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Recipes for a Digital Cosmos
How much matter should be put into the recipe? What types of matter and
proportions? Most cosmological models assume that the total amount of all
kinds of matter corresponds to the closure
density, the density required to just balance the
outward expansion of the universe against the an eventual collapse.
limit, the proportions of the different types of matter can be varied. The
choice of mix critically affects the outcome.
First of all, there's luminous, baryonic
matter. This is the matter we're most familiar with. Made
chiefly of hydrogen and helium gas formed in the first few minutes after the
Big Bang, luminous baryonic matter interacts with and emits electromagnetic
radiation we can detect on Earth.
Then, there's dark matter. This
mysterious, invisible matter is believed make up more
than 90 percent of the mass of the universe and to dominate its gravitational
field. Although some of the dark matter is believed to be ordinary baryonic
matter that simply fails to emit radiation detectable on earth, most is
presumed to be in the form of extremely massive, slow-moving particles (cold
dark matter) or light-weight particles that move near the speed of light (hot
dark matter). But how much "cold" or "hot" dark matter should be
included? Or should both be incorporated and if so, in what ratios?
Until recombination--the era 300,000 years after the Big Bang in which
photons were freed from matter-- the
universe was enormously dense and hot. The radiation that was released at
that time is still around, detectable today as cosmic background radiation.
Along with this radiation are the distinctive ultraviolet radiation signatures left by
the very oldest (Population III) stars, primeval galaxies, and quasars, as well as the
X-ray radiation that bathes galaxies. This radiation field interacts with
baryonic matter and must be taken into account when developing more
comprehensive models of cosmic evolution.
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Copyright, (c) 1995: Board of Trustees, University of
NCSA. Last modified 10/6/95.