Linearized multipole solutions and their representation

TL;DR

This paper introduces a new family of static axisymmetric vacuum solutions to Einstein's equations, representing deviations from spherical symmetry via relativistic multipole moments, and characterizes these solutions using a Newtonian dumbbell model.

Contribution

It develops a generalized family of Weyl solutions with explicit multipole moments and links them to a simple Newtonian density profile, extending the understanding of relativistic multipole representations.

Findings

Derived bounds on multipole moments for positive-definite density profiles.

Established correspondence between multipole moments and the properties of the relativistic solutions.

Connected the density conditions to the existence of an infinite-redshift surface.

Abstract

The monopole solution of the Einstein vacuum field equations (Schwarzschild`s solution) in Weyl coordinates involves a metric function that can be interpreted as the gravitational potential of a bar of length $2m$ with constant linear density. The question addressed in this work is whether similar representations can be constructed for Weyl solutions other than the spherically symmetric one. A new family of static solutions of the axisymmetric vacuum field equations generalizing the M-Q$^{(1)}$ solution is developed. These represent slight deviations from spherical symmetry in terms of the relativistic multipole moments (RMM) we wish the solution to contain. A Newtonian object referred to as a dumbbell can be used to describe these solutions in a simple form by means of the density of this object, since the physical properties of the relativistic solution are characterized by its behaviour. The density profile of the dumbbell, which is given in terms of the RMM of the solution, allows us to distinguish general multipole Weyl solutions from the constant-density Schwarzschild solution. The range of values of the multipole moments that generate positive-definite density profiles are also calculated. The bounds on the multipole moments that arise from this density condition are identical to those required for a well-behaved infinite-redshift surface $g_{00}=0$.