title: Resilience of the standard predictions for primordial tensor modes.
abstract: We show that the prediction for the primordial tensor power spectrum cannot be modified at leading order in derivatives. Indeed, one can always set to unity the speed of propagation of gravitational waves during inflation by a suitable disformal transformation of the metric, while a conformal one can make the Planck mass time independent. Therefore, the tensor amplitude unambiguously fixes the energy scale of inflation. Using the effective field theory of inflation, we check that predictions are independent of the choice of frame, as expected. The first corrections to the standard prediction come from two parity violating operators with three derivatives. Also the correlator ⟨γγγ⟩ is standard and only receives higher derivative corrections. These results hold also in multifield models of inflation and in alternatives to inflation and make the connection between a (quasi-)scale-invariant tensor spectrum and inflation completely robust.
authors.affiliation: Abdus Salam International Centre for Theoretical Physics Strada Costiera 11, 34151 Trieste, Italy.
authors.forename: Paolo
authors.initials: P
authors.lastname: Creminelli
authors.affiliation: CEA, Institut de Physique Théorique, 91191 Gif-sur-Yvette cédex, France and CNRS, Unité de recherche associée-2306, 91191 Gif-sur-Yvette cédex, France and Université Paris Sud, 15 rue George Clémenceau, 91405 Orsay, France.
authors.forename: Jérôme
authors.initials: J
authors.lastname: Gleyzes
authors.affiliation: Department of Theoretical Physics and Center for Astroparticle Physics (CAP), 24 quai E. Ansermet, 1211 Geneva 4, Switzerland.
authors.forename: Jorge
authors.initials: J
authors.lastname: Noreña
authors.affiliation: CEA, Institut de Physique Théorique, 91191 Gif-sur-Yvette cédex, France and CNRS, Unité de recherche associée-2306, 91191 Gif-sur-Yvette cédex, France.
authors.forename: Filippo
authors.initials: F
authors.lastname: Vernizzi