Algebraic classification of 2+1 geometries: a new approach

Abstract

We present a convenient method of algebraic classification of 2+1 spacetimes into the types I, II, D, III, N and O, without using any field equations. It is based on the 2+1 analogue of the Newman-Penrose curvature scalars ${\Psi }_A$ of distinct boost weights, which are specific projections of the Cotton tensor onto a suitable null triad. The algebraic types are then simply determined by the gradual vanishing of such Cotton scalars, starting with those of the highest boost weight. This classification is directly related to the specific multiplicity of the Cotton-aligned null directions (CANDs) and to the corresponding Bel-Debever criteria. Using a bivector (that is 2-form) decomposition, we demonstrate that our method is fully equivalent to the usual Petrov-type classification of 2+1 spacetimes based on the eigenvalue problem and determining the respective canonical Jordan form of the Cotton-York tensor. We also derive a simple synoptic algorithm of algebraic classification based on the key polynomial curvature invariants. To show the practical usefulness of our approach, we perform the classification of several explicit examples, namely the general class of Robinson-Trautman spacetimes with an aligned electromagnetic field and a cosmological constant, and other metrics of various algebraic types.

final version accepted as an OPEN ACCESS publication in Class. Quantum Grav. (references and additional observations added), 36 pages, 1 figure, 8 tables

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Cited by in corpus (4)

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• Robinson-Trautman spacetimes in (2+1) dimensions
• Interpreting gravitational fields of Topologically Massive Gravity using geodesic deviation
• Extremal rotating BTZ black holes cannot be dressed in (anti-)self-dual Maxwell field