  We evaluate the quantum backreaction due to a gauge field coupled to a pseudo
scalar field driving a slow-roll inflationary stage, the so-called axion
inflation. The backreaction is evaluated for the first time using a gauge
invariant approach, going to second order in perturbation theory and taking in
consideration inflaton fluctuations as well as scalar perturbations of the
metric. Within our gauge invariant, but observer-dependent approach, we
naturally consider as physical observer the one comoving with the inflaton
field. Considering the effective expansion rate consequent to the gauge fields
bacreaction, we observe that the backreaction effect becomes significant quite
rapidly, moving the system out of the perturbative regime and into what is
often referred to the strong backreaction regime. This behavior also applies to
the parameter which dictates the production of the gauge fields. The space-time
dynamic is initially mainly influenced by the helicity contribution, with a
significant response to the energy density which occurs much later with respect
to the number of e-folds. As a final result, we see that the evaluated
backreaction prolongs the inflationary period much more compared to the
scenarios previously studied neglecting scalar metric perturbations.
