Abstract

Magnetic fields are ubiquitous in the Universe: they have been observed in galaxies, in clusters of galaxies and also in the inter-cluster medium. However, the origin of these fields is still unclear. One possibility to explain the initial seed magnetic fields is that they originated in the primordial universe. A stochastic background of primordial magnetic fields generated before recombination can leave imprints on the temperature anisotropy of the cosmic microwave background. In this talk I will present an analytical derivation of the CMB temperature anisotropy induced at large scales from a primordial magnetic field. I will discuss the contradictory results presented in the literature regarding this effect. I will show that the leading order term in the CMB spectrum is strongly suppressed due to the presence of free-streaming neutrinos and that consequently the magnetic Sachs Wolfe spectrum from a causal magnetic field behaves as l(l+1)Cl~l^2.