Acausal exact vacuum solutions in nonlocal gravity

TL;DR

This paper demonstrates that certain G"odel-type universes with closed time-like curves are exact vacuum solutions in nonlocal gravity, highlighting implications for causality and quantum consistency.

Contribution

It explicitly finds G"odel-type solutions in nonlocal gravity and discusses their quantum and causality implications, emphasizing the role of matter in breaking degeneracy.

Findings

G"odel-type universes are exact vacuum solutions in nonlocal gravity.

Causality violation occurs at the nonlocal scale, not necessarily at macroscopic scales.

Quantum effects make the transition from flat to G"odel universe negligible.

Abstract

Nonlocal gravity is a promising super-renormalizable or finite quantum gravity theory consistent with unitarity. In this paper, we focus on the classical equations of motion and explicitly show that a particular subclass of G\"{o}del-type Universes, where closed time-like curves are allowed, is an exact solution of nonlocal gravity in vacuum. The result is consistent with a well defined theory at quantum level, but it is realized only with a special, although large, class of nonlocal form factors. Therefore, by itself the renormalizability requirement is not a sufficient guiding principle in vacuum whether we want to avoid the causality violation. From the physical point of view, the causality violation takes place from the non locality fundamental scale to macroscopic scales. Therefore, it is the presence of matter to break the classical degeneracy between the Minkowski and the G\"{o}del Universe. Finally, we have shown that at the non-perturbative quantum level the transition from a flat to a G\"{o}del Universe is ridiculously small.