The tilt of primordial gravitational waves spectra from BICEP2

Abstract

In this paper we constrain the tilt of the spectrum of primordial gravitational waves from Background Imaging of Cosmic Extragalactic Polarization (BICEP2) data only. We find r = 0.21_-0.10^+0.04 and n_t = -0.06_-0.23^+0.25 (at 68% C.L.) which implies that a scale-invariant primordial gravitational waves spectra is consistent with BICEP2 nicely. Our results provide evidence for supporting inflation model, and the alternative models, for example the ekpyrotic model which predicts n_t = 2, are ruled out at high significance level.