Gravitational waves and the scale of inflation

We revisit alternative mechanisms of gravitational wave production during inflation and argue that they generically emit a non-negligible amount of scalar fluctuations. We find the scalar power is larger than the tensor power by a factor of order 1/ε². For an appreciable tensor contribution, the associated scalar emission completely dominates the zero-point fluctuations of the inflaton, resulting in a tensor-to-scalar ratio r~ε². A more quantitative result can be obtained if one further assumes that gravitational waves are emitted by localized subhorizon processes, giving rₘₐₓ0.3ε². However, ε is generally time dependent, and this result for r depends on its instantaneous value during the production of the sources, rather than just its average value, somewhat relaxing constraints from the tilt nₛ. We calculate the scalar 3-point correlation function in the same class of models and show that non-Gaussianity cannot be made arbitrarily small, i.e. fNL1, independently of the value of r. Possible exceptions in multifield scenarios are discussed.