Work

Birkhoff’s theorem and uniqueness: A peek beyond general relativity

Year: 2025

Type: article

Abstract: In General Relativity, Birkhoff’s theorem asserts that any spherically symmetric vacuum solution must be static and asymptotically flat. In this paper, we study the validity of Birkhoff’s theorem for a broad class of modified gravity theories in four spacetime dimensions, including quadratic and higher-order gravity models. We demonstrate that the Schwarzschild spacetime remains the unique Einstein branch solution outside any spherically symmetric configuration of these theories. Consequently, unlike black holes, the breakdown of junction conditions at the surface of the star further implies that the actual spacetime metric outside a horizonless star in these modified theories cannot simultaneously be spherically symmetric and remain within the Einstein branch. This insight offers a unique observational probe for theories beyond General Relativity.

Source: Physics Letters B

DOI: 10.1016/j.physletb.2025.139974

Authors: Rajes Ghosh, Akash K. Mishra, Avijit Chowdhury

Institutions: Tata Institute of Fundamental Research, International Centre for Theoretical Sciences, Saha Institute of Nuclear Physics, Indian Institute of Technology Guwahati

Language: English

Topic: Cosmology and Gravitation Theories

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: diamond

Funders: Arthritis National Research Foundation, Science and Engineering Research Board, Indian Association for the Cultivation of Science

Award: G7554586862