The apparent discrepancy between the value of the tensor-to-scalar ratio reported by the BICEP2 collaboration, r=0.20+0.07−0.05 at 68% CL, and the Planck upper limit, r<0.11 at 95% CL, has attracted a great deal of attention. In this short note, we show that this discrepancy is mainly due to an `apples to oranges' comparison. The result reported by BICEP2 was measured at a pivot scale k∗=0.05/Mpc, assuming n_t=0, whereas the Planck limit was provided at k_∗=0.002/Mpc, assuming the slow-roll consistency relation n_t=−r/8. One should obviously compare the BICEP2 and Planck results under the same circumstances. By imposing n_t=0, the Planck constraint at k_∗=0.05/Mpc becomes r<0.135 at 95 CL, which can be compared directly with the BICEP2 result. Once a plausible dust contribution to the BICEP2 signal is taken into account (DDM2 model), r is reduced to r=0.16+0.06−0.05 and the discrepancy becomes of order 1.3σ only.

## Address

Département de Physique Théorique

Université de Genève

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