Nonlinear dynamics of axion inflation: A detailed lattice study

We study in detail the fully inhomogeneous nonlinear dynamics of axion inflation, identifying three regimes: weak, mild, and strong backreaction, depending on the duration of inflation. We use lattice techniques that explicitly preserve gauge invariance and shift symmetry, and which we validate against other computational methods of the linear dynamics and of the homogeneous backreaction regime. Notably, we demonstrate that the latter fails to accurately describe the truly local dynamics of strong backreaction. We investigate the convergence of simulations of local backreaction, determining the requirements to achieve an accurate description of the dynamics, and providing useful parametrizations of the delay of the end of inflation. Additionally, we identify key features emerging from a proper local treatment of strong backreaction: the dominance of magnetic energy against the electric counterpart, the excitation of the longitudinal mode, and the generation of a scale-dependent chiral (im)balance. Our results underscore the necessity to accurately capture the local nature of the nonlinear dynamics of the system, in order to correctly assess phenomenological predictions, such as, e.g., the production of gravitational waves and primordial black holes.