Instability in axion inflation with strong backreaction from gauge modes

We perform an analytical study of the stability of the background solution [1] of the model in which an inflaton, through an axionic coupling to a U(1) gauge field, causes an amplification of the gauge field modes that strongly backreact on its dynamics. To this goal, we study the evolution of the gauge field modes coupled to the inflaton zero mode, treating perturbatively the deviation of the inflaton velocity from its mean-field value. As long as the system is in the strong backreaction regime we find that the inflaton velocity performs oscillations of increasing amplitude about the value it would have in the approximation of constant velocity, confirming an instability that has been observed in numerical studies.

Automated summary

We have performed an analytical study on the stability of the background solution of a model where an inflaton, with an axionic coupling to a U(1) gauge field, amplifies the gauge field modes that backreact on its dynamics. By perturbatively treating the deviation of the inflaton velocity from its mean-field value, we have found that as long as the system is in the strong backreaction regime, the inflaton velocity exhibits increasing amplitude oscillations around the value it would have in the approximation of constant velocity, confirming the instability observed in numerical studies.