The weak-gravity bound and the need for spin in asymptotically safe matter-gravity models

TL;DR

This paper uncovers a weak-gravity bound in scalar-gravity systems within the asymptotic safety framework, showing that including spinning matter like fermions and vectors is necessary for consistent fixed points.

Contribution

It demonstrates that a weak-gravity bound exists in scalar-gravity models and highlights the importance of spinning matter to achieve asymptotic safety.

Findings

Weak-gravity bound restricts gravitational strength in scalar-gravity systems.

Including fermions and vectors allows fixed points below the critical gravitational strength.

Beyond the bound, complex fixed points emerge, hindering asymptotic safety.

Abstract

We discover a weak-gravity bound in scalar-gravity systems in the asymptotic-safety paradigm. The weak-gravity bound arises in these systems under the approximations we make, when gravitational fluctuations exceed a critical strength. Beyond this critical strength, gravitational fluctuations can generate complex fixed-point values in higher-order scalar interactions. Asymptotic safety can thus only be realized at sufficiently weak gravitational interactions. We find that within truncations of the matter-gravity dynamics, the fixed point lies beyond the critical strength, unless spinning matter, i.e., fermions and vectors, is also included in the model.