Imperfect fluid description of modified gravities

TL;DR

This paper presents a unified framework to describe scalar-tensor and f(R) modified gravity theories as imperfect fluids, revealing symmetries and extending the effective fluid approach to various spacetime geometries.

Contribution

It generalizes the imperfect fluid description to a broad class of modified gravities, including scalar-tensor and f(R) theories, and explores their symmetries in a unified manner.

Findings

Effective fluid description recovers known results for scalar-tensor gravity.

Describes the imperfect fluid equivalent of f(R) gravity.

Identifies symmetries of electrovacuum solutions in the fluid framework.

Abstract

The Brans-Dicke-like field of scalar-tensor gravity can be described as an imperfect fluid in an approach in which the field equations are regarded as effective Einstein equations. After completing this approach we recover, as a special case, the known effective fluid for a scalar coupled nonminimally to the Ricci curvature and we describe the imperfect fluid equivalent of f(R) gravity. A symmetry of electrovacuum Brans-Dicke gravity is translated into a symmetry of the corresponding effective fluid. The discussion is valid for any spacetime geometry.