No evidence for the blue-tilted power spectrum of relic gravitational
  waves
  In this paper, we constrain the tilt of the power spectrum of relic
gravitational waves by combining the data from BICEP2/Keck array and Planck
(BKP) and the Laser Interferometer Gravitational-Waves Observatory (LIGO). From
the data of BKP B-modes, the constraint on the tensor tilt is
$n_t=0.66^{+1.83}_{-1.44}$ at the $68%$ confidence level. By further adding the
LIGO upper limit on the energy density of gravitational waves, the constraint
becomes $n_t=-0.76^{+1.37}_{-0.52}$ at the $68%$ confidence level. We conclude
that there is no evidence for a blue-tilted power spectrum of relic
gravitational waves and either sign of the index of tensor power spectrum is
compatible with the data.
