmological constant--an effect first
postulated by Einstein--and from other forms of background energy, which
theorists have named quintessence or X-matter. Although no one knows just
what physical processes might produce these forms of energy, they would
behave differently. The cosmological constant would deliver an unchanging
push, while quintessence and X-matter could have varied over time, and
energy from quintessence could actually flow and bunch up, affecting
different parts of the universe differently.

So far, says Garnavich, the unrelenting push of the cosmological constant
fits the data best. But the handful of distant supernovae observed so far
"certainly doesn't do much in restricting what exactly the [form of the]
quintessence is." The most that can be said, he explains, is that one form
of quintessence seems to be ruled out: defects in the fabric of space,
called light nonabelian strings, that might have been left over from the
big bang. The Perlmutter group is now analyzing 40 supernovae, which could
give a clearer picture of the mysterious energy.

But whatever form the acceleration energy takes, there appears to be just
enough of it to combine with matter and give the critical density of mass
and energy that is predicted by leading theories of the big bang. To gauge
the total, Garnavich, with CfA's Saurabh Jha and others, added the
supernovae data to observations of the cosmic microwave background
radiation, often referred to as the big bang's afterglow. Slight ripples in
the background reflect conditions in the early universe and yield clues to
basic cosmic parameters. The result is just the right density to make the
universe geometrically "flat"--the kind of universe predicted by the
simplest versions of inflation, the theory of how a sort of spark in the
primordial nothingness could have set off the big bang.

Everything that researchers have concluded so far from these distant
beacons rests on a crucial assumption: that the redshifts actually are
caused by universal expansion. Most cosmologists don't question this
assumption, but a few mavericks have proposed alternative explanations for
the reddening of distant objects--for example, a sapping of the photons'
energies as they traverse great distances.

Type Ia's offer a way to distinguish among these possibilities, because the
physics of the explosions force them to brighten and dim on a predictable
schedule. That "light curve" should appear to be stretched out for
supernovae rushing away from Earth, because the light carrying news of
later and later events would have to travel longer and longer distances.

By examining the light curves of about 40 supernovae, Berkeley's Gerson
Goldhaber and others in the Perlmutter group found spectacular confirmation
that they really are speeding away from Earth: Events that actually take a
month on Earth were stretched to almost 7 weeks for the most distant of the
supernovae. Although no one was surprised by the result, says Goldhaber,
it's one more example of the light a standard candle can shed on the
cosmos.
 
 

Volume 280, Number 5366 Issue of 15 May 1998, pp. 1008 - 1009
©1998 by The American Association for the Advancement of Science.