The Evolution of Primordial Magnetic Fields due to Magnetohydrodynamic Turbulence, and their Cosmological Applications*

We study the generation of primordial magnetic fields (PMFs), with an emphasis on the observable signatures. We look at the following:

(i) PMFs generated during inflation which have a scale-invariant spectrum. We see that this spectrum changes temporally at small scales and becomes a steeper weak-turbulence spectrum at progressively larger scales. We show numerically, this happens at the turbulent-diffusive scale which increases with the square root of time. We also show that the infrared cutoff scale of perturbations produced during inflation determines the observables signatures of these magnetic fields on the CMB.

(ii) PMFs generated during the Electroweak Phase Transition (EWPT). We study the evolution of these fields from the time of generation till the epoch of recombination by magnetohydrodynamic (MHD) simulations. Various initial conditions, i.e., magnetically dominant or subdominant, different helicities, spectral shapes, etc., are studied and compared with bounds from Big Bang Nucleosynthesis (BBN) and TeV Blazar observations.

The observable signatures of PMFs on primordial gravitational waves (PGWs) is also being studied.