Revisiting the Birkhoff theorem from a dual null point of view

TL;DR

This paper offers a modern dual null approach to the general Birkhoff theorem in general relativity, clarifying its conditions, simplifying solutions, and exploring implications for black hole regions.

Contribution

It introduces a dual null formalism for the most general Birkhoff theorem, making it more accessible and applicable in black hole and cosmological contexts.

Findings

Presented a family of solutions consistent with the theorem

Demonstrated how the formalism simplifies solving dynamical equations

Analyzed the role of trapped and untrapped regions in spacetime

Abstract

The Birkhoff theorem is a well-known result in general relativity and it is used in many applications. However, its most general version, due to Bona, is almost unknown and presented in a form less accessible to the relativist and cosmologist community. Moreover, many wield it mistakenly as a simple transposition of Newton's iron sphere theorem. In the present work, we propose a modern, dual null, presentation --- useful in many explorations, including black holes --- of the theorem that renders accessible most of the results of Bona's version. In addition, we discuss the fluid contents admissible for the application of the theorem, beyond a vacuum, and we demonstrate how the formalism greatly simplifies solving the dynamical equations and allows one to express the solution as a power expansion in $r$. We present a family of solutions that share the properties predicted by the Birkhoff theorem and discuss the existence of trapped and antitrapped regions. The formalism manifestly shows how the type of region --- trapped or untrapped --- determines the character of the Killing vector.