CFTs Blueshift Tensor Fluctuations Universally

TL;DR

Conformal sectors in cosmology universally blueshift tensor fluctuations, potentially detectable via CMB and gravitational wave experiments, providing constraints on new physics beyond the Standard Model.

Contribution

This work calculates the universal effect of conformal sectors on tensor fluctuations, linking conformal symmetry constraints to observable gravitational wave signatures.

Findings

Limits from Stage-IV CMB experiments can bound the size of conformal sectors.

A conformal sector larger than 10^15 is detectable and constrains N-Naturalness.

The effect is a universal consequence of conformal symmetry in hidden sectors.

Abstract

The strong constraints of conformal symmetry cause any nearly-conformal sector to blueshift tensor fluctuations in cosmology. Hidden sectors with approximate conformal symmetry, which may be quite large, are a well-motivated extension of physics beyond the Standard Models of particle physics and cosmology. They can therefore lead to a detectable shift in the tensor tilt for next-generation CMB and gravitational wave experiments. We compute the leading-order contribution to the in-in graviton two-point function from virtual loops in such sectors to demonstrate this universal effect. In units where a single conformally-coupled scalar is 1, limits from Stage-IV CMB experiments could bound the size of this extra sector to be smaller than about 10^15. This would be sufficient to rule out N-Naturalness as a complete resolution of the hierarchy problem.