On Synthetic Gravitational Waves from Multi-field Inflation
  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.
