Showing papers by "Paolo Pani published in 2013"
TL;DR: In this paper, it has been shown that the Schwarzschild geometry is linearly unstable for small tensor masses, against a spherically symmetric mode, both against the spherical mode and against long-lived superradiant modes.
Abstract: Massive bosonic fields of arbitrary spin are predicted by general extensions of the standard model. It has been recently shown that there exists a family of bimetric theories of gravity—including massive gravity—which are free of Boulware-Deser ghosts at the nonlinear level. This opens up the possibility to describe consistently the dynamics of massive spin-2 particles in a gravitational field. Within this context, we develop the study of massive spin-2 fluctuations—including massive gravitons—around Schwarzschild and slowly rotating Kerr black holes. Our work has two important outcomes. First, we show that the Schwarzschild geometry is linearly unstable for small tensor masses, against a spherically symmetric mode. Second, we provide solid evidence that the Kerr geometry is also generically unstable, both against the spherical mode and against long-lived superradiant modes. In the absence of nonlinear effects, the observation of spinning black holes bounds the graviton mass μ to be μ≲5×10-23 eV.
248 citations
TL;DR: In this article, it was shown that the quasi-adiabatic inspiral is mainly driven by DM accretion and by dynamical friction, rather than by radiation reaction, and that the frequency and amplitude of the GW signal produced during the latest stages of the inspiral are nearly constant.
Abstract: The nature and properties of dark matter (DM) are both outstanding issues in physics. Besides clustering in halos, the universal character of gravity implies that self-gravitating compact DM configurations—predicted by various models—might be spread throughout the universe. Their astrophysical signature can be used to probe fundamental particle physics, or to test alternative descriptions of compact objects in active galactic nuclei. Here, we discuss the most promising dissection tool of such configurations: the inspiral of a compact stellar-size object and consequent gravitational-wave (GW) emission. The inward motion of this "test probe" encodes unique information about the nature of the supermassive configuration. When the probe travels through some compact region we show, within a Newtonian approximation, that the quasi-adiabatic inspiral is mainly driven by DM accretion and by dynamical friction, rather than by radiation reaction. When accretion dominates, the frequency and amplitude of the GW signal produced during the latest stages of the inspiral are nearly constant. In the exterior region we study a model in which the inspiral is driven by GW and scalar-wave emission, described at a fully relativistic level. Resonances in the energy flux appear whenever the orbital frequency matches the effective mass of the DM particle, corresponding to the excitation of the central object's quasinormal frequencies. Unexpectedly, these resonances can lead to large dephasing with respect to standard inspiral templates, to such an extent as to prevent detection with matched filtering techniques. We discuss some observational consequences of these effects for GW detection.
153 citations
TL;DR: In this article, a post-Newtonian-affine approach is used to model tidal deformations in compact binaries and show that the moment of inertia, the Love number, and the spin-induced quadrupole moment of an isolated neutron star are related through functions which are practically independent of the equation of state.
Abstract: Neutron stars are extremely relativistic objects which abound in our universe and yet are poorly understood, due to the high uncertainty on how matter behaves in the extreme conditions which prevail in the stellar core. It has recently been pointed out that the moment of inertia $I$, the Love number $\ensuremath{\lambda}$, and the spin-induced quadrupole moment $Q$ of an isolated neutron star, are related through functions which are practically independent of the equation of state. These surprising universal $I\ensuremath{-}\ensuremath{\lambda}\ensuremath{-}Q$ relations pave the way for a better understanding of neutron stars, most notably via gravitational-wave emission. Gravitational-wave observations will probe highly dynamical binaries and it is important to understand whether the universality of the $I\ensuremath{-}\ensuremath{\lambda}\ensuremath{-}Q$ relations survives strong-field and finite-size effects. We apply a post-Newtonian-affine approach to model tidal deformations in compact binaries and show that the $I\ensuremath{-}\ensuremath{\lambda}$ relation depends on the inspiral frequency, but is insensitive to the equation of state. We provide a fit for the universal relation, which is valid up to a gravitational wave frequency of $\ensuremath{\sim}900\text{ }\text{ }\mathrm{Hz}$ and accurate to within a few percent. Our results strengthen the universality of $I\ensuremath{-}\ensuremath{\lambda}\ensuremath{-}Q$ relations, and are relevant for gravitational-wave observations with advanced ground-based interferometers. We also discuss the possibility of using the Love-compactness relation to measure the neutron-star radius with an uncertainty $\ensuremath{\lesssim}10%$ from gravitational-wave observations.
