The Effective Strength of Gravity, the Scale of Inflation (and how KK gravitons evade the Higuchi Bound)

TL;DR

This paper discusses how the effective strength of gravity and the inflation scale can differ from the Planck scale due to hidden species, affecting the interpretation of primordial tensor measurements.

Contribution

It clarifies the distinction between different gravitational scales and explains how hidden species influence inflationary scale inference.

Findings

Effective gravitational interactions are characterized by a scale $M_*$ different from $M_{\rm pl}$.

Hidden species can alter the inferred scale of inflation without affecting observable CMB quantities.

The paper addresses and clarifies recent concerns about these effects in the context of inflationary physics.

Abstract

For any given momentum transfer, gravitational interactions have a strength set by a characteristic scale $M_*$ inferred from amplitudes calculated in an effective theory with a strong coupling scale $M_{**}$. These are in general different from each other and $M_{\rm pl}$, the macroscopic strength of gravity as determined by (laboratory scale) Cavendish experiments. During single field inflation, $M_*$ can differ from $M_{\rm pl}$ due to the presence of any number of (hidden) universally coupled species between laboratory and inflationary scales. Although this has no effect on dimensionless (i.e. observable) quantities measured at a fixed scale such as the amplitude and spectral properties of the CMB anisotropies, it complicates the inference of an absolute scale of inflation given any detection of primordial tensors. In this note we review and elaborate upon these facts and address concerns raised in a recent paper.