PUB - Publikationen an der Universität Bielefeld

We propose a novel scenario to generate primordial magnetic fields duringinflation induced by an oscillating coupling of the electromagnetic field tothe inflaton. This resonant mechanism has two key advantages over previousproposals. First of all, it generates a narrow band of magnetic fields at anyrequired wavelength, thereby allaying the usual problem of a strongly bluespectrum and its associated backreaction. Secondly, it avoids the need for astrong coupling as the coupling is oscillating rather than growing or decayingexponentially. Despite these major advantages, we find that the backreaction isstill far too large during inflation if the generated magnetic fields arerequired to have a strength of ${\cal O}(10^{-15}\, \Gauss)$ today onobservationally interesting scales. We provide a more general no-go argument,proving that this problem will apply to any model in which the magnetic fieldsare generated on subhorizon scales and freeze after horizon crossing.