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

TL;DR

This paper examines how prior assumptions influence constraints on cosmic tensor perturbations, revealing significant dependence on the chosen priors and scales, which affects the limits on the tensor-to-scalar ratio and tensor spectral index.

Contribution

It analyzes the impact of prior choices on constraints of tensor perturbations, highlighting the importance of prior assumptions and scale selection in cosmological parameter estimation.

Findings

Dropping the inflationary consistency relation broadens the allowed range of nT.

The upper limit on r increases significantly when the consistency relation is not assumed.

Constraints on tensor parameters depend strongly on the pivot scale chosen.

Abstract

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. We assess the size of this effect and determine the optimal scale for constraining the tensor amplitude, both with and without the consistency relation.