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Recipes for a Digital Cosmos


How much matter should be put into the recipe? What types of matter and in what 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. Within this 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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NCSA. Last modified 10/6/95.