**
Gravitomagnetic response of an irrotational body to an applied tidal field**

**Philippe Landry, Eric Poisson**

*Physical Review D* **91**, 104026 (2015)

The deformation of a nonrotating body resulting from the application of a tidal field is measured by two sets of Love numbers associated with the gravitoelectric and gravitomagnetic pieces of the tidal field, respectively. The gravitomagnetic Love numbers were previously computed for fluid bodies, under the assumption that the fluid is in a strict hydrostatic equilibrium that requires the complete absence of internal motions. A more realistic configuration, however, is an irrotational state that establishes, in the course of time, internal motions driven by the gravitomagnetic interaction. We recompute the gravitomagnetic Love numbers for this irrotational state, and show that they are dramatically different from those associated with the strict hydrostatic equilibrium: While the Love numbers are positive in the case of strict hydrostatic equilibrium, they are negative in the irrotational state. Our computations are carried out in the context of perturbation theory in full general relativity, and in a post-Newtonian approximation that reproduces the behavior of the Love numbers when the body's compactness is small.

**
Tidal deformation of a slowly rotating material body. External metric**

**Philippe Landry, Eric Poisson**

*Physical Review D* **91**, 104018 (2015)

We construct the external metric of a slowly rotating, tidally deformed material body in general relativity. The tidal forces acting on the body are assumed to be weak and to vary slowly with time, and the metric is obtained as a perturbation of a background metric that describes the external geometry of an isolated, slowly rotating body. The tidal environment is generic and characterized by two symmetric-tracefree tidal moments E_{ab} and B_{ab}, and the body is characterized by its mass M, its radius R, and a dimensionless angular-momentum vector chi^a << 1. The perturbation accounts for all couplings between chi^a and the tidal moments. The body's gravitational response to the applied tidal field is measured in part by the familiar gravitational Love numbers K^{el}_2 and K^{mag}_2, but we find that the coupling between the body's rotation and the tidal environment requires the introduction of four new quantities, which we designate as rotational-tidal Love numbers. All these Love numbers are gauge invariant in the usual sense of perturbation theory, and all vanish when the body is a black hole.

**
Tidal deformation of a slowly rotating black hole**

**Eric Poisson**

*Physical Review D* **91**, 044004 (2015)

In the first part of this article I determine the geometry of a slowly rotating black hole deformed by generic tidal forces created by a remote distribution of matter. The metric of the deformed black hole is obtained by integrating the Einstein field equations in a vacuum region of spacetime bounded by r < r_max, with r_max a maximum radius taken to be much smaller than the distance to the remote matter. The tidal forces are assumed to be weak and to vary slowly in time, and the metric is expressed in terms of generic tidal quadrupole moments E_{ab} and B_{ab} that characterize the tidal environment. The metric incorporates couplings between the black hole's spin vector and the tidal moments, and captures all effects associated with the dragging of inertial frames. In the second part of the article I determine the tidal moments by immersing the black hole in a larger post-Newtonian system that includes an external distribution of matter; while the black hole's internal gravity is allowed to be strong, the mutual gravity between the black hole and the external matter is assumed to be weak. The post-Newtonian metric that describes the entire system is valid when r > r_min, where r_min is a minimum distance that must be much larger than the black hole's radius. The black-hole and post-Newtonian metrics provide alternative descriptions of the same gravitational field in an overlap r_min < r < r_max, and matching the metrics determine the tidal moments, which are expressed as post-Newtonian expansions carried out through one-and-a-half post-Newtonian order. Explicit expressions are obtained in the specific case in which the black hole is a member of a post-Newtonian two-body system.

**
Gravitational self-force in nonvacuum spacetimes**

**Peter Zimmerman, Eric Poisson**

*Physical Review D* **90**, 084030 (2014)

The gravitational self-force has thus far been formulated in background spacetimes for which the metric is a solution to the Einstein field equations in vacuum. While this formulation is sufficient to describe the motion of a small object around a black hole, other applications require a more general formulation that allows for a nonvacuum background spacetime. We provide a foundation for such extensions, and carry out a concrete formulation of the gravitational self-force in two specific cases. In the first we consider a particle of mass m and scalar charge q moving in a background spacetime that contains a background scalar field. In the second we consider a particle of mass m and electric charge e moving in an electrovac spacetime. The self-force incorporates all couplings between the gravitational perturbations and those of the scalar or electromagnetic fields. It is expressed as a sum of local terms involving tensors defined in the background spacetime and evaluated at the current position of the particle, as well as tail integrals that depend on the past history of the particle. Because such an expression is rarely a useful starting point for an explicit evaluation of the self-force, we also provide covariant expressions for the singular potentials, expressed as local expansions near the world line; these can be involved in the construction of effective extended sources for the regular potentials, or in the computation of regularization parameters when the self-force is computed as a sum over spherical-harmonic modes.

