Welcome
to the Dark Side: Delighted to See You
Linda Rowan and Robert Coontz
Dark stars, the dark age, dark
matter, and dark
energy are the major components of the dark side of the universe: 96%
of
the universe consists of mass and energy we can't see and don't really
understand. Fortunately, the badly outnumbered 4% of luminous matter
feels
the dark side through gravity and other forces. Stellar struggles with
the dark side, which we can see through gravity and electromagnetic
emissions,
have much to tell us about the bulk of the universe. Here five intrepid
astronomers and two News writers review what we know or think we know
about
these epic battles throughout cosmic time.
Perhaps the best-understood component of
the dark side
is the dark stars called black holes. Begelman (p. 1898)
explains that black holes are common. By studying black holes in the
center
of the Milky Way and other galaxies, astronomers have discovered that
their
masses are correlated with certain types and masses of galaxies,
suggesting
that either the black hole knows about the structure of the stars in
the
galaxy or the stars know about the black hole through other indirect
forces.
The answer to this "who-saw-who-first" question may hold the key to
explaining
how black holes and galaxies form.
Long ago, the universe was dark and
there were no stars.
Miralda-Escudé (p. 1904)
reviews what we know about this dark age. He concentrates on the hints
of light at either end of the dark age: the cosmic microwave background
radiation that dispersed right after the big bang, at a redshift of
1100,
and the first stars that formed about 75 million years later, at a
redshift
of about 38. Although we have not seen the first population of stars,
we
can observe stars as far back as a redshift of about 6. During the dark
age, dark matter clumped together, creating density fluctuations that
could
collapse and form stars.
The first stars formed from this dark
matter, which provided
the blueprint, the DNA, for cosmic structure and its rate of evolution.
Ostriker and Steinhardt (p. 1909) discuss the possible types of dark
matter
(it is not a double helix) that may now account for about 26% of the
universe.
The "cold dark matter" model says that dark matter is made up of cold
particles
such as neutralinos or other weakly interacting massive particles
(WIMPs).
More sinister-sounding varieties, such as self-annihilating or
repulsive
dark matter, may also exist and battle against WIMPs to determine the
fate
of ordinary matter. Irion's News article (p. 1894)
describes how researchers plan to map the distribution of dark matter
throughout
the universe by analyzing its subtle effects on the light from distant
galaxies.
Nowadays, about 70% of the universe is
dominated by dark
energy, which is the dark-side component we understand the least.
Evidence
for dark energy comes from hundreds of type Ia supernovae, detected as
far back as a redshift of 1.8. As Kirshner explains (p. 1914),
supernovae show that the expansion of the universe has been
accelerating
over the past 7 billion years, and the acceleration is caused by dark
energy.
By extending observations of supernovae further back in time, we should
be able to see when the universe shifted gears from deceleration caused
by clumpy, gravitationally attractive dark matter to acceleration
caused
by less clumpy, gravitationally repulsive dark energy. As Seife reports
(p. 1896),
cosmologists hope that this cosmic tipping point, along with a better
understanding
of the physical properties of dark energy, will provide beachheads for
future forays into this murkiest province of dark-side science.
Science Online sheds further
light on darkness
with links to missions, experiments, and papers bearing on various
topics
in cosmology (www.sciencemag.org/feature/data/darkside/).
May the dark energy be with you as we
struggle to understand
the darkness of space and time.
Related articles in Science:
- 01-The Warped
Side of Dark Matter
- Robert Irion
Science 2003 300: 1894-1896. (in
News) -
02-Dark Energy
Tiptoes Toward the Spotlight
- Charles Seife
Science 2003 300: 1896-1897. (in
News) -
03-Evidence
for Black Holes
- Mitchell C. Begelman
Science 2003 300: 1898-1903. (in
Review) -
04-The Dark Age
of the Universe
- Jordi Miralda-Escudé
Science 2003 300: 1904-1909. (in
Review) -
05-New Light
on Dark Matter
- Jeremiah P. Ostriker and Paul
Steinhardt
Science 2003 300: 1909-1913. (in
Review) -
06-Throwing
Light on Dark Energy
- Robert P. Kirshner
Science 2003 300: 1914-1918. (in
Review)
Volume
300, Number 5627, Issue of 20 Jun 2003, p. 1893.
Copyright
© 2003 by The American Association for the Advancement of Science.
All rights reserved.
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