The Tilt of Primordial Gravitational Waves Spectra from BICEP2
  In this paper we constrain the tilt of the spectra 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 strong evidence for supporting inflation
model, and the alternative models, for example the ekpyrotic model which
predicts $n_t=2$, are ruled out at more than $5\sigma$ significance.
