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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 evolve the cosmos must solve the mathematical equations that describe the following physical processes:

Gravity

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 particle is 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

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.

Radiative Transfer

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

Atomic physics governs the ionization of primordial gas atoms by radiation or heating, as well as the reverse process, linked to cooling: recombination with electrons to yield neutral atomic gas. Acting together, radiative transfer and atomic physics control the microscopic behavior of the baryonic gas.

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 starbirth, as 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 accurate simulations.

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