How appropriate are the gravitational entropy proposals for traversable wormholes?

TL;DR

This paper evaluates the applicability of two gravitational entropy proposals to traversable wormholes, finding that their validity varies and depends on the specific definition used, impacting the physical realism of such wormholes.

Contribution

It compares two different gravitational entropy definitions in the context of traversable wormholes, highlighting their applicability and limitations.

Findings

The geometric approach applies broadly but lacks thermodynamic basis.

The CET proposal is limited to Petrov type D and N spacetimes.

Gravitational entropy validity depends on the chosen definition.

Abstract

In this paper we have examined the validity of some proposed definitions of gravitational entropy (GE) in the context of traversable wormhole solutions of the Einstein field equations. Here we have adopted two different proposals of GE and checked for their applicability in the case of these wormholes. The first one is the phenomenological approach proposed by Rudjord et al \cite{entropy1} and expanded by Romero et al in \cite{entropy2}, which is a purely geometric method of measuring gravitational entropy. The latter one is the Clifton-Ellis-Tavakol (CET) proposal \cite{CET} for the gravitational entropy which arises in relativistic thermodynamics and is based on the Bel-Robinson tensor, which represents the effective super-energy-momentum tensor of free gravitational fields. Considering some of the Lorentzian traversable wormholes along with the Brill solution for NUT wormholes and the AdS wormholes, we have evaluated the gravitational entropy for these systems. Incidentally, the application of the CET proposal can provide unique gravitational entropies for spacetimes of Petrov type D and N only, whereas the geometric method can be applied to almost every kind of spacetime, although it has no relation with thermodynamics. For any traversable wormhole to be physically realistic, it should have a viable GE. We found that the GE proposals do give us a consistent measure of GE in several of them. This means that the existence of a viable gravitational entropy strictly depends on its definition.