On the prior dependence of constraints on the tensor-to-scalar ratio

Understand

We investigate the prior dependence of constraints on cosmic tensor perturbations.

• Commonly imposed is the strong prior of the single-field inflationary consistency equation, relating the tensor spectral index nT to the tensor-to-scalar ratio r.
• Dropping it leads to significantly different constraints on nT, with both positive and negative values allowed with comparable likelihood, and substantially increases the upper limit on r on scales k = 0.01 Mpc^-1 to 0.05 Mpc^-1, by a factor of ten or more.
• Even if the consistency equation is adopted, a uniform prior on r on one scale does not correspond to a uniform one on another; constraints therefore depend on the pivot scale chosen.