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Adding the Right Physics
Aside from picking a recipe, cosmological models must also add in
the physics that determines how the chief components--dark matter, luminous
matter and radiation --interact over multiple distance scales. Codes to
the cosmos must solve the mathematical equations that describe the following
- Gravity is at work at all cosmic scales, from solar systems to
galaxies to superclusters. Although the basic equations of motion describing the
motion of two objects traveling along elliptical orbits about their center of mass
(Newton's classical two-body problem) are easily solved, the solution for
many bodies is much more difficult, requiring numerical methods.
Dark Matter (Collisionless) Dynamics
- Dark matter can be represented as collisionless particles. The
gravitational interactions of "mass clouds" of these particles are calculated
as they move in response to each other's gravity. The force on any
computed by summing up the pairwise force from all the other particles. The
resulting changes in the density distribution in turn alters the
movements of the clouds.
- Gas dynamics, also known as "hydrodynamics", is a set of
well-established physical laws that govern the gross behavior of baryonic,
primordial gas, particularly its
flow or motion in response to pressure, gravity, heating or cooling. The same
basic laws control the flow of air under and over an airplane's wing, or the
rise of hot air that send a balloon aloft.
- The universe is bathed in diffuse, electromagnetic
radiation left over from earlier epochs, and
from astronomical sources such as quasars and primeval galaxies. There is
X-ray, extreme ultraviolet and visible radiation, now shifted by cosmic
expansion to the microwave region of the spectrum. The higher energy
radiation interacts with electrically neutral gas atoms, stripping away their electrons
or "ionizing" them, resulting in an electrically charged plasma.
- Atomic physics governs the ionization of primordial gas atoms by
heating, as well as the reverse process, linked to cooling: recombination
electrons to yield neutral atomic gas. Acting together, radiative
atomic physics control the microscopic behavior of the baryonic
Galaxy and star formation
- When it comes to simulating the birth and evolution of individual
galaxies and stars, all the above physics must be modeled separately at
suitably small scales according to various prescriptions. These
prescriptions take account of the specifics of galaxy formation and
well as stellar evolution, death, and the recycling of matter and energy.
Because star formation is still poorly understood and cannot be resolved
within multiple-scale cosmology simulations, cosmologists use various
prescriptions that are physically motivated but nonetheless ad hoc. Star
birth occurs on galactic scales when galaxies first form. Large amounts of energy
are released when billions of stars within a given galaxy begin burning their nuclear fuel.
This energy in turn has a substantial feedback effect on the gas out of which the galaxy
condensed in the first place. Cosmologists believes this caused powerful galactic winds
and intergalactic shock waves, both of which must also be incorporated into the most
Return to Cooking the Cosmos
to Cosmic Code Building
Copyright, (c) 1995: Board of Trustees, University of
NCSA. Last modified 10/6/95.