Black hole factory: a review of double-null formalism

In this review paper, we comprehensively summarize numerical applications of double-null formalism for studying dynamics within the theory of gravity. By using the double-null coordinates, we can investigate dynamical black holes and gravitational phenomena within spherical symmetry, including gravitational collapse, formation of horizons and singularities, as well as evaporations. This formalism can be extended to generic situations, where we can change dimensions, topologies, the gravity sector, as well as the matter sector. We also discuss its possible implications for black hole physics and particle astrophysics. This strong numerical tool will have lots of future applications for various research areas including general relativity, string theory, and various approaches to quantum gravity.