Bounds on the interior geometry and pressure profile of static fluid spheres

TL;DR

This paper extends classical bounds on the interior geometry and pressure profiles of static fluid spheres in relativistic stellar models, providing new inequalities under various geometric and physical constraints.

Contribution

It generalizes the Buchdahl--Bondi bound by incorporating additional constraints on the metric, gravity, and density-pressure profiles of static fluid spheres.

Findings

Derived new bounds on the internal compactness 2m(r)/r

Established inequalities for the pressure profile beyond the central pressure

Extended the classical bounds to more general geometric and physical conditions

Abstract

It is a famous result of relativistic stellar structure that (under mild technical conditions) a static fluid sphere satisfies the Buchdahl--Bondi bound 2M/R <= 8/9; the surprise here being that the bound is not 2M/R <= 1. In this article we provide further generalizations of this bound by placing a number of constraints on the interior geometry (the metric components), on the local acceleration due to gravity, on various combinations of the internal density and pressure profiles, and on the internal compactness 2m(r)/r of static fluid spheres. We do this by adapting the standard tool of comparing the generic fluid sphere with a Schwarzschild interior geometry of the same mass and radius -- in particular we obtain several results for the pressure profile (not merely the central pressure) that are considerably more subtle than might first be expected.