Tides and traversability in gravastars and other related geometries

TL;DR

This paper investigates tidal effects and traversability issues in gravastars and related geometries, revealing conditions under which large tides can be avoided and analyzing the impact on objects crossing thin-shell regions.

Contribution

It provides a detailed analysis of tidal effects in various gravastar models, including non-trivial topologies, and clarifies when large tides are unavoidable or can be mitigated.

Findings

Large radial tides are unavoidable in gravastar shells but can be avoided in non-trivial topologies.

Transverse tides are most problematic in shells satisfying the flare-out condition.

Objects traversing gravastar shells experience compression due to tidal effects.

Abstract

Tidal effects related to the traversability across thin shells are examined in spherically symmetric geometries. We focus mainly on shells separating inner from outer regions of gravastars (de Sitter -- i.e. $\Lambda>0$ -- interior and Schwarzschild exterior of mass parameter $M$), but we also examine other related geometries by including the possibility of a negative cosmological constant and, besides, non trivial topologies where the shell separates two outer regions. The analysis is developed for radially traversing objects and for tides in both radial and transverse directions, which present difficulties of somewhat different nature. Transverse tides across shells which satisfy the flare-out condition are the most troublesome, while shells in trivial topologies, i.e. geometries with one asymptotic region, are more indulgent with the issue of large tides. Besides, contradicting other cases analyzed in previous works, we find that large radial tides cannot be avoided when traveling across the shell in the gravastar solution, but in non-trivial topologies they can. We study with special attention the traversability in practice of the transition layer in the thin-shell gravastar solution. In particular, a finite object which traverses radially the shell in a gravastar with $\sqrt{\Lambda}\ll 1/M$ undergoes a compression effect in both the transverse and the radial directions due to the tides associated to the thin layer. The results are interpreted in terms of the total momentum transfer obtained by integrating the travel time of the object.