A no-go theorem shows that negative effective mass squared for the vector field in vector-tensor gravity always accompanies ghost or gradient instabilities, blocking spontaneous vectorization in stationary axisymmetric black holes.
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Black hole hair in generalized scalar-tensor gravity: An explicit example
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abstract
In a recent Letter we have shown that in shift-symmetric Horndeski theory the scalar field is forced to obtain a nontrivial configuration in black hole spacetimes, unless a linear coupling with the Gauss-Bonnet invariant is tuned away. As a result, black holes generically have hair in this theory. In this companion paper, we first review our argument and discuss it in more detail. We then present actual black hole solutions in the simplest case of a theory with the linear scalar-Gauss-Bonnet coupling. We generate exact solutions numerically for a wide range of values of the coupling and also construct analytic solutions perturbatively in the small coupling limit. Comparison of the two types of solutions indicates that non-linear effects that are not captured by the perturbative solution lead to a finite area, as opposed to a central, singularity. Remarkably, black holes have a minimum size, controlled by the length scale associated with the scalar-Gauss-Bonnet coupling. We also compute some phenomenological observables for the numerical solution for a wide range of values of the scalar-Gauss-Bonnet coupling. Deviations from the Schwarzschild geometry are generically very small.
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Numerical relativity simulations of black hole scattering in Einstein-scalar-Gauss-Bonnet gravity agree closely with effective-one-body analytic predictions.
Rotating black holes with primary scalar hair in beyond Horndeski gravity produce shadows whose diameter increases for negative Q and whose distortion increases for positive Q, with EHT bounds on M87* restricting but not ruling out the (a, Q) parameter space.
A covariant framework reveals non-closed scalar charges with bulk contributions in ESGB black holes that become closed under shift symmetry and interpret spontaneous scalarization via the Smarr formula.
Leading-order cubic-curvature corrections to scalar quasinormal modes of black holes with spins up to 0.99M are computed numerically for modes up to l=5 with relative errors below 10^{-4}.
Scalar hair sourced by black holes in de Sitter spacetime grows temporally and spatially on superhorizon scales due to the dynamics of a minimally coupled massless scalar field in expanding spacetime, carrying a steady outward energy flux.
Numerical solutions show that leading effective-field-theory corrections to the Kerr metric grow with spin and are largest near extremality.
In scalar Gauss-Bonnet gravity, black hole solutions below a tunable minimum mass lose hyperbolicity in perturbations, corresponding to EFT breakdown, but scalar charge stays bounded above.
A review summarizing modified theories of gravity, their effects on compact objects, existing bounds from astrophysical observations, and the promise of future gravitational wave tests for strong-field gravity.
citing papers explorer
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No-go theorem for spontaneous vectorization
A no-go theorem shows that negative effective mass squared for the vector field in vector-tensor gravity always accompanies ghost or gradient instabilities, blocking spontaneous vectorization in stationary axisymmetric black holes.
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Black-Hole Scattering in Einstein-scalar-Gauss-Bonnet: Numerical Relativity Meets Analytics
Numerical relativity simulations of black hole scattering in Einstein-scalar-Gauss-Bonnet gravity agree closely with effective-one-body analytic predictions.
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Rotating Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity
Rotating black holes with primary scalar hair in beyond Horndeski gravity produce shadows whose diameter increases for negative Q and whose distortion increases for positive Q, with EHT bounds on M87* restricting but not ruling out the (a, Q) parameter space.
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Non-closed scalar charge in four-dimensional Einstein-scalar-Gauss-Bonnet black hole thermodynamics
A covariant framework reveals non-closed scalar charges with bulk contributions in ESGB black holes that become closed under shift symmetry and interpret spontaneous scalarization via the Smarr formula.
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Ringing of rapidly rotating black holes in effective field theory
Leading-order cubic-curvature corrections to scalar quasinormal modes of black holes with spins up to 0.99M are computed numerically for modes up to l=5 with relative errors below 10^{-4}.
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On cosmological properties of black-hole hair in linearly coupled scalar-Gauss-Bonnet theory
Scalar hair sourced by black holes in de Sitter spacetime grows temporally and spatially on superhorizon scales due to the dynamics of a minimally coupled massless scalar field in expanding spacetime, carrying a steady outward energy flux.
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Leading effective field theory corrections to the Kerr metric at all spins
Numerical solutions show that leading effective-field-theory corrections to the Kerr metric grow with spin and are largest near extremality.
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Minimum mass, maximum charge and hyperbolicity in scalar Gauss-Bonnet gravity
In scalar Gauss-Bonnet gravity, black hole solutions below a tunable minimum mass lose hyperbolicity in perturbations, corresponding to EFT breakdown, but scalar charge stays bounded above.
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Testing General Relativity with Present and Future Astrophysical Observations
A review summarizing modified theories of gravity, their effects on compact objects, existing bounds from astrophysical observations, and the promise of future gravitational wave tests for strong-field gravity.