Isocurvature perturbations and tensor mode in light of Planck and BICEP2
  We investigate the degeneracy of the isocurvature perturbations and the primordial gravitational waves, by using recent observations of the cosmic microwave background (CMB) reported by Planck and BICEP2 collaborations. We show that the tension in the bound on the tensor-to-scalar ratio $r$ between Planck and BICEP2 can be resolved by introducing the anti-correlated isocurvature perturbations. Quantitatively, we find that with the anti-correlated isocurvature perturbations the constraints on $r$ from Planck alone and BICEP2 results can be consistent at 68 % C.L. 
Masahiro Kawasaki, Toyokazu Sekiguchi, Tomo Takahashi and Shuichiro   Yokoyama
astro-ph.CO gr-qc hep-ph hep-th