MAGNETOHYDRODYNAMICS IN THE EARLY UNIVERSE AND THE DAMPING OF NONLINEAR ALFVEN WAVES - ART. NO. 083502

The evolution and viscous damping of cosmic magnetic fields in the ear ly universe is analyzed. Using the fact that the fluid, electromagneti c, and shear viscous energy-momentum tensors are all conformally invar iant, the evolution is transformed from the expanding universe setting into that in hat spacetime. Particular attention is paid to the evolu tion of nonlinear Alfven modes. For a small enough magnetic field, whi ch satisfies our observational constraints, these wave modes either os cillate negligibly or: when they do oscillate, become overdamped. Henc e they do not suffer Silk damping on galactic and subgalactic scales. The smallest scale which survives damping depends on the field strengt h and is of the order of a dimensionless Alfven velocity times the usu al baryon-photon Silk damping scale. After recombination, nonlinear ef fects can convert the Alfven-type mode into compressional, gravitation ally unstable waves and seed cosmic structures if the cosmic magnetic field is sufficiently strong. [S0556-2821(98)02718-0].