36723416
Gravitational wave signatures of the absence of an event horizon: Nonradial oscillations of a thin-shell gravastar
Gravitational waves from compact objects provide information about their structure, probing deep into strong-gravity regions. Here we illustrate how the presence or absence of an event horizon can produce qualitative differences in the gravitational waves emitted by ultracompact objects. In order to set up a straw-man ultracompact object with no event horizon, but which is otherwise almost identical to a black hole, we consider a nonrotating thin-shell model inspired by Mazur and Mottola's gravastar, which has a Schwarzschild exterior, a de Sitter interior and an infinitely thin shell with finite tension separating the two regions. As viewed from the external space-time, the shell can be located arbitrarily close to the Schwarzschild radius, so a gravastar might seem indistinguishable from a black hole when tests are only performed on its external metric. We study the linearized dynamics of the system, and, in particular, the junction conditions connecting internal and external gravitational perturbations. As a first application of the formalism we compute polar and axial oscillation modes of a thin-shell gravastar. We show that the quasinormal mode spectrum is completely different from that of a black hole, even in the limit when the surface redshift becomes infinite. Polar quasinormal modes depend on the equation of state of matter on the shell and can be used to distinguish between different gravastar models. Our calculations suggest that low-compactness gravastars could be unstable when the sound speed on the shell v_s/c ≳ 0.92.
118376658
Gravitational signature of Schwarzschild black holes in dynamical Chern-Simons gravity
Dynamical Chern-Simons gravity is an extension of general relativity in which the gravitational field is coupled to a scalar field through a parity-violating Chern-Simons term. In this framework, we study perturbations of spherically symmetric black hole spacetimes, assuming that the background scalar field vanishes. Our results suggest that these spacetimes are stable, and small perturbations die away as a ringdown. However, in contrast to standard general relativity, the gravitational waveforms are also driven by the scalar field. Thus, the gravitational oscillation modes of black holes carry imprints of the coupling to the scalar field. This is a smoking gun for Chern-Simons theory and could be tested with gravitational-wave detectors, such as LIGO or LISA. For negative values of the coupling constant, ghosts are known to arise, and we explicitly verify their appearance numerically. Our results are validated using both time evolution and frequency domain methods.
118397658
Anti de Sitter black holes and branes in dynamical Chern-Simons gravity: perturbations, stability and the hydrodynamic modes
Dynamical Chern-Simons (DCS) theory is an extension of General Relativity in which the gravitational field is coupled to a scalar field through a parity violating term. We study perturbations of anti-de Sitter black holes and branes in such a theory, and show that the relevant equations reduce to a set of coupled ODEs which can be solved efficiently through a series expansion. We prove numerically that black holes and branes in DCS gravity are stable against gravitational and scalar perturbations in the entire parameter space. Furthermore, by applying the AdS/CFT duality, were late black hole perturbations to hydrodynamic quantities in the dual field theory, which is a (2 + 1)-dimensional isotropic fluid with broken spatial parity. The Chern-Simons term does not affect the entropy to viscosity ratio and the relaxation time, but instead quantities that enter the shear mode at order q4 in the small momentum limit, for example the Hall viscosity and other quantities related to second and third order hydrodynamics. We provide explicit corrections to the gravitational hydrodynamic mode to first relevant order in the couplings.
119202445
Instability of hyper-compact Kerr-like objects
Viable alternatives to astrophysical black holes include hyper-compact objects without horizon, such as gravastars, boson stars, wormholes and superspinars. The authors have recently shown that typical rapidly spinning gravastars and boson stars develop a strong instability. That analysis is extended in this paper to a wide class of horizonless objects with approximate Kerr-like geometry. A detailed investigation of wormholes and superspinars is presented, using plausible models and mirror boundary conditions at the surface. Like gravastars and boson stars, these objects are unstable with very short instability timescales. This result strengthens previous conclusions that observed hyper-compact astrophysical objects with large rotation are likely to be black holes.
118714531
Infrared behavior of scalar condensates in effective holographic theories
A bstractWe 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.
119258640
Eddington-inspired Born-Infeld gravity: Phenomenology of nonlinear gravity-matter coupling
Viable corrections to the matter sector of Poisson's equation may result in qualitatively different astrophysical phenomenology, for example the gravitational collapse and the properties of compact objects can change drastically. We discuss a class of modified non-relativistic theories and focus on a relativistic completion, Eddington-inspired Born-Infeld gravity. This recently proposed theory is equivalent to General Relativity in vacuum, but its non-trivial coupling to matter prevents singularities in early cosmology and in the non-relativistic collapse of non-interacting particles. We extend our previous analysis, discussing further developments. We present a full numerical study of spherically symmetric non-relativistic gravitational collapse of dust. For any positive coupling, the final state of the collapse is a regular pressureless star rather than a singularity. We also argue that there is no Chandrasekhar limit for the mass of non-relativistic white dwarf in this theory. Finally, we extend our previous results in the fully relativistic theory by constructing static and slowly rotating compact stars governed by nuclear-physics inspired equations of state. In the relativistic theory, there exists an upper bound on the mass of compact objects, suggesting that black holes can still be formed in the relativistic collapse.
56041533
GEMS: Underwater spectrometer for long-term radioactivity measurements
Abstract GEMS (Gamma Energy Marine Spectrometer) is a prototype of an autonomous radioactivity sensor for underwater measurements, developed in the framework for a development of a submarine telescope for neutrino detection (KM3NeT Design Study Project). The spectrometer is highly sensitive to gamma rays produced by 40 K decays but it can detect other natural (e.g., 238 U, 232 Th) and anthropogenic radio-nuclides (e.g., 137 Cs). GEMS was firstly tested and calibrated in the laboratory using known sources and it was successfully deployed for a long-term (6 months) monitoring at a depth of 3200 m in the Ionian Sea (Capo Passero, offshore Eastern Sicily). The instrument recorded data for the whole deployment period within the expected specifications. This monitoring provided, for the first time, a continuous time-series of radioactivity in deep-sea.
123140529
DoI position resolution in a continuous LaBr3(Ce) scintillation crystal for γ-ray imaging
A Monte Carlo simulation of a 50 mm × 50 mm × 4 mm continuous crystal has been developed to investigate the correlation between the scintillation light width ( σ ) and the Depth of Interaction ( DoI ) within the crystal. Our studies are based on a LaBr 3 (Ce) crystal, in order to take advantage of its high light yield to reduce the statistic uncertainties on the estimators of the σ . The first one, the standard deviation of the light distribution, has demonstrated a poor, though linear, correlation to DoI , that is not experimentally detectable. Otherwise, the second estimator, N I , is the ratio of the total number of photoelectrons to the maximum number of photoelectrons collected from a single anode, in a scintillation event. N I has been found to have the best correlation to DoI , that provides 2 mm resolution.