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Author : George Musser
Date    :


If astronomers are right and the universe is expanding at a quickening pace,
the cosmos will grow to untold size. But our world will shrink. The vast
distances between galaxies will become ever vaster, until not even a
spaceship traveling at the speed of light could cross them. The relatively
nearby Coma cluster of galaxies, for instance, will be cut off from the Milky
Way after some 60 billion years. Eventually we will be solitary prisoners in
our cosmic neighborhood. "If you want to see Coma, go now," recommends
cosmologist Glenn D. Starkman of Case Western Reserve University. "Time is
running out."

A year and a half ago few scientists thought about the oddities of life in an
accelerating universe, as opposed to in the traditional, decelerating one.
But observations of distant supernovae, as well as confirmation of
discrepancies in the amount of matter in space, have stretched cosmologists'
minds. The latest finds have boosted the two main arguments for acceleration
and hinted at its exotic antigravitational causes.

The supernovae, acting as rafts on the cosmic currents, provide the most
direct probe of the expansion rate. Acceleration could account for the
anomalous faintness of these stellar explosions. But might not they seem
dimmer for more prosaic reasons, such as dust absorption and changes in
stellar composition over time?

Image by: Christopher D. Impey, University of Arizona; Space Telescope
Science Institute
GRAVITATIONAL LENS creates four images of a single quasar. The paucity of
such lenses may mean that the cosmological constant, if it exists, is not
really constant.

Last October's detection of SN1998eq helps to allay this concern. Nicknamed
for the composer Tomaso Albinoni by the Supernova Cosmology Project--whose
leader, Saul Perlmutter of Lawrence Berkeley National Laboratory, plays the
violin--the supernova is the most distant yet found. It is not as anomalously
dim as nearer explosions, and that is difficult to attribute to prosaic
effects, which should steadily increase with distance. But it is easy to
explain in an accelerating universe, because acceleration decreases with
distance: earlier on in cosmic history, the density of matter was higher and
gravity stronger.

To address unorthodox speculation that the anomalous dimness might be caused
by light getting "tired" on its long journey, Perlmutter and other supernova
hunters point out that distant supernovae appear to fade more slowly than
nearby ones. This "time dilation" is a natural consequence of cosmic
expansion, which, by stretching light waves, both reddens them and drags out
their arrival on the earth. The putative tiring of light might change its
color and predicted brightness, but not the apparent passage of time.

The second main argument for acceleration is the discrepancy between the
observed amount of matter in the universe and the amount needed to give space
a Euclidean geometry. A hitherto unknown type of energy may plug this gap,
perhaps the infamous cosmological constant or its inconstant cousin,
"quintessence," either of which could exert an antigravity force.

The weak link in this second argument has been the assumption that space is
Euclidean [see "Is Space Finite?"]. Observations of the cosmic microwave
background radiation at a telescope in Saskatoon, Canada, several years ago
suggested that it is. Since then, the case has been solidifying. The latest
evidence comes from two South Pole telescopes, Python and Viper, run by
scientists at Carnegie Mellon University and the University of Chicago; from
the California Institute of Technology's Owens Valley Radio Observatory; and
from a reanalysis of the balloon-borne Medium Scale Anisotropy Measurement
(MSAM), sent aloft by researchers at Chicago and the National Aeronautics and
Space Administration Goddard Space Flight Center.

Some data, however, refuse to go along quietly with the accelerating
scenario. The main counterevidence involves gravitational lensing, the
bending of light from one celestial body by the gravity of another. One type
of distortion, multiple galaxy images, should be common if the volume of
space is large, as in an accelerating universe. Yet various studies, most
recently by Emilio E. Falco and his colleagues at the Harvard-Smithsonian
Center for Astrophysics, have found only a handful of image clones. Another
type of distortion, sweeping arcs of light, depends on the concentration of
galaxy clusters and should be fairly rare in an accelerating universe. But
according to Matthias Bartelmann of the Max Planck Institute for Astrophysics
in Garching and his colleagues, such arcs are widespread.

The lensing observations might be easier to reconcile if the accelerating
force varies with position or time--a scenario that quintessence conveniently
brings about. Groups led by Perlmutter, by Peter M. Garnavich of Harvard and
by Limin Wang of Columbia University have combined all the available data to
deduce what properties quintessence could have. If the force does differ from
place to place, the local universe might be just a small pocket of
accelerating space, as Starkman and his colleagues have described. As
galaxies are pushed apart, they eventually leave the pocket and begin to

But if the observers find that the acceleration persists out much farther
than Albinoni, any variations will matter little to us. Almost all the
galaxies we now see will come to recede at light speed, and solitude will
indeed be our fate. Because of time dilation, we will watch the ghosts of
departed galaxies slow to an adagio. "The galaxies will rotate more and more
sluggishly, stars will evolve more slowly, and everything will look redder,"
Starkman says. The cosmos will have been paralyzed by its haste.

--George Musser





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