137 citations
TL;DR: Two mechanisms that can render Kerr black holes unstable in scalar-tensor gravity are uncovered, both associated with the presence of matter in the vicinity of the black hole and the fact that this introduces an effective mass for the scalar.
Abstract: We uncover two mechanisms that can render Kerr black holes unstable in scalar-tensor gravity, both associated with the presence of matter in the vicinity of the black hole and the fact that this introduces an effective mass for the scalar. Our results highlight the importance of understanding the structure of spacetime in realistic, astrophysical black holes in scalar-tensor theories.
131 citations
TL;DR: In a stationary electrovacuum, asymptotically flat black holes in scalar-tensor theories of gravity are described by the Kerr-Newman family of solutions as discussed by the authors.
Abstract: In a stationary electrovacuum, asymptotically flat black holes in scalar-tensor theories of gravity are described by the Kerr-Newman family of solutions, just as in general relativity We show that there exist two mechanisms which can render Kerr black holes unstable when matter is present in the vicinity of the black hxole, as this induces an effective mass for the scalar The first mechanism is a tachyonic instability that appears when the effective mass squared is negative, triggering the development of scalar hair---a black-hole version of ``spontaneous scalarization'' The second instability is associated with superradiance and is present when the effective mass squared is positive and when the black-hole spin exceeds a certain threshold The second mechanism is also responsible for a resonant effect in the superradiant scattering of scalar waves, with amplification factors as large as ${10}^{5}$ or more
129 citations
TL;DR: In this paper, a detailed study of compact boson stars and their gravitational-wave signatures in a fully relativistic setting is performed, a study which was lacking in the existing literature in many respects.
Abstract: Compact bosonic field configurations, or boson stars, are promising dark matter candidates which have been invoked as an alternative description for the supermassive compact objects in active galactic nuclei. Boson stars can be comparable in size and mass to supermassive objects, and they might be hard to distinguish by electromagnetic observations. However, boson stars do not possess an event horizon, and their global spacetime structure is different from that of a black hole. This leaves a characteristic imprint in the gravitational-wave emission, which can be used as a discriminant between black holes and other horizonless compact objects. Here we perform a detailed study of boson stars and their gravitational-wave signatures in a fully relativistic setting, a study which was lacking in the existing literature in many respects. We construct several fully relativistic boson star configurations, and we analyze their geodesic structure and free oscillation spectra, or quasinormal modes. We explore the gravitational and scalar response of boson star spacetimes to an inspiraling stellar-mass object and compare it to its black hole counterpart. We find that a generic signature of compact boson stars is the resonant-mode excitation by a small compact object on stable quasicircular geodesic motion.
124 citations
TL;DR: In this article, the authors present an overview of these applications and introduce extensions of the standard semianalytical methods to construct and solve the linearized field equations in curved space-time.
Abstract: Black-hole perturbation theory is a useful tool to investigate issues in astrophysics, high-energy physics, and fundamental problems in gravity. It is often complementary to fully-fledged nonlinear evolutions and instrumental to interpret some results of numerical simulations. Several modern applications require advanced tools to investigate the linear dynamics of generic small perturbations around stationary black holes. Here, we present an overview of these applications and introduce extensions of the standard semianalytical methods to construct and solve the linearized field equations in curved space–time. Current state-of-the-art techniques are pedagogically explained and exciting open problems are presented.
114 citations
TL;DR: In this article, it was shown that black holes can support massive graviton hair in theories of massive gravity, and that these hairy solutions are, most likely, the generic end state of the recently discovered monopole instability of Schwarzschild black holes.
Abstract: No-hair theorems exclude the existence of nontrivial scalar and massive vector hair outside four-dimensional, static, asymptotically flat black-hole spacetimes. We show, by explicitly building nonlinear solutions, that black holes can support massive graviton hair in theories of massive gravity. These hairy solutions are, most likely, the generic end state of the recently discovered monopole instability of Schwarzschild black holes in massive graviton theories.
88 citations
TL;DR: In this article, the imprint of super-radiant instabilities of nonevaporating primordial black holes (PBHs) on the spectrum of the cosmic microwave background (CMB) was considered.
