Simpler than vacuum: Antiscalar alternatives to black holes

TL;DR

This paper introduces antiscalar solutions as simpler, horizonless alternatives to black holes, with distinct observational signatures like larger shadows and different orbital frequencies, potentially testable through strong-field observations.

Contribution

It derives the exact rotational antiscalar spacetime and compares its properties to black holes, highlighting its simplicity and potential observational differences.

Findings

Antiscalar metrics lack event horizons and ergospheres.

Static antiscalar fields are thermodynamically stable.

Predicted shadows are 5% larger, and ISCO frequency differs by 6.92%.

Abstract

The Janis-Newman-Winicour and Papapetrou metrics represent counterparts to the Schwarzschild black hole with scalar and antiscalar background fields, correspondingly (where "anti" is to be understood as in "anti-de Sitter"). There is also a scalar counterpart (the Krori-Bhattacharjee metric) to the Kerr black hole. Here we study analytical connections between these solutions and obtain the exact rotational generalization of the antiscalar Papapetrou spacetime as a viable alternative to the Kerr black hole. The antiscalar metrics appear to be the simplest ones as they do not reveal event horizons and ergospheres, and they do not involve an extra parameter for scalar charge. Static antiscalar field is thermodynamically stable and self-consistent, but this is not the case for the scalar Janis-Newman-Winicour solution; besides, antiscalar thermodynamics is reducible to black-hole thermodynamics. Lensing, geodetic and Lense-Thirring effects are found to be practically indistinguishable between antiscalar and vacuum solutions in weak fields. Only strong-field observations might provide a test for the existence of antiscalar background. In particular, the antiscalar solution predicts 5% larger shadows of supermassive compact objects, as compared to the vacuum solution. Another measurable aspect is the 6.92% difference in the frequency of the innermost stable circular orbit, characterizing the upper cut-off in the gravitational wave spectrum.