Primordial magnetic field generated in natural inflation

Abstract

We study the simple gauge invariant model ${f^2}FF$ as a way to generate primordial magnetic fields (PMF) in natural inflation (NI). We compute both magnetic and electric spectra generated by the ${f^2}FF$ model in NI for different values of model parameters and find that both de Sitter and power law expansion lead to the same results at sufficiently large number of e-foldings. We also find that the necessary scale invariance property of the PMF cannot be obtained in NI in first order of slow roll limits under the constraint of inflationary potential, $V\left( 0 \right) \simeq 0$. Furthermore, if this constraint is relaxed to achieve scale invariance, then the model suffers from the backreaction problem for the co-moving wave number, $k \lesssim 8.0\times 10^{-7} \mathrm {Mpc^{-1}}$ and Hubble parameter, $H_i \gtrsim 1.25\times 10^{-3} \mathrm {M_\mathrm{{Pl}}}$. The former can be considered as a lower bound of k and the later as an upper bound of $H_i$ for a model which is free from the backreaction problem. Further, we show that there is a narrow range of the height of the potential $\Lambda$ around ${\Lambda _{\min }} \approx 0.00874{M_{\mathrm{{Pl}}}}$ and of k around ${k_{\min }} \sim 0.0173\mathrm{{Mp}}{\mathrm{{c}}^{ - 1}}$, at which the energy of the electric field can fall below the energy of the magnetic field. The range of k lies within some observable scales. However, the relatively short range of k presents a challenge to the viability of this model.