We present a complete discussion of the boundary term in action functional general relativity when includes null segments addition to more usual timelike and spacelike segments. confirm that ambiguities appear contribution from segment, because it depends on an arbitrary choice parametrization for generators. also show similar codimension-two surface at which segment is joined another (spacelike, timelike, or null) segment. The ambiguity can be tamed by insisting generators affinely...

Scalar-tensor theories of gravity are natural phenomenological alternatives to General Relativity, where the gravitational interaction is mediated by a scalar degree freedom, besides usual tensor gravitons. In regions parameter space these constraints from both solar system experiments and binary-pulsar observations satisfied, we show that binaries neutron stars present marked differences Relativity in late-inspiral merger phases. particular, phenomena related spontaneous scalarization...

We study the merger of binary neutron stars using different realistic, microphysical nuclear equations state, as well incorporating magnetic field and neutrino cooling effects. In particular, we concentrate on influence equation state gravitational wave signature also its role, in combination with electromagnetic effects, determining properties hypermassive star resulting from merger, production neutrinos, characteristics ejecta system. The find are consistent other recent studies that soft...

We describe the behavior of 5-dimensional black strings, subject to Gregory-Laflamme instability. Beyond linear level, evolving strings exhibit a rich dynamics, where at intermediate stages horizon can be described as sequence 3-dimensional spherical holes joined by string segments. These segments are themselves instability, resulting in self-similar cascade, ever-smaller satellite form connected ever-thinner This is akin formation low-viscosity fluid streams Rayleigh-Plateau The simulation...

We investigate the influence of magnetic fields upon dynamics of, and resulting gravitational waves from, a binary neutron-star merger in full general relativity coupled to ideal magnetohydrodynamics. consider two scenarios: one where stars have aligned poloidal without. Both mergers result strongly differentially rotating object. In comparison nonmagnetized scenario, delay stars. During after we observe phenomena driven by field, including Kelvin-Helmholtz instabilities shear layers,...

Many astrophysical processes involving magnetic fields and quasi-stationary are well described when assuming the fluid as a perfect conductor. For these systems, ideal-magnetohydrodynamics (MHD) description captures dynamics effectively number of well-tested techniques exist for its numerical solution. Yet, there several which highly dynamical resistive effects can play an important role. The modeling such non-ideal MHD flows is significantly more challenging resistivity expected to change...

We present a straightforward approach for estimating the final black hole spin of binary coalescence with arbitrary initial masses and spins. Making some simple assumptions, we estimate angular momentum to be sum individual spins plus orbital test particle orbiting at last stable orbit around Kerr parameter hole. The formula obtain is able reproduce reasonable accuracy results from available numerical simulations, but, more importantly, it can used investigate what configurations might give...

Supermassive black holes are found at the centers of most galaxies and their inspiral is a natural outcome when merge. The these systems utmost astrophysical importance as prodigious producers gravitational waves in possible role energetic electromagnetic events. We study such binary hole coalescence under influence an external magnetic field produced by expected circumbinary disk surrounding them. Solving Einstein equations to describe spacetime using force-free approach for fields tenuous...

We present a framework to study generic neutron-star binaries in scalar-tensor theories of gravity. Our formalism achieves this goal by suitably interfacing post-Newtonian orbital evolution (described set ordinary differential equations) with nonlinear algebraic equations, which provide description the scalar charge each binary's component along terms isolated-star data. validate semianalytical procedure comparing its results those fully general-relativistic simulations, and use it...

We study the merger of binary neutron stars with different mass ratios adopting three realistic, microphysical nuclear equations state, as well incorporating neutrino cooling effects. In particular, we concentrate on influence equation state gravitational wave signature and also its role, in combination cooling, determining properties resulting hypermassive star, neutrinos produced, ejected material. The ejecta find are consistent other recent studies that small produce more than equal cases...

We construct boson stars in global anti--de Sitter (AdS) space and study their stability. Linear perturbation results suggest that the ground state along with first three excited are stable. evolve some of these solutions nonlinear stability light recent work [9], arguing a weakly turbulent instability drives scalar perturbations AdS to black hole formation. However, evolutions nonlinearly stable immune for sufficiently small perturbation. Furthermore, studies find other families initial...

We study head-on collisions of boson stars in three dimensions. consider evolutions two which may differ their phase or have opposite frequencies but are otherwise identical. Our studies show that these differences result different late time behavior and gravitational wave output.

