  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.
