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We show that gravitational wave radiative patterns from a point test particle falling radially into a Schwarzschild black hole, as derived by Davis, Ruffini, Press and Price [M. Davis et al., Phys. Rev. Lett. 27, 1466 (1971).], are present in the nonlinear regime of head-on mergers of black holes. We use the Bondi-Sachs characteristic formulation and express the gravitational wave luminosity and the net momentum flux in terms of the news functions. We then evaluate the ($-2$)-spin-weighted $\ell$-multipole decomposition of these quantities via exact expressions valid in the nonlinear regime and defined at future null infinity. Our treatment is made in the realm of Robinson-Trautman dynamics, with characteristic initial data corresponding to the head-on merger of two black holes. We consider mass ratios in the range $0.01\le \alpha \le 1$. We obtain the exponential decay with $\ell$ of the total energy contributed by each multipole $\ell$, with an accurate linear correlation in the log-linear plot of the points up to $\alpha \simeq 0.7$. Above this mass ratio the contribution of the odd modes to the energy decreases faster than that of the even modes, leading to the breaking of the linear correlation; for $\alpha =1$ the energy in all odd modes is zero. The dominant contribution to the total radiated energy comes from the quadrupole mode $\ell =2$ corresponding, for instance, to about $\simeq 84\%$ for small mass ratios up to $\simeq 99.8\%$ for the limit case $\alpha =1$. The total rescaled radiated energy $E_W^{\text{total}}/m_0{\alpha }^2$ decreases linearly with decreasing $\alpha$, yielding for the point particle limit $\alpha \to 0$ the value $\simeq 0.0484$, about 5 times larger than the result of Davis et al. [1]. The mode decomposition of the net momentum flux and of the associated gravitational wave impulses results in an adjacent-even-odd mode-mixing pattern. We obtain that the impulses contributed by each $(\ell ,\ell +1)$ mixed mode also accurately satisfy the exponential decay with $\ell$, for the whole mass ratio domain considered, $0.01\le \alpha <1$. The (2, 3) mode contributions to the total impulses are dominant. The mode-mixing effect can also be seen in the decomposition of the net kick velocity imparted to the system by the gravitational wave emission. The mixed mode impulses reach a maximum at $\alpha \simeq 0.7$; for $\alpha >0.7$ the impulses decrease and are zero in the equal mass case, due to the decrease to zero of the odd modes of the news functions.

In the general theory of relativity, gravitational waves have two possible polarizations, which are transverse and traceless with helicity $\pm 2$. Some alternative theories contain additional helicity 0 and helicity $\pm 1$ polarization modes. Here, we consider a hypothetical “pure vector” theory in which gravitational waves have only two possible polarizations, with helicity $\pm 1$. We show that if these polarizations are allowed to rotate as the wave propagates, then for certain source locations on the sky, the strain outputs of three ideal interferometric gravitational wave detectors can exactly reproduce the strain outputs predicted by general relativity.

In the present work, we have studied the spectrum of the primordial gravitational waves due to magnetic instability in the presence of neutrino asymmetry. The magnetic instability generates a helical magnetic field on a large scale. The anisotropic stress generated by the magnetic field is shown to be a source of primordial gravitational waves at the time of matter-neutrino decoupling. We expect that the theoretically predicted gravitational waves by this mechanism may be detected by the Square Kilometer Array or pulsar time array observations. We also compare our findings with the results obtained by the earlier work where the effect of magnetic instability was not considered.

