The Primordial Magnetic Field (PMF) Generated in Large Field Inflation   (LFI), Natural Inflation (NI) and $R^2$-Inflation by $f^{2}FF$ Model
  Large scale magnetic fields seem to be present in almost all astrophysical systems and scales from planets to superclusters of galaxies and in very low density intergalactic media. The upper limit of primordial magnetic fields (PMF) has been set by recent observations by the Planck observatory (2015) to be of the order of a few nG. The simple model ${f^2}FF$ used to generate the PMF during the inflation era. It is based on the breaking of conformal symmetry of electromagnetism during inflation. It is attractive because it is stable under perturbations and leads to a scale invariant PMF. However, it may suffer from two problems: Backreaction and strong coupling. In the first case, the electromagnetic energy may exceed the energy of inflation, ${\rho _{{\rm{Inf}}}}$. In the second case, the effective electric charges become excessively large if we want to retrieve the standard electromagnetism at the end of inflation. In this research, we investigate the generation of PMF under three different models of inflation in order to avoid the backreaction problem. We compute magnetic and electric spectra generated by the ${f^2}FF$ model in the context of large field inflation (LFI), natural inflation (NI) and ${R^2}$-inflation, for all possible values of model parameters for de Sitter and power law expansion of inflation. The results of the research show that the scale invariant PMF can be generated in these models and the problem of backreaction may be avoided in some observational ranges. In $R^2$-inflation, which is preferred by the recent results of Planck 2015, we calculate the upper of the scale invariant PMF generated by ${f^2}FF$ and in turns we find that the upper limit of present magnetic field, ${B_0} < 8.058 \times {10^{ - 9}}{\rm{G}}$. It is in the same order of magnitude of PMF, reported by Planck, 2015. 
Anwar Saleh AlMuhammad
astro-ph.CO gr-qc