On Synthetic Gravitational Waves from Multi-field Inflation

Abstract: We revisit the possibility of producing observable tensor modes through a continuous particle production process during inflation. Particularly, we focus on the multi-field realization of inflation where a spectator pseudo-scalar $\sigma$ induces a significant amplification of the ${\rm U}(1)$ gauge fields through the coupling $\propto \sigma F_{\mu\nu}\tilde{F}^{\mu\nu}$. In this model, both the scalar $\sigma$ and the Abelian gauge fields are gravitationally coupled to the inflaton sector, therefore they can only affect the primordial scalar and tensor fluctuations through their mixing with gravitational fluctuations. Recent studies on this scenario show that the sourced contributions to the scalar correlators can be dangerously large to invalidate a large tensor power spectrum through the particle production mechanism. In this paper, we re-examine these recent claims by explicitly calculating the dominant contribution to the scalar power and bispectrum. Particularly, we show that once the current limits from CMB data are taken into account, it is still possible to generate a signal as large as $r \approx 10^{-3}$ and the limitations on the model building are more relaxed than what was considered before.