Supercomputing the Cosmos

The set of equations describing how a filamentary, electrically conducting, magnetized plasma evolves is a mathematician's nightmare. Because of this complexity, effective solution had to wait for the advent of supercomputers. Thanks to the remarkable similarities of plasmas at all size scales, what is leaned in modeling nuclear weapons has great relevance to modeling galaxies.

Plasma theorists often use a method called particle simulation. Some hundreds of millions of particles can be used to represent, say, a galaxy. But since a system similar to the Milky Way may contain 1065 free electrons and ions, each particle in the simulation actually represents a cloud of real ones. These "superparticles" are assumed to be in a magnetic field similar to that between the planets in the solar system but extended over much greater distances. The computer then calculates how the particles move according to the laws of electromagnetism (and gravity).

The simplest simulation traces the interactions of two plasma current filaments made up of fast-moving electrons. (Because of their greater mass, positively charged ions move more slowly but serve to set up a background that causes the electrical stream to undergo many types of fluid-like instabilities).

Because electrons spiral around magnetic-field lines, each filament has a circular current component. Furthermore, the movements and interactions of electrons cause them to give off energy in the form of synchrotron radiation in amounts that are calculated to match quite closely those of such strong extragalactic radio sources as Cygnus A. (High-energy electrons interacting with magnetic fields characteristically give off radiation call synchrotron radiation).

The calculations provide information on the power levels and shapes of sources as well as their polarization. All these properties can be compared with results from radio telescopes.

Computer simulations by the author suggest that double radio galaxies evolve from filamentary plasma, announcing their birth through a double-beam pattern or radiation that persists for up to 10 million years. In the simulations, the radiation patterns grow more complex as they fade, but the plasma filaments do not disappear. Instead, suggest the simulations, the double radio galaxies and quasars might change first into "peculiar" and Seyfert galaxies, then into normal and barred spirals.

Calculations are now good enough that we can compare their detailed predictions with observations of how a galaxy's rotation velocity varies with distance from its center. Additionally, fine-detail magnetic-field maps produced by simulations can be closely compared with those traced by radio telescopes.

—A. P. and G. C. S.


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