Overspinning naked singularities in AdS$_3$ spacetime

TL;DR

This paper investigates the causal structure and geodesic behavior of overspinning naked singularities in AdS$_3$ spacetime, extending the understanding of BTZ geometries beyond black holes and conical singularities.

Contribution

It provides a complete analysis of geodesics in overspinning BTZ geometries, including new results on circular geodesics and the classification of noncircular geodesics.

Findings

Null and spacelike geodesics can reach infinity passing near the singularity.

Timelike geodesics cannot reach infinity and either loop or fall into the singularity.

Spatial projections of geodesics can have self-intersections and some are closed.

Abstract

The BTZ black hole belongs to a family of locally three-dimensional anti-de Sitter (AdS$_3$) spacetimes labeled by their mass $M$ and angular momentum $J$. The case $M \ell \geq |J|$, where $\ell$ is the anti-de Sitter radius, provides the black hole. Extending the metric to other values of of $M$ and $J$ leads to geometries with the same asymptotic behavior and global symmetries, but containing a naked singularity at the origin. The case $M \ell \leq -|J|$ corresponds to spinning conical singularities that are reasonably well understood. Here we examine the remaining case, that is $-|J|<M\ell<|J|$. These naked singularities are mathematically acceptable solutions describing classical spacetimes. They are obtained by identifications of the covering pseudosphere in $\mathbb{R}^{2,2}$ and are free of closed timelike curves. Here we study the causal structure and geodesics around these \textit{overspinning} geometries. We present a review of the geodesics for the entire BTZ family. The geodesic equations are completely integrated, and the solutions are expressed in terms of elementary functions. Special attention is given to the determination of circular geodesics, where new results are found. According to the radial bounds, eight types of noncircular geodesics appear in the BTZ spacetimes. For the case of overspinning naked singularity, null and spacelike geodesics can reach infinity passing by a point nearest to the singularity, others extend from the central singularity to infinity, and others still have a radial upper bound and terminate at the singularity. Timelike geodesics cannot reach infinity; they either loop around the singularity or fall into it. The spatial projections of the geodesics (orbits) exhibit self-intersections, whose number is determined for null and spacelike geodesics, and it is found a special class of timelike geodesics whose spatial projections are closed.