G\"{o}del-type universes in f(R) gravity

TL;DR

This paper investigates G"odel-type solutions in $f(R)$ gravity, showing that such theories can admit solutions with causality violation, extending known results from general relativity to modified gravity frameworks.

Contribution

It proves that all perfect-fluid G"odel-type solutions in $f(R)$ gravity with $df/dR > 0$ are isometric to G"odel geometry, allowing causality violation, and derives a general expression for the critical radius.

Findings

$f(R)$ gravity admits G"odel-type solutions with causality violation.

The critical radius for causality violation depends on the specific $f(R)$ model and matter content.

A specific viable $f(R)$ model can have both causal and noncausal G"odel-type solutions.

Abstract

The $f(R)$ gravity theories provide an alternative way to explain the current cosmic acceleration without a dark energy matter component. If gravity is governed by a $f(R)$ theory a number of issues should be reexamined in this framework, including the violation of causality problem on nonlocal scale. We examine the question as to whether the $f(R)$ gravity theories permit space-times in which the causality is violated. We show that the field equations of these $f(R)$ gravity theories do not exclude solutions with breakdown of causality for a physically well-motivated perfect-fluid matter content. We demonstrate that every perfect-fluid G\"{o}del-type solution of a generic $f(R)$ gravity satisfying the condition $df/dR > 0$ is necessarily isometric to the G\"odel geometry, and therefore presents violation of causality. This result extends a theorem on G\"{o}del-type models, which has been established in the context of general relativity. We also derive an expression for the critical radius $r_c$ (beyond which the causality is violated) for an arbitrary $f(R)$ theory, making apparent that the violation of causality depends on both the $f(R)$ gravity theory and the matter content. As an illustration, we concretely take a recent $f(R)$ gravity theory that is free from singularities of the Ricci scalar and is cosmologically viable, and show that this theory accommodates noncausal as well as causal G\"odel-type solutions.