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 ^ { -1 } , assuming n _ { t } = 0 , whereas the Planck limit was provided at k _ { * } = 0.002 Mpc ^ { -1 } , 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 ^ { -1 } 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 1.3 \sigma only .