Primordial non-Gaussianity of gravitational waves in Hořava-Lifshitz gravity

In this paper, we study the 3-point correlation function of primordial gravitational waves generated in the de Sitter background in the framework of the general covariant Hořava-Lifshitz gravity with an arbitrary coupling constant λ. We find that, at cubic order, the interaction Hamiltonian receives contributions from four terms built of the three-dimensional Ricci tensor R_ij of the leaves t=constant. In particular, the 3D Ricci scalar R yields the same k dependence as that in general relativity, but with different magnitude due to coupling with the U(1) field A and a UV history. Interestingly, the two terms R_ijRij and (^xiRjk) (x _iR_jk) exhibit peaks at the squeezed limit. We show that this is due to the effects of the polarization tensors. The signal generated by the fourth term, RjiRkjRik, favors the equilateral shape when spins of the three tensor fields are the same, but peaks in between the equilateral and squeezed limits when spins are mixed. The consistency with the recently released Planck observations on non-Gaussianity is also discussed and it is found that (H/M _*)2(H/M_pl)≤10^-8, where M _* denotes the suppression energy of high-order operators, M_pl the Planck mass, and H the energy of inflation.