Spherically symmetric black holes and affine-null metric formulation of Einstein’s equations

• Published in: Physical review. D 2021-10, Vol.104 (8), p.1, Article 084048
• Main Authors: Gallo, Emanuel, Kozameh, Carlos, Mädler, Thomas, Moreschi, Osvaldo M., Perez, Alejandro
• Format: Article
• Language: English
• Subjects: Black holes; Boundary value problems; Coordinates; Einstein equations; General Relativity and Quantum Cosmology; Horizon; Infinity; Mathematical analysis; Physics; Singularity (mathematics); Spacetime
• ISSN: 2470-0010; 2470-0029
• DOI: 10.1103/PhysRevD.104.084048

The definition of well-behaved coordinate charts for black hole spacetimes can be tricky, as they can lead for example to either unphysical coordinate singularities in the metric (e.g., r = 2M in the Schwarzschild black hole) or to an implicit dependence of the chosen coordinates to physical relevant coordinates (e.g., the dependence of the null coordinates in the Kruskal metric). Here we discuss two approaches for coordinate choices in spherically symmetric spacetimes allowing us to explicitly discuss "solitary" and spherically symmetric black holes from a regular horizon to null infinity. The first approach relies on a construction of a regular null coordinate system (where regular is meant as being defined from the horizon to null infinity) given an explicit solution of the Einstein-matter equations. The second approach is based on an affine-null formulation of the Einstein equations and the respective characteristic initial value problem. In particular, we present a derivation of the Reissner-Nordström black holes expressed in terms of these regular coordinates.