We present a new method for extracting the instantaneous orbital axis only from gravitational wave strains of precessing binary systems observed from a particular observer direction. This method enables us to reconstruct the coprecessing frame waveforms only from observed strains for the ideal case with the high signal-to-noise ratio. Specifically, we do not presuppose any theoretical model of the precession dynamics and coprecessing waveforms in our method. We test and measure the accuracy of our method using the numerical relativity simulation data of precessing binary black holes taken from the SXS Catalog. We show that the direction of the orbital axis is extracted within $\approx 0.07\mathrm{rad}$ error from gravitational waves emitted during the inspiral phase. The coprecessing waveforms are also reconstructed with high accuracy; the mismatch (assuming white noise) between them and the original coprecessing waveforms is typically a few times ${10}^{-3}$ including the merger-ringdown phase, and can be improved by an order of magnitude focusing only on the inspiral waveform. In this method, the coprecessing frame waveforms are not only the purely technical tools for understanding the complex nature of precessing waveforms but also direct observables.

Impulsive, nondiverging, Petrov-Segre type-N gravitational wave solutions to a general massive three-dimensional gravity in the de Sitter, anti–de Sitter, and flat Minkowski backgrounds are constructed in a unified manner by using the exterior algebra of differential forms.

The scrutiny of black hole perturbations and its application to testing gravity theories have been a very crucial frontier in modern physics. In this paper we study the quasinormal modes (QNM) of massless scalar perturbations for various charged black holes in the Palatini-type theories of gravity. Specifically, we consider the Palatini $f(R)$ theory coupled with Born-Infeld nonlinear electrodynamics and the Eddington-inspired-Born-Infeld gravity (EiBI) coupled with Maxwell electromagnetic fields. Special attention is paid to Einstein-Born-Infeld black holes, EiBI charged black holes and Born-Infeld charged black holes within $R\pm R^2$ gravity. These charged black holes are shown to be stable and their quasi-normal frequencies, including the frequencies in the eikonal limit, are calculated by using the Wentzel-Kramers-Brillouin (WKB) method up to the 6th order. We compare the results with the Reissner-Nordström charged black hole and prove that both the changes in the gravity sector and the matter sector of the action alter the QNM spectra.

A model of spontaneous Lorentz violation in four dimension is given, which seems to provide a Lorentz invariant effective theory. An SU(2) Yang-Mills gauge field and an auxiliary U(1) vector field generate gravity and other interactions when they have vacuum expectation values. The emergent gravity is equivalent to conventional general relativity up to the third order terms in the Lagrangian. The coupling to matter, including spin-$1/2$ fermions, is also given correctly to this level. It remains to be seen whether this formalism reproduces the properties of black holes and other consequences obtained from Einstein’s theory.

We analyze the spin stability of a binary black hole coalescence when the binary system is described by the Post-Newtonian (PN) equations in the adiabatic regime. The main idea in this work is to make a massive exploration of the solution space in search of chaos. For that, we evolve the PN equations using a CUDA implementation of the RKF78 scheme and study the dynamical behavior of the system. Each initial spin configuration run in the GPU is composed by more than 80000 simulations. The chaos indicator used to characterize the degree of separation of two infinitesimally close trajectories is the Lyapunov exponent. We find zones in the solution space where the separation between nearby trajectories reaches several orders of magnitude bigger than the initial separation. Also, we note that the chaotic behavior can be observed in forward as well as in backward evolution.

The gravitational waves (GWs) from the merger events of binary black hole (BH-BH) and binary neutron star (NS-NS) have been detected by LIGO-Virgo (O1 and O2). Besides, GWs from the NS-BH mergers have been recently reported and might be hopefully confirmed in the near future. The mass distributions of these merger events are poorly understood now; thus, LIGO-Virgo adopted a simple cut rule for chirp mass ($\mathcal{M}$) to distinguish the GW event candidates: $\mathcal{M}<2.1M_{\odot }$ for NS-NS, $2.1M_{\odot }<\mathcal{M}<4.35M_{\odot }$ for NS-BH, and $\mathcal{M}>4.35M_{\odot }$ for BH-BH. We tested its validity by simulating the chirp mass ($\mathcal{M}$) distributions in two synthetic models, i.e., Model Galaxy and Model LIGO, in which the masses of BHs and NSs are observed by the electromagnetic spectrum observations in our Galaxy and inferred by LIGO-Virgo detection (O1 and O2), respectively. The simulation shows that it is unsuitable for Model LIGO due to the BHs inferred by LIGO-Virgo are usually bigger than those in our Galaxy, and $\mathcal{M}$ of NS-BH events would distribute in the range of $2.1M_{\odot }<\mathcal{M}<7.3M_{\odot }$, which partially overlaps with those of BH-BH events. Therefore, we suggest that the new searching round of LIGO-Virgo (e.g., O3) should carefully seek out the underlying NS-BH candidates in the range of $2.1M_{\odot }<\mathcal{M}<7.3M_{\odot }$.

