Constraints on the primordial magnetic field from sigma(8)

A primordial magnetic field (PMF) can affect the evolution of density-field fluctuations in the early universe. In this paper we constrain the PMF amplitude B-lambda and power spectral index n(B) by comparing calculated density-field fluctuations with observational data, i.e. the number density fluctuation of galaxies. We show that the observational constraints on cosmological density fluctuations, as parametrized by sigma(8), lead to strong constraints on the amplitude and spectral index of the PMF.