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## Cosmic Code Building

Cosmologists today are conjuring "digital recipes" for mimicking the
universe's structure and history. By running these "recipes" on powerful
supercomputers, they can try out alternative models and compare the results
against what the astronomers can actually observe. No longer is cosmology
closeted from the real universe.

**Jeremiah Ostriker, Princeton University,
on-camera**

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But for a cosmological model to be scientifically meaningful--that is,
to make useful predictions that can be tested against observation--it must
satisfy the following requirements:

- The model chosen is assumed to evolve within a
homogeneous, smoothspacetime. As described by Einstein's Theory of General Relativity the shape and fate of this spacetime is
intimately bound up with the total amount of matter it contains.

- A computer model must yield elaborate,
**hierarchical
structures** whose
emergence in time and distribution in space can be clearly tracked.

- There must be a specific set of initial conditions from
which a model is to
evolve in the computer. These conditions set the range of variations in the
density fluctuations out of which structures can later emerge.

- The make-up of matter, as well as the underlying physics
governing its behavior, must be defined. Computer models include varying combinations of
baryonic matter (the ordinary stuff of the universe), hot and/or cold dark
matter as well as the radiation permeating the universe. These constituents
comprise the numerical recipe embodied in a code.

Modern cosmology rests upon a coherent theoretical foundation, namely the
Big Bang Theory coupled to
inflation. Within this framework, several competing models of cosmic
evolution can be accommodated, each selecting a particular recipe as well as
values for certain critical quantities (for example, the rate at which the
universe is expanding, or the Hubble Constant) and the initial conditions.
Recent observations have provided some directions for cooking up the cosmic
recipe, but even so, deciding on which ingredients to use, and how much,
is no trivial task!
Each layer of complexity added to a recipe raises the computational
demands for
running it. As such, the development of cosmology codes constitutes a
computational grand
challenge.

What are the cosmological recipes and what physics
should be included?

How can the resulting equations be solved across multiple
distance scales, and how can cosmologists visualize the results?

What types of computers are best suited for the job?

Return to What Does It Take to Put the Universe in a Box?

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#### Copyright, (c) 1995: Board of Trustees, University of
Illinois

*NCSA. Last modified 10/6/95.*