General cylindrical waves are the simplest axisymmetrical gravitational waves that contain both $+$ and $\times$ modes of polarization. In this paper, we have studied the evolution of general cylindrical gravitational waves in the realm of the characteristic scheme with a numerical code based on the Galerkin-Collocation method. The investigation consists of the numerical realization of concepts such as Bondi mass and the news functions adapted to cylindrical symmetry. The Bondi mass decays due to the presence of the news functions associated with both polarization modes. We have interpreted each polarization mode as channels from which mass is extracted. Under this perspective, we have presented the enhancement effect of the polarization mode $+$ due to the nonlinear interaction with the mode $\times$. After discussing the role of matter in cylindrical symmetry, we have extended the numerical code to include electromagnetic fields.

In a recent paper [M. Chernicoff et al., J. High Energy Phys. 08 (2018) 087], we explored the space of solutions of the exotic massive 3D gravity theory proposed in [M. Özkan et al., J. High Energy Phys. 08 (2018) 035]. We showed that the theory admits a rich space of vacua, including asymptotically anti de-Sitter (AdS) geometries obeying different types of boundary conditions. The examples include black holes dressed with low decaying gravitons. Based on what happens in other theories of massive gravity, we conjectured that such geometries appear on a curve of the parameter space (chiral curve) where the exotic massive gravity on AdS with sufficiently strong boundary conditions results dual to a 2D chiral conformal field theory. Here, we show that this expectation is consistent with the conserved charges and thermodynamical properties of the black holes of the theory, which have recently been computed [Mann et al.,Energy of asymptotically AdS black holes in exotic massive gravity and its log-extension, arXiv:1812.09525.]. When the boundary conditions are relaxed relative to the standard Brown-Henneaux boundary conditions, the theory exhibits solutions consistent with the definition of the so-called log-gravity. The asymptotic behavior of these solutions presents a logarithmic term in the Fefferman-Graham expansion that, nonetheless, is compatible with the AdS asymptotic symmetries. This long range interaction is due to a mode of the massive gravity that becomes massless precisely on the chiral curve. We construct exact solutions exhibiting this behavior, which admit to be interpreted as fully backreacting gravitational waves propagating on an extremal black hole, and carrying nonvanishing gravitational energy. We also discuss other vacua of the theory, such as $\mathrm{warped}\text{−}{\mathrm{AdS}}_3$ black holes, gravitational waves on such backgrounds, ${\mathrm{AdS}}_2\times S^1$ spaces, and black holes in ${\mathrm{dS}}_3$ space.

We investigate the gravitational memory effect for linearized perturbations off of Minkowski space in odd spacetime dimensions $d$ by examining the effects of gravitational radiation from classical point particle scattering. We also investigate analogous memory effects for electromagnetic and scalar radiation. We find that there is no gravitational memory effect in all odd dimensions. For scalar and electromagnetic fields, there is no memory effect for $d\ge 7$; for $d=3$ there is an infinite momentum memory effect, whereas for $d=5$ there is no momentum memory effect but the displacement of a test particle will grow unboundedly with time. Our results are further elucidated by analyzing the memory effect for any slowly moving source of compact spatial support in odd dimensions.

