Production of Axions by Cosmic Magnetic Helicity

Abstract

We investigate the effects of an external magnetic helicity production on the evolution of the cosmic axion field. It is shown that a helicity larger than (few \times 10^{-15} G)^2 Mpc, if produced at temperatures above a few GeV, is in contradiction with the existence of the axion, since it would produce too much of an axion relic abundance.

4 pages, 2 figures, major changes in the first part of the paper, reference added, to be published in Phys. Rev. Lett