Cosmic microwave background polarization, Faraday rotation, and stochastic gravitational-wave backgrounds

A magnetic field, coherent over the horizon size at the decoupling and strong enough to rotate the polarization plane of the cosmic microwave background radiation, can be generated from the electromagnetic vacuum fluctuations amplified by the space-time evolution of the dilaton coupling. The possible relevance of this result for superstring-inspired cosmological models is discussed. Particular attention will be paid to the connection between Faraday rotation signals and stochastic gravitational-wave backgrounds.