Indistinguishable Macroscopic Behaviour of Palatini Gravities and General Relativity

TL;DR

This paper demonstrates that Palatini modified gravity theories are practically indistinguishable from General Relativity on large scales, but they predict small deviations in particle physics, which are tightly constrained by experiments.

Contribution

It shows that Palatini models do not differ from General Relativity in cosmological and astrophysical predictions, but they do imply testable modifications at the particle physics level.

Findings

Palatini gravity and General Relativity are indistinguishable macroscopically.

No violation of the Weak Equivalence Principle in Palatini theories.

Experimental constraints strongly limit Palatini gravity modifications.

Abstract

We show that, within some modified gravity theories, such as the Palatini models, the non-linear nature of the field equations implies that the usual naive averaging procedure (replacing the microscopic energy-momentum by its cosmological average) could be invalid. As a consequence, the relative motion of particles in Palatini theories is actually indistinguishable from that predicted by General Relativity. Moreover, there is no WEP violation. Our new and most important result is that the cosmology and astrophysics, or put more generally, the behaviours on macroscopic scales, predicted by these two theories are the same, and as a result the naturalness problems associated with the cosmological constant are not alleviated. Palatini gravity does however predict alterations to the internal structure of particles and the particle physics laws, e.g., corrections to the hydrogen energy levels. Measurements of which place strong constraints on the properties of viable Palatini gravities.