Predictive Power of Strong Coupling in Theories with Large Distance Modified Gravity

TL;DR

This paper demonstrates that consistent large-distance modified gravity theories inherently exhibit strong coupling effects, making their deviations from Einsteinian gravity source-dependent and testable through both cosmological and local gravitational experiments.

Contribution

It shows that all ghost-free large-distance modified gravity theories must have strong coupling, leading to source-dependent modifications and providing a simple parametrization for testing.

Findings

Strong coupling is universal in ghost-free large-distance modified gravity.

Modification effects depend on source mass and distance, becoming prominent at shorter scales.

Theories are testable via cosmological and precision local gravitational measurements.

Abstract

We consider theories that modify gravity at cosmological distances, and show that any such theory must exhibit a strong coupling phenomenon, or else it is either inconsistent or is already ruled out by the solar system observations. We show that all the ghost-free theories that modify dynamics of spin-2 graviton on asymptotically flat backgrounds, automatically have this property. Due to the strong coupling effect, modification of the gravitational force is source-dependent, and for lighter sources sets in at shorter distances. This universal feature makes modified gravity theories predictive and potentially testable not only by cosmological observations, but also by precision gravitational measurements at scales much shorter than the current cosmological horizon. We give a simple parametrization of consistent large distance modified gravity theories and their predicted deviations from the Einsteinian metric near the gravitating sources.