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/epsilon(2). 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 similar to epsilon(2). A more quantitative result can be obtained if one further assumes that gravitational waves are emitted by localized subhorizon processes, giving r(max) similar or equal to 0.3 epsilon(2). However, epsilon 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 n(s). 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. f(NL) >= 1, independently of the value of r. Possible exceptions in multifield scenarios are discussed.