Gravitational wave science is dependent upon expensive numerical simulations, which provide the foundational understanding of binary merger radiation needed to interpret observations of massive binary black holes. The high cost of these simulations limits large-scale campaigns to explore the binary black hole parameter space. Surrogate models have been developed to efficiently interpolate between simulation results, but these models require a sufficiently comprehensive sample to train on. Acquisition functions can be used to identify points in the domain for simulation. We develop a new acquisition function which accounts for the cost of simulating new points. We show that when applied to a 3D domain of binary mass ratio and dimensionless nonprecessing spins, the accumulated cost of simulation is reduced by a factor of about 10.

In this manuscript, we examine the gravitational radiation emitted by binary systems using an Unruh-DeWitt detector coupled to gravitons. Recoil is incorporated into the system via a kinetic energy term in the energy gap of the detector. We find a splitting of the gravitational wave frequency due to the recoil. Implications for the recoil velocity and force are discussed.

We report long-term simulations of black hole-neutron star binary mergers where the neutron star possesses an asymmetric magnetic field dipole. Focusing on the scenario where the neutron star is tidally disrupted by the black hole, we track the evolution of the binary up to $\approx 100\mathrm{ms}$ after the merger. We uncover more than one episode of thermally driven winds being launched along a funnel wall in all these cases beginning from $\approx 25\mathrm{ms}$ after the merger. On the other hand, we are unable to conclude presently whether the amount of ejected mass increases with the degree of asymmetry. A large-scale magnetic field configuration in the poloidal direction is formed along the funnel wall accompanied by the generation of a large Poynting flux. The magnetic field in the accretion disk around the black hole remnant is amplified by both magnetic winding and the nonaxisymmetric magnetorotational instability (MRI). The MRI growth is estimated to be in the ideal magnetohydrodynamics (MHD) regime and thus would be free from significant effects induced by potential neutrino radiation. However, the asymmetry in the magnetic field leads to increased turbulence, which causes the vertical magnetic field in the accretion disk to grow largely in a nonlinear manner.

We present a formulation of Einstein-Maxwell vacuum fields due to plane fronted electromagnetic waves sharing their wave fronts with gravitational waves. This is based on a recent geometrical reconstruction of plane fronted wave fields by the authors that clearly identifies the cases in which the wave fronts collide or do not collide. In the former case our construction suggests an explicit example of a searchlight beam, accompanied by gravitational radiation, which sweeps across the sky. This gravitoelectromagnetic searchlight and its properties are described in detail.

We describe and demonstrate a method to control a detuned movable-mirror Fabry-Pérot cavity using radiation pressure in the presence of a strong optical spring. At frequencies below the optical spring resonance, self-locking of the cavity is achieved intrinsically by the optomechanical (OM) interaction between the cavity field and the movable end mirror. The OM interaction results in a high rigidity and reduced susceptibility of the mirror to external forces. However, due to a finite delay time in the cavity, this enhanced rigidity is accompanied by an antidamping force, which destabilizes the cavity. The cavity is stabilized by applying external feedback in a frequency band around the optical spring resonance. The error signal is sensed in the amplitude quadrature of the transmitted beam with a photodetector. An amplitude modulator in the input path to the cavity modulates the light intensity to provide the stabilizing radiation pressure force.

We follow the life of a generic primordial perturbation mode (scalar or tensor) subject to modified dispersion relations (MDR), as its proper wavelength is stretched by expansion. A necessary condition ensuring that traveling waves can be converted into standing waves is that the mode starts its life deep inside the horizon and in the trans-Planckian regime, then leaves the horizon as the speed of light corresponding to its growing wavelength drops, to eventually become cis-Planckian whilst still outside the horizon, and finally reenter the horizon at late times. We find that scalar modes in the observable range satisfy this condition, thus ensuring the viability of MDR models in this respect. For tensor modes we find a regime in which this does not occur, but in practice it can only be realized for wavelengths in the range probed by future gravity wave experiments if the quantum gravity scale experienced by gravity waves goes down to the PeV range. In this case traveling—rather than standing—primordial gravity waves could be the telltale signature of MDR scenarios.