NUT wormholes

TL;DR

This paper explores NUT-charged black holes as a new setting for traversable wormholes, showing they can be geodesically complete and free of certain causality violations without exotic matter.

Contribution

It demonstrates that NUT black holes can host traversable wormholes with geodesic completeness and no need for exotic matter, addressing issues like singularities and causality.

Findings

Wormholes with NUT charge are geodesically complete.

No closed timelike or null geodesics exist in these spacetimes.

Timelike and null geodesics connect asymptotic regions with manageable tidal forces.

Abstract

We show that supercritically charged black holes with NUT provide a new setting for traversable wormholes. This does not require exotic matter, a price being the Misner string singularities. Without assuming time periodicity to make Misner strings unobservable, we show that, contrary to expectations, geodesics do not stop there. Moreover, since there is no central singularity the space-time turns out to be geodesically complete. Another unpleasant feature of spacetimes with NUTs is the presence of regions where the azimuthal angle $\varphi$ becomes timelike, signalling the appearance of closed timelike curves (CTCs). We show that among them there are no closed timelike or null geodesics, so the freely falling observers should not encounter causality violations. Considering worldlines of charged particles, we find that, although these can become closed in the vicinity of the wormhole throat for large enough charge-to-mass ratio, the non-causal orbits are still disconnected from the distant zones. All these findings support our feeling that wormholes with NUTs deserve to be taken seriously. Integrating the geodesic equations completely, we demonstrate the existence of timelike and null geodesics connecting two asymptotic regions of the wormhole, such that the tidal forces in the throat are reasonably small. We discuss bounds on the NUT charge which follow from the Schwinger pair creation and ionization thresholds and speculate that such NUT wormholes could be present in some galactic centers.