Magnetohydrodynamic Effects of a First-Order Cosmological Phase Transition

The author analyzes the spectrum, amplitude, and evolution of a tangled primeval magnetic field generated by macroscopic dynamo activity arising during a first-order phase transition in the early universe. The effects of the field on the matter distribution at late times could be much larger than that of the adiabatic or isothermal density fluctuations generated by the transition. With optimistic assumptions, magnetic fields generated during a transition at $T\sim 0.2$ GeV could lead to the collapse of gas clouds of $\simeq {10}^{1-5}{M}_{\odot }$ after recombination, and to the formation of pregalactic stars or quasars.