Observable consequences of quantum gravity: Can light fermions exist?

TL;DR

This paper explores how the requirement for light fermions in our universe constrains quantum gravity theories, especially within the asymptotic safety framework, linking observable matter properties to fundamental gravity models.

Contribution

It demonstrates that the existence of light fermions imposes restrictions on UV completions of quantum gravity, with a focus on asymptotically safe gravity.

Findings

UV completions are restricted by the need for light fermions.

Asymptotically safe gravity favors universes with light fermions.

Effective theory analysis connects matter properties to quantum gravity constraints.

Abstract

Any theory of quantum gravity must ultimately be connected to observations. This demand is difficult to be met due to the high energies at which we expect the quantum nature of gravity to become manifest. Here we study, how viable quantum gravity proposals can be restricted by investigating the interplay of gravitational and matter degrees of freedom. Specifically we demand that a valid quantum theory of gravity must allow for the existence of light (compared to the Planck scale) fermions, since we observe these in our universe. Within the effective theory framework, we can thus show that UV completions for gravity are restricted, regardless of the details of the microscopic theory. Specialising to asymptotically safe quantum gravity, we find indications that universes with light fermions are favoured within this UV completion for gravity.