Primordial gravitational waves from excited states
  We show that a scalar excited state with large occupation numbers during
inflation leads to an enhancement of tensor modes and a characteristic pattern
of order-one oscillations in the associated stochastic gravitational wave
background (SGWB) sourced during inflation. An effective excited state, i.e. a
departure from the Bunch-Davies vacuum, can emerge dynamically as the result of
a transient non-adiabatic evolution, e.g. a sharp feature along the
inflationary history. We provide an explicit example in a multifield context
where the sharp feature triggering the excited state is identified with a
strong turn in the inflationary trajectory. En passant, we derive a universal
expression for the tensor power spectrum sourced at second order by an
arbitrary number of scalar degrees of freedom during inflation, crucially
taking into account the nontrivial structure of the Hilbert space in multifield
setups. The SGWB sourced during inflation can overcome the standard
scalar-induced SGWB sourced at horizon re-entry of the fluctuations after
inflation, while being less constrained by perturbativity and backreaction
bounds. In addition, one may entertain the possibility of detecting both since
they peak at different frequencies exhibiting oscillations with distinct
periods.
