Nonperturbative production of massless scalars during inflation and generation of gravitational waves

Abstract

We discuss the possibility of a feature in the spectrum of inflationary gravitational waves sourced by a scalar field χ whose vacuum fluctuations are amplified by a rapidly time dependent mass. Unlike previous work which has focused on the case in which the mass of the field χ vanishes only for an instant before becoming massive again, we study a system where the scalar field becomes and remains massless through the end of inflation. After applying appropriate constraints to our parameters, we find, for future CMB experiments, a small contribution to the tensor-to-scalar ratio which can be at most of the order r ∼ 10−5. At smaller scales probed by gravitational interferometers, on the other hand, the energy density in the gravitational waves produced this way might be above the projected sensitivity of LISA, ΩGW h2 ∼ 10−13, in a narrow region of parameter space. If there is more than one χ species, then these amplitudes are enhanced by a factor equal to the number of those species.