Abstract: We consider the imprint of super-radiant instabilities of nonevaporating primordial black holes (PBHs) on the spectrum of the cosmic microwave background (CMB). In the radiation-dominated era, PBHs are surrounded by a roughly homogeneous cosmic plasma which endows photons with an effective mass through the plasma frequency. In this setting, spinning PBHs are unstable to a spontaneous spindown through the well-known ``black hole bomb'' mechanism. At the linear level, the photon density is trapped by the effective photon mass and grows exponentially in time due to super-radiance. As the plasma density declines due to cosmic expansion, the associated energy around PBHs is released and dissipated in the CMB. We evaluate the resulting spectral distortions of the CMB in the redshift range ${10}^{3}\ensuremath{\lesssim}z\ensuremath{\lesssim}2\ifmmode\times\else\texttimes\fi{}{10}^{6}$. Using the existing COBE/FIRAS bounds on CMB spectral distortions, we derive upper limits on the fraction of dark matter that can be associated with spinning PBHs in the mass range ${10}^{\ensuremath{-}8}{M}_{\ensuremath{\bigodot}}\ensuremath{\lesssim}M\ensuremath{\lesssim}0.2{M}_{\ensuremath{\bigodot}}$. For maximally spinning PBHs, our limits are much tighter than those derived from microlensing or other methods. Future data from the proposed PIXIE mission could improve our limits by several orders of magnitude.
86 citations
TL;DR: Quite remarkably, the axial and polar sectors of the gravitoelectromagnetic perturbations are isospectral to linear order in the spin, the first, fully consistent stability analysis of the Kerr-Newman metric.
Abstract: The most general stationary black-hole solution of Einstein-Maxwell theory in vacuum is the Kerr-Newman metric, specified by three parameters: mass M, spin J, and charge Q. Within classical general relativity, one of the most important and challenging open problems in black-hole perturbation theory is the study of gravitational and electromagnetic fields in the Kerr-Newman geometry, because of the indissoluble coupling of the perturbation functions. Here we circumvent this long-standing problem by working in the slow-rotation limit. We compute the quasinormal modes up to linear order in J for any value of Q and provide the first, fully consistent stability analysis of the Kerr-Newman metric. For scalar perturbations the quasinormal modes can be computed exactly, and we demonstrate that the method is accurate within 3% for spins J/J(max) ≲ 0.5, where J(max) is the maximum allowed spin for any value of Q. Quite remarkably, we find numerical evidence that the axial and polar sectors of the gravitoelectromagnetic perturbations are isospectral to linear order in the spin. The extension of our results to nonasymptotically flat space-times could be useful in the context of gauge-gravity dualities and string theory.
85 citations
TL;DR: In this article, the authors present an overview of these applications and introduce extensions of the standard semianalytical methods to construct and solve the linearized field equations in curved spacetime.
Abstract: Black-hole perturbation theory is a useful tool to investigate issues in astrophysics, high-energy physics, and fundamental problems in gravity. It is often complementary to fully-fledged nonlinear evolutions and instrumental to interpret some results of numerical simulations. Several modern applications require advanced tools to investigate the linear dynamics of generic small perturbations around stationary black holes. Here, we present an overview of these applications and introduce extensions of the standard semianalytical methods to construct and solve the linearized field equations in curved spacetime. Current state-of-the-art techniques are pedagogically explained and exciting open problems are presented.
TL;DR: Pani et al. as discussed by the authors studied scalar, electromagnetic and gravitational perturbations of Kerr-Newman black holes in the slow-rotation limit, and showed that the axial and polar sectors are isospectral at first order in the spin.
Abstract: In Einstein-Maxwell theory, according to classic uniqueness theorems, the most general stationary black-hole solution is the axisymmetric Kerr-Newman metric, which is defined by three parameters: mass, spin and electric charge. The radial and angular dependence of gravitational and electromagnetic perturbations in the Kerr-Newman geometry do not seem to be separable. In this paper we circumvent this problem by studying scalar, electromagnetic and gravitational perturbations of Kerr-Newman black holes in the slow-rotation limit. We extend (and provide details of) the analysis presented in a recent Letter [P. Pani, E. Berti, and L. Gualtieri, Phys. Rev. Lett. 110, 241103 (2013)]. Working at linear order in the spin, we present the first detailed derivation of the axial and polar perturbation equations in the gravito-electromagnetic case, and we compute the corresponding quasinormal modes for any value of the electric charge. Our study is the first self-consistent stability analysis of the Kerr-Newman metric, and in principle it can be extended to any order in the small rotation parameter. We find numerical evidence that the axial and polar sectors are isospectral at first order in the spin and speculate on the possible implications of this result.
TL;DR: In this paper, the authors explore the possibility of modifying Einstein's theory by adding solely non-dynamical fields, with the minimal requirement that the theory satisfies the weak equivalence principle and admits a covariant Lagrangian formulation.
