The Effective Planck Mass and the Scale of Inflation

TL;DR

This paper explores how the effective strength of gravity during inflation affects the inference of the inflationary energy scale, highlighting uncertainties due to unknown particles and their impact on cosmological measurements.

Contribution

It analyzes the complications in determining the inflation energy scale caused by the variable effective Planck mass influenced by unknown particles.

Findings

The effective Planck mass during inflation may differ from the current measured value.

Unknown particles between laboratory and inflation scales introduce uncertainties.

Implications for cosmological observables depend on the spectrum of these particles.

Abstract

Observable quantities in cosmology are dimensionless, and therefore independent of the units in which they are measured. This is true of all physical quantities associated with the primordial perturbations that source cosmic microwave background anisotropies such as their amplitude and spectral properties. However, if one were to try and infer an absolute energy scale for inflation-- a priori, one of the more immediate corollaries of detecting primordial tensor modes-- one necessarily makes reference to a particular choice of units, the natural choice for which is Planck units. In this note, we discuss various aspects of how inferring the energy scale of inflation is complicated by the fact that the effective strength of gravity as seen by inflationary quanta necessarily differs from that seen by gravitational experiments at presently accessible scales. The uncertainty in the former relative to the latter has to do with the unknown spectrum of universally coupled particles between laboratory scales and the putative scale of inflation. These intermediate particles could be in hidden as well as visible sectors or could also be associated with Kaluza-Klein resonances associated with a compactification scale below the scale of inflation. We discuss various implications for cosmological observables.