**
Relativistic theory of surficial Love numbers**

**Philippe Landry, Eric Poisson**

*Physical Review D* **89**, 124011 (2014)

A relativistic theory of surficial Love numbers, which characterize the surface deformation of a body subjected to tidal forces, was initiated by Damour and Nagar. We revisit this effort in order to extend it, clarify some of its aspects, and simplify its computational implementation. First, we refine the definition of surficial Love numbers proposed by Damour and Nagar and formulate it directly in terms of the deformed curvature of the body’s surface, a meaningful geometrical quantity. Second, we develop a unified theory of surficial Love numbers that applies equally well to material bodies and black holes. Third, we derive a compactness-dependent relation between the surficial and (electric-type) gravitational Love numbers of a perfect-fluid body and show that it reduces to the familiar Newtonian relation when the compactness is small. And fourth, we simplify the tasks associated with the practical computation of the surficial and gravitational Love numbers for a material body.

**
Self-force on a charge outside a five-dimensional black hole**

**Matthew J.S. Beach, Eric Poisson, Bernhard G. Nickel**

*Physical Review D* **89**, 124014 (2014)

We compute the electromagnetic self-force acting on a charged particle held in place at a fixed position r outside a five-dimensional black hole described by the Schwarzschild-Tangherlini metric. Using a spherical-harmonic decomposition of the electrostatic potential and a regularization prescription based on the Hadamard Green’s function, we express the self-force as a convergent mode sum. The self-force is first evaluated numerically, and next presented as an analytical expansion in powers of R/r, with R denoting the event-horizon radius. The power series is then summed to yield a closed-form expression. Unlike its four-dimensional version, the self-force features a dependence on a regularization parameter s that can be interpreted as the particle’s radius. The self-force is repulsive at large distances, and its behavior is related to a model according to which the force results from a gravitational interaction between the black hole and the distribution of electrostatic field energy attached to the particle. The model, however, is shown to become inadequate as r becomes comparable to R, where the self-force changes sign and becomes attractive. We also calculate the self-force acting on a particle with a scalar charge, which we find to be everywhere attractive. This is to be contrasted with its four-dimensional counterpart, which vanishes at any r.

**
The Schwarzschild metric: It's the coordinates, stupid!**

**Pierre Fromholz, Eric Poisson, Clifford M. Will**

*American Journal of Physics* **82**, 295 (2014)

Every general relativity textbook emphasizes that coordinates have no physical meaning. Nevertheless, a coordinate choice must be made in order to carry out real calculations, and that choice can make the difference between a calculation that is simple and one that is a mess. We give a concrete illustration of the maxim that "coordinates matter" using the exact Schwarzschild solution for a vacuum, static, spherical spacetime. We review the standard textbook derivation, Schwarzschild's original 1916 derivation, and a derivation using the Landau-Lifshitz formulation of the Einstein field equations. The last derivation is much more complicated, has one aspect for which we have been unable to find a solution, and gives an explicit illustration of the fact that the Schwarzschild geometry can be described in infinitely many coordinate systems.

**
Electromagnetic self-force on a static charge in Schwarzschild-de
Sitter spacetimes**

**Joseph Kuchar, Eric Poisson, Ian Vega**

*Classical and Quantum Gravity* **30**, 235033 (2013)

We compute the self-force acting on an electric charge at rest in Schwarzschild-de Sitter spacetimes, allowing the cosmological constant to be either positive or negative. In the case of a positive cosmological constant, we show that the self-force is always positive, representing a repulsion from the black hole, and monotonically decreasing with increasing distance from the black hole. The spectrum of results is richer in the case of a negative cosmological constant. Here the self-force is not always positive --- it is negative when the black-hole and cosmological scales are comparable and the charge is close to the black hole --- and not always monotonically decreasing --- it is actually monotonically increasing when the cosmological scale is sufficiently small compared to the black-hole scale. The self-force also approaches a constant asymptotic value when the charge is moved to large cosmological distances; this feature can be explained in terms of an interaction between the charge and the conformal boundary at infinity, which acts as a grounded conductor.

**
Self-force as a cosmic censor
**

**Peter Zimmerman, Ian Vega, Eric Poisson, Roland Haas**

*Physical Review D* **87**, 041501(R) (2013)

We examine Hubeny's scenario according to which a near-extremal Reissner-Nordstrom black hole can absorb a particle and be driven toward an over-extremal state in which the charge exceeds the mass, signaling the destruction of the black hole. Our analysis incorporates the particle's electromagnetic self-force and the energy radiated to infinity in the form of electromagnetic waves. With these essential ingredients, our sampling of the parameter space reveals no instances of an overcharged final state, and we conjecture that the self-force acts as a cosmic censor, preventing the destruction of a near-extremal black hole by the absorption of a charged particle. We argue, on the basis of the third law of black-hole mechanics, that this conclusion is robust and should apply to attempts to overspin a Kerr black hole.