PROBING PRIMORDIAL MAGNETIC FIELDS USING Lyα CLOUDS

From previous studies of the effect of primordial magnetic fields on early structure formation, we know that the presence of primordial magnetic fields during early structure formation could induce more perturbations at small scales (at present 1-10 h(-1) Mpc) as compared to the usual Lambda CDM theory. Matter power spectra over these scales are effectively probed by cosmological observables such as shear correlation and Ly alpha clouds. In this paper we discuss the implications of primordial magnetic fields on the distribution of Ly alpha clouds. We simulate the line-of-sight density fluctuation including the contribution coming from the primordial magnetic fields. We compute the evolution of Ly alpha opacity for this case and compare our theoretical estimates of Ly alpha opacity with the existing data to constrain the parameters of the primordial magnetic fields. We also discuss the case when the two density fields are correlated. Our analysis yields an upper bound of roughly 0.3-0.6 nG on the magnetic field strength for a range of nearly scale-invariant models, corresponding to a magnetic field power spectrum index n similar or equal to -3.