On the stability of scalar-vacuum space-times

TL;DR

This paper analyzes the stability of static, spherically symmetric scalar-vacuum space-times, demonstrating that all such solutions with zero potential are unstable under spherical perturbations, including wormholes and black holes.

Contribution

It introduces a general methodology for studying radial perturbations in scalar-vacuum solutions with arbitrary potentials, and proves the instability of all solutions with V(φ)=0.

Findings

All static solutions with V(φ)=0 are unstable under spherical perturbations.

The methodology effectively regularizes the effective potential at throats, enabling stability analysis.

Anti-Fisher wormholes and black holes are confirmed to be unstable.

Abstract

We study the stability of static, spherically symmetric solutions to the Einstein equations with a scalar field as the source. We describe a general methodology of studying small radial perturbations of scalar-vacuum configurations with arbitrary potentials V(\phi), and in particular space-times with throats (including wormholes), which are possible if the scalar is phantom. At such a throat, the effective potential for perturbations V_eff has a positive pole (a potential wall) that prevents a complete perturbation analysis. We show that, generically, (i) V_eff has precisely the form required for regularization by the known S-deformation method, and (ii) a solution with the regularized potential leads to regular scalar field and metric perturbations of the initial configuration. The well-known conformal mappings make these results also applicable to scalar-tensor and f(R) theories of gravity. As a particular example, we prove the instability of all static solutions with both normal and phantom scalars and V(\phi) = 0 under spherical perturbations. We thus confirm the previous results on the unstable nature of anti-Fisher wormholes and Fisher's singular solution and prove the instability of other branches of these solutions including the anti-Fisher "cold black holes".