Abstract: Modifications of general relativity usually include extra dynamical degrees of freedom, which to date remain undetected. Here we explore the possibility of modifying Einstein’s theory by adding solely nondynamical fields. With the minimal requirement that the theory satisfies the weak equivalence principle and admits a covariant Lagrangian formulation, we show that the field equations generically have to include higher-order derivatives of the matter fields. This has profound consequences for the viability of these theories. We develop a parametrization based on a derivative expansion and show that—to next-to-leading order—all theories are described by just two parameters. Our approach can be used to put stringent, theory-independent constraints on such theories, as we demonstrate using the Newtonian limit as an example.
TL;DR: In this article, it was shown that general relativity black holes are stable in this limit and that the spectrum of massive gravitational perturbations is isospectral, where the mass of the graviton is fixed in terms of the cosmological constant.
Abstract: Recent nonlinear completions of Fierz-Pauli theory for a massive spin-2 field include nonlinear massive gravity and bimetric theories. The spectrum of black-hole solutions in these theories is rich and comprises the same vacuum solutions of Einstein's gravity enlarged to include a cosmological constant. It was recently shown that Schwarzschild (de Sitter) black holes in these theories are generically unstable against spherical perturbations. Here we show that a notable exception is partially massless gravity, where the mass of the graviton is fixed in terms of the cosmological constant by ${\ensuremath{\mu}}^{2}=2\ensuremath{\Lambda}/3$ and a new gauge invariance emerges. We find that general relativity black holes are stable in this limit. Remarkably, the spectrum of massive gravitational perturbations is isospectral.
TL;DR: In this paper, a strong connection between tidal acceleration and superradiant scattering around spinning black holes is presented. But, this process is known for many years under a different guise: superradiance and it is not impossible that imprints of such a mechanism have already been observed.
Abstract: Tidal effects have long ago locked the Moon in a synchronous rotation with the Earth and progressively increase the Earth–Moon distance. This ‘tidal acceleration’ hinges on dissipation. Binaries containing black holes may also be tidally accelerated, dissipation being caused by the event horizon—a flexible, viscous one-way membrane. In fact, this process is known for many years under a different guise: superradiance. Here, we provide compelling evidence for a strong connection between tidal acceleration and superradiant scattering around spinning black holes. In general relativity, tidal acceleration is obscured by the gravitational-wave emission. However, when coupling to light scalar degrees of freedom is allowed, an induced dipole moment produces a ‘polarization acceleration’, which might be orders of magnitude stronger than tidal quadrupolar effects. Consequences for optical and gravitational-wave observations are intriguing and it is not impossible that imprints of such a mechanism have already been observed.
TL;DR: In this article, the infrared spectrum of scalar-dressed, asymptotically anti de Sitter (AdS) black brane (BB) solutions of effective holographic models is investigated.
Abstract: We investigate the infrared behavior of the spectrum of scalar-dressed, asymptotically Anti de Sitter (AdS) black brane (BB) solutions of effective holographic models. These solutions describe scalar condensates in the dual field theories. We show that for zero charge density the ground state of these BBs must be degenerate with the AdS vacuum, must satisfy conformal boundary conditions for the scalar field and it is isolated from the continuous part of the spectrum. When a finite charge density is switched on, the ground state is not anymore isolated and the degeneracy is removed. Depending on the coupling functions, the new ground state may possibly be energetically preferred with respect to the extremal Reissner-Nordstrom AdS BB. We derive several properties of BBs near extremality and at finite temperature. As a check and illustration of our results we derive and discuss several analytic and numerical, BB solutions of Einstein-scalar-Maxwell AdS gravity with different coupling functions and different potentials. We also discuss how our results can be used for understanding holographic quantum critical points, in particular their stability and the associated quantum phase transitions leading to superconductivity or hyperscaling violation.
TL;DR: In this article, the infrared spectrum of scalar-dressed, asymptotically anti de Sitter (AdS) black brane (BB) solutions of effective holographic models is investigated.
Abstract: We investigate the infrared behavior of the spectrum of scalar-dressed, asymptotically Anti de Sitter (AdS) black brane (BB) solutions of effective holographic models. These solutions describe scalar condensates in the dual field theories. We show that for zero charge density the ground state of these BBs must be degenerate with the AdS vacuum, must satisfy conformal boundary conditions for the scalar field and it is isolated from the continuous part of the spectrum. When a finite charge density is switched on, the ground state is not anymore isolated and the degeneracy is removed. Depending on the coupling functions, the new ground state may possibly be energetically preferred with respect to the extremal Reissner-Nordstrom AdS BB. We derive several properties of BBs near extremality and at finite temperature. As a check and illustration of our results we derive and discuss several analytic and numerical, BB solutions of Einstein-scalar-Maxwell AdS gravity with different coupling functions and different potentials. We also discuss how our results can be used for understanding holographic quantum critical points, in particular their stability and the associated quantum phase transitions leading to superconductivity or hyperscaling violation.
