Mercury,
September/October 2003 Table of Contents
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Image
Courtesy of NASA/STScI/Ann Feild.
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here for a larger version of this image)
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by
Amanda Gefter
About
70% of the universe’s energy consists of a force that accelerates
cosmic expansion, and scientists know virtually nothing about it.
Astronomical observations and bold theories may help solve the mystery.
Some
mysteries grow deeper the closer you look. From the unexpected supernovae
announcements of 1998, a new world has unfolded before our eyes.
Like Alice emerging from the rabbit hole and finding herself in
Wonderland, we must untangle the meaning of a strange new terrain
ruled by the covert forces of dark energy. The latest observational
and theoretical exploration of this mystery will point the way to
a more fundamental understanding of cosmic evolution and the inner
workings of the universe itself.
The
story begins, of course, at the beginning. In the first fraction
of a second following its birth, the universe apparently expanded
exponentially in a dramatic event called inflation, increasing its
size by a factor of 1050 in far less than the blink of
an eye. The expansion slowed as time passed, allowing galaxies and
clusters of galaxies to coalesce. With gravity applying the brakes,
the expansion of the cosmos should still be slowing down. So when
two teams of observers, the Supernova Cosmology Project and the
High-Z Supernovae Search team, began their work in the mid 1990s,
they were trying to measure the current rate of deceleration.
Using
Type Ia supernovae, which result from the thermonuclear explosions
of white dwarfs locked in binary systems (see "Searching
for Supernovae To Be," Jan/Feb 2003), the teams were able to
determine the recent expansion history of the universe. Type Ia
supernovae shine with near uniformity, so by comparing their apparent
brightness with their intrinsic brightness, the teams can infer
a supernova’s distance. A supernova’s spectrum reveals
how its light has been stretched, or redshifted, by the expansion
of space itself. When the two teams completed their analyses, they
were confronted with the astonishing result that the expansion of
the universe was not slowing down as expected — it was speeding
up.
And
so cosmology’s adventures in Wonderland began. What could
be causing space to expand faster and faster when it should be slowing
down? Most cosmologists think there is some mysterious energy lurking
in the shadows, some devious and furtive "antigravity"
force called dark energy. Although cosmologists were at first loath
to admit the existence of dark energy, it seems to be the most natural
solution to the puzzle.
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