G\"{o}del-type universes in unimodular gravity

Abstract

In this paper, the G\"{o}del-type universes are examined within the framework of unimodular gravity. Since the existence of G\"{o}del solutions is intrinsically related to the presence of a cosmological constant in general relativity, one can naturally wonder how the acausal structure of the G\"{o}del solutions behaves in different cosmological scenarios. One of the simplest, but important, frameworks within this context is the unimodular gravity, in which the cosmological constant emerges as an integration constant rather than a coupling constant. Hence, the validity of G\"{o}del-type solutions is scrutinized within the unimodular approach, examining whether this theory can address the known issue of causality violation in G\"{o}del universes. In detail, it is demonstrated that for certain gravitational sources, both causal and non-causal regions are permissible.