Introductory Remarks

Our aim in the talks given at the XXV Canary Islands Winter School of Astrophysics in 2013 was to present models for the generation of primordial magnetic fields in the early Universe and the imprints that these leave in cosmological observables, with an emphasis on Cosmic Microwave Background (CMB) anisotropies.

Introduction

The origin of the large-scale magnetic fields (LSMF) observed in galaxies, clusters of galaxies, with also hints of their presence in cosmic voids and on even larger scales in filaments, is an open issue of great importance in modern astrophysics (seeWidrow, 2002, for reviews). Both large-scale and stochastic components are present in magnetic fields observed in galaxies (usually dependent on the morphology of the host) with magnitudes from a few to several microGauss (μG). In clusters of galaxies, stochastic magnetic fields from a few to several μG strength are present with a correlation scale of the order of magnitude of ten kiloparsec (Clarke et al., 2001; Enslin & Vogt, 2006). More recently, the presence of intergalactic magnetic fields even in cosmic voids of the large-scale structure was proposed as a possible explanation for the gamma-ray observations of a couple of blazars (Neronov & Vovk, 2010; Taylor et al., 2011; Vovk et al., 2012; Tavecchio et al., 2010, 2011; Dolag et al., 2011). As a possible explanation for the lack of TeV photons observed in the high-energy spectrum from these blazars a lower bound of 10−18−10−15 G was derived for such pervasive intergalactic magnetic fields.

In light of this multitude of observations of LSMF of different magnitudes and different coherence lengths, a primordial hypothesis for generating the seed magnetic fields, which are amplified afterwards by adiabatic compression and dynamo during structure formation, is a viable possibility (Widrow, 2002), also taking in consideration the recent observations of strong magnetic fields in galaxies at high redshift (Bernet et al., 2008; Wolfe et al., 2008). Recently, initial seeds motivated as primordial magnetic fields (PMF) have been used in N-body simulations to reproduce magnetic fields in clusters of galaxies (Govoni et al., 2013; Xu et al., 2012).