Geometry and dynamics of a tidally deformed black hole

TL;DR

This paper derives the metric of a nonrotating black hole under tidal deformation using an expansion in the external spacetime's curvature radius, providing insights into the black hole's horizon geometry and area change due to tides.

Contribution

It presents a detailed metric expansion for tidally deformed black holes in a geometrically meaningful coordinate system, including horizon properties and area increase calculations.

Findings

The metric is expanded up to order R^{-4} in tidal strength.

The horizon remains a stationary null hypersurface with an isolated horizon structure.

The black hole's surface area increases due to tidal interactions.

Abstract

The metric of a nonrotating black hole deformed by a tidal interaction is calculated and expressed as an expansion in the strength of the tidal coupling. The expansion parameter is the inverse length scale R^{-1}, where R is the radius of curvature of the external spacetime in which the black hole moves. The expansion begins at order R^{-2}, and it is carried out through order R^{-4}. The metric is parameterized by a number of tidal multipole moments, which specify the black hole's tidal environment. The tidal moments are freely-specifiable functions of time that are related to the Weyl tensor of the external spacetime. The metric is presented in a light-cone coordinate system that possesses a clear geometrical meaning: The advanced-time coordinate $v$ is constant on past light cones that converge toward the black hole; the angles theta and phi are constant on the null generators of each light cone; and the radial coordinate r is an affine parameter on each generator, which decreases as the light cones converge toward the black hole. The coordinates are well-behaved on the black-hole horizon, and they are adjusted so that the coordinate description of the horizon is the same as in the Schwarzschild geometry: r = 2M. At the order of accuracy maintained in this work, the horizon is a stationary null hypersurface foliated by apparent horizons; it is an isolated horizon in the sense of Ashtekar and Krishnan. As an application of our results we examine the induced geometry and dynamics of the horizon, and calculate the rate at which the black-hole surface area increases as a result of the tidal interaction.