The late stage of an inspiraling neutron-star binary gives rise to strong gravitational wave emission due its highly dynamic, gravity. Moreover, interactions between the stellar magnetospheres can produce considerable electromagnetic radiation. We study this scenario using fully general relativistic, resistive magnetohydrodynamic simulations. show that these extract kinetic energy from system, dissipate heat, and power radiative Poynting flux, as well develop current sheets. Our results...

We model two mergers of orbiting binary neutron stars, the first forming a black hole and second differentially rotating star. extract gravitational waveforms in wave zone. Comparisons to post-Newtonian analysis allow us compute orbital kinematics, including trajectories eccentricities. verify our code by evolving single stars extracting radial perturbative modes, which compare very well results from perturbation theory. The Einstein equations are solved first-order reduction generalized...

For a real massless scalar field in general relativity with negative cosmological constant, we uncover large class of spherically symmetric initial conditions that are close to anti--de Sitter space (AdS) but whose numerical evolution does not result black hole formation. According the AdS/conformal theory (CFT) dictionary, these bulk solutions dual states strongly interacting boundary CFT fail thermalize at late times. Furthermore, as stationary, they define dynamical configurations do...

The interaction of black holes with ambient magnetic fields is important for a variety highly energetic astrophysical phenomena. We study this within the force-free approximation in which tenuous plasma assumed to have zero inertia. Blandford and Znajek (BZ) used approach demonstrate conversion some hole's energy into electromagnetic Poynting flux stationary axisymmetric single hole systems. adopt extend it examine asymmetric and, most importantly, dynamical systems by implementing fully...

Throughout the Universe many powerful events are driven by strong gravitational effects that require general relativity to fully describe them. These include compact binary mergers, black hole accretion and stellar collapse, where velocities can approach speed of light, extreme fields --$\Phi_{\rm Newt}/c^2 \simeq 1$-- mediate interactions. Many these processes trigger emission across a broad range electromagnetic spectrum. Compact binaries further source wave could directly be detected in...

We incorporate realistic, tabulated equations of state into fully relativistic simulations magnetized neutron stars along with a neutrino leakage scheme which accounts for cooling via emission. Both these improvements utilize open-source code (GR1D) and tables from the Web site stellarcollapse.org. Our implementation makes use novel method calculation optical depth, simplifies its distributed adaptive mesh refinement. present various tests without magnetization preliminary results both...

We describe the full nonlinear development of superradiant instability for a charged massless scalar field coupled to general relativity and electromagnetism, in vicinity Reissner-Nordstr\"om-anti--de Sitter black hole. The presence negative cosmological constant provides natural context considering perfectly reflecting boundary conditions studying dynamics as interacts repeatedly with At early times, small perturbations grow expected from linearized studies. Backreaction then causes hole...

The measurement of multiple ringdown modes in gravitational waves from binary black hole mergers will allow for testing the fundamental properties holes general relativity and to constrain modified theories gravity. To enhance ability Advanced LIGO/Virgo perform such tasks, we propose a coherent mode stacking method search chosen target within collection merger events. We first rescale each signal so that them has same frequency then sum waveforms constructively. A crucial element realize...

Gravitational-wave observations of black hole ringdowns are commonly used to characterize binary merger remnants and test general relativity. These analyses assume linear perturbation theory, in particular that the ringdown can be described terms quasinormal modes even for times approaching merger. Here we investigate a nonlinear effect during ringdown, namely how mode excited at early excite additional as it is absorbed by hole. This third-order secular effect: change black-hole mass causes...

A bstract The dynamics of black hole horizons has recently been linked to that Carrollian fluids. This results in a dictionary between geometrical quantities and those fluid with unusual properties due its underlying symmetries. In this work we explore relation dynamical settings the interest shedding light on either side by relevant observations. particular: discuss how null surface where evolves is affected behavior; fluid’s equilibration are tied teleological considerations; connection...

The majority of extensions to general relativity (GR) display mathematical pathologies-higher derivatives, character change in equations that can be classified within partial differential equation theory, and even unclassifiable ones-that cause severe difficulties study them, especially dynamical regimes. We present here an approach enables their consistent treatment extraction physical consequences. illustrate this method the context single merging black holes a highly challenging beyond GR theory.

We study electromagnetic and gravitational properties of anti–de Seitter (AdS) black shells (also referred to as AdS bubbles)—a class quantum gravity motivated hole mimickers, that in the classical limit are described ultracompact matter. find their remarkably similar holes. then discuss extent which these objects distinguishable from holes, both for intrinsic interest within shell model, a guide efforts other subclasses exotic compact (ECOs). photon rings lensing band characteristics,...