Numerical simulations of single and binary black holes in scalar-tensor theories: circumventing the no-hair theorem

Scalar-tensor theories are a compelling alternative to general relativity and one of the most accepted extensions of Einstein's theory. Black holes in these theories have no hair, but could grow "wigs" supported by time-dependent boundary conditions or spatial gradients. Time-dependent or spatially varying fields lead in general to nontrivial black hole dynamics, with potentially interesting experimental consequences. We carry out a numerical investigation of the dynamics of single and binary black holes in the presence of scalar fields. In particular we study gravitational and scalar radiation from black-hole binaries in a constant scalar-field gradient, and we compare our numerical findings to analytical models. In the single black hole case we find that, after a short transient, the scalar field relaxes to static configurations, in agreement with perturbative calculations. Furthermore we predict analytically (and verify numerically) that accelerated black holes in a scalar-field gradient emit scalar radiation. For a quasicircular black-hole binary, our analytical and numerical calculations show that the dominant component of the scalar radiation is emitted at twice the binary's orbital frequency.Comment: 21 pages, 6 figures, matches version accepted in Physical Review

Tensor-multi-scalar theories: relativistic stars and 3+1 decomposition

Gravitational theories with multiple scalar fields coupled to the metric and each other --- a natural extension of the well studied single-scalar-tensor theories --- are interesting phenomenological frameworks to describe deviations from general relativity in the strong-field regime. In these theories, the $N$-tuple of scalar fields takes values in a coordinate patch of an $N$-dimensional Riemannian target-space manifold whose properties are poorly constrained by weak-field observations. Here we introduce for simplicity a non-trivial model with two scalar fields and a maximally symmetric target-space manifold. Within this model we present a preliminary investigation of spontaneous scalarization for relativistic, perfect fluid stellar models in spherical symmetry. We find that the scalarization threshold is determined by the eigenvalues of a symmetric scalar-matter coupling matrix, and that the properties of strongly scalarized stellar configurations additionally depend on the target-space curvature radius. In preparation for numerical relativity simulations, we also write down the $3+1$ decomposition of the field equations for generic tensor-multi-scalar theories.Comment: 32 pages, 8 figures, 1 table, invited contribution to the Classical and Quantum Gravity Focus Issue "Black holes and fundamental fields". v3: version in press in CQG, with various improvements in response to the referees' comments. In particular, the 3+1 decomposition now allows for matte

Light scalar field constraints from gravitational-wave observations of compact binaries

Scalar-tensor theories are among the simplest extensions of general relativity. In theories with light scalars, deviations from Einstein's theory of gravity are determined by the scalar mass m_s and by a Brans-Dicke-like coupling parameter \omega_{BD}. We show that gravitational-wave observations of nonspinning neutron star-black hole binary inspirals can be used to set lower bounds on \omega_{BD} and upper bounds on the combination m_s/\sqrt{\omega_{BD}}$. We estimate via a Fisher matrix analysis that individual observations with signal-to-noise ratio \rho would yield (m_s/\sqrt{\omega_{BD}})(\rho/10)<10^{-15}, 10^{-16} and 10^{-19} eV for Advanced LIGO, ET and eLISA, respectively. A statistical combination of multiple observations may further improve these bounds.Comment: 9 pages, 4 figures. Matches version accepted in Physical Review

Gravitational Higgs mechanism in neutron star interiors

We suggest that nonminimally coupled scalar fields can lead to modifications of the microphysics in the interiors of relativistic stars. As a concrete example, we consider the generation of a non-zero photon mass in such high-density environments. This is achieved by means of a light gravitational scalar, and the scalarization phase transition in scalar-tensor theories of gravitation. Two distinct models are presented, and phenomenological implications are briefly discussed

Testing General Relativity with Present and Future Astrophysical Observations

One century after its formulation, Einstein's general relativity has maderemarkable predictions and turned out to be compatible with all experimentaltests. Most of these tests probe the theory in the weak-field regime, and thereare theoretical and experimental reasons to believe that general relativityshould be modified when gravitational fields are strong and spacetime curvatureis large. The best astrophysical laboratories to probe strong-field gravity areblack holes and neutron stars, whether isolated or in binary systems. We reviewthe motivations to consider extensions of general relativity. We present a(necessarily incomplete) catalog of modified theories of gravity for whichstrong-field predictions have been computed and contrasted to Einstein'stheory, and we summarize our current understanding of the structure anddynamics of compact objects in these theories. We discuss current bounds onmodified gravity from binary pulsar and cosmological observations, and wehighlight the potential of future gravitational wave measurements to inform uson the behavior of gravity in the strong-field regime