TL;DR: In this paper, the authors consider a non-linear approach in which two point particles traveling in an AdS-soliton background suddenly collide to form an object at rest (presumably a black hole for large enough center-of-mass energies).
Abstract: We study the gravitational dual of a high-energy collision in a confining gauge theory. We consider a linearized approach in which two point particles traveling in an AdS-soliton background suddenly collide to form an object at rest (presumably a black hole for large enough center-of-mass energies). The resulting radiation exhibits the features expected in a theory with a mass gap: late-time power law tails of the form t^(-3/2), the failure of Huygens' principle and distortion of the wave pattern as it propagates. The energy spectrum is exponentially suppressed for frequencies smaller than the gauge theory mass gap. Consequently, we observe no memory effect in the gravitational waveforms. At larger frequencies the spectrum has an upward-stairway structure, which corresponds to the excitation of the tower of massive states in the confining gauge theory. We discuss the importance of phenomenological cutoffs to regularize the divergent spectrum, and the aspects of the full non-linear collision that are expected to be captured by our approach.
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TL;DR: In this paper, the authors discuss recent developments in gravitational fields with sources, regular black holes, quasiblack holes, and analogue black holes related to the talks presented at the corresponding Parallel Session AT3 of the 13th Marcel Grossmann Meeting.
Abstract: We discuss recent developments in gravitational fields with sources, regular black holes, quasiblack holes, and analogue black holes, related to the talks presented at the corresponding Parallel Session AT3 of the 13th Marcel Grossmann Meeting.
TL;DR: In this article, it was shown that the quasi-adiabatic evolution of the inspiral is mainly driven by DM accretion into the small compact object and by dynamical friction, rather than by gravitational-wave radiation reaction.
Abstract: The nature and properties of dark matter (DM) are both outstanding issues in physics. Besides clustering in halos, the universal character of gravity implies that self-gravitating compact DM configurations might be spread throughout the universe. The astrophysical signature of these objects may be used to probe fundamental particle physics, or even to provide an alternative description of compact objects in active galactic nuclei. Here we discuss the most promising dissection tool of these configurations: the inspiral of a compact stellar-size object and consequent gravitational-wave emission. The inward motion of this "test probe" encodes unique information about the nature of the central, supermassive DM configuration. When the probe travels through some compact DM profile we show that, within a Newtonian approximation, the quasi-adiabatic evolution of the inspiral is mainly driven by DM accretion into the small compact object and by dynamical friction, rather than by gravitational-wave radiation-reaction. These effects circularize the orbits and leave a peculiar imprint on the gravitational waves emitted at late time. When accretion dominates, the frequency and the amplitude of the gravitational-wave signal produced during the latest stages of the inspiral are nearly constant. In the exterior region we study a relativistic model in which the inspiral is driven by the emission of gravitational and scalar waves. Resonances in the energy flux appear whenever the orbital frequency matches the mass of the DM particle and they correspond to the excitation of the central object's quasinormal frequencies. Unexpectedly, these resonances can lead to large dephasing with respect to standard inspiral templates, to such an extent as to prevent detection with matched filtering techniques. We discuss some observational consequences of these effects for gravitational-wave detection.
01 Jan 2013
TL;DR: In this paper, the authors show that objects with large rotation have a strong ergoregion instability when rapidly spinning, and that these instability scales can be of the order of 0.1 seconds to 1 week for objects with m assM = 1 10 6 M and angularm om entum J > 0:4M 2.
Abstract: M ost ofthe properties ofblack holes can be m im icked by horizonless com pact objects such as gravastarsand boson stars.W e show thatthese ultra-com pactobjectsdevelop a strong ergoregion instability when rapidly spinning. Instability tim escales can be ofthe order of0.1 seconds to 1 week forobjects with m assM = 1 10 6 M and angularm om entum J > 0:4M 2 . Thisprovidesa strong indication that ultra-com pact objects with large rotation are black holes. Explosive events due to ergoregion instability have a well-de ned gravitational-wave signature. These eventscould be detected by next-generation gravitational-wave detectorssuch asAdvanced LIG O orLISA.