2022-0901

• [2208.14467] Supergravity p-branes with scalar charge
Abstract: Standard dilatonic supergravity p-branes have scalar charges that are not independent parameters, but are determined by the brane tension and Page charges. This feature can be traced to the no-hair theorem in the four-dimensional Einstein-scalar gravity, implying that more general solutions with independent scalar charges can have naked singularities. Since singular branes are also of interest as tentative classical counterparts of unstable tachyonic branes and/or brane-antibrane systems, it is worth investigating branes with independent scalar charges in more detail. Here we study singular branes associated with the Fisher-Janis-Newman-Winicour solution of four-dimensional gravity. In the case of codimension three, we also construct singular branes endowed with a Zipoy-Voorhees-type oblateness parameter. It is expected that such branes will not be supersymmetric in the string theory. We demonstrate this in the special case of NS5-branes of type II theory. We analyze geodesics and test scalar perturbations of new solutions focusing on possible quantum healing of classical singularities.
 

2022-0902

• [2209.00160] Circular orbits of test particles interacting with massless linear scalar field of the naked singularity***
Abstract: We study effects of the particles coupling with scalar field (SF) on the distribution of stable circular orbits (SCO) around the naked singularity described by the well-known Fisher-Janis-Newman-Winicour solution. The power-law and exponential models of the particle--SF interaction are analyzed. The focus is on the non-connected SCO distributions. We show that coupling between particles and SF can essentially complicate the topology of the SCO distributions. In particular, it can lead to new non-overlapping SCO regions, which are separated by unstable orbits and/or by regions where the circular orbits do not exist.

2022-0905

• [2209.00679] First few overtones probe the event horizon geometry
Abstract: It is broadly believed that quasinormal modes (QNMs) cannot tell the black-hole near-horizon geometry, because usually the low-lying modes are determined by the scattering of perturbations around the peak of the effective potential. Using the general parametrization of the black-hole spacetimes respecting the generic post-Newtonian asymptotic, we will show that tiny modifications of the Schwarzschild/Kerr geometry in a small region near the event horizon lead to almost the same Schwarzschild/Kerr fundamental mode, but totally different first few overtones. Having in mind that the first several overtones affect the quasinormal (QN) ringing at its early and intermediate stage [M. Giesler, M. Isi, M. Scheel, and S. Teukolsky, Phys. Rev. X 9, 041060 (2019)], we argue that the near-horizon geometry could in principle be studied via the first few overtones of the QN spectrum, which is important because corrections to the Einstein theory must modify precisely the near-horizon geometry, keeping the known weak field regime. We discuss the connection of this observation with the so called ``overtones' instability'' recently studied in [J. Jaramillo et. al. Phys. Rev. Lett. 128, 211102 (2022)].

• [2209.00719] A spherically symmetric gravitational solution of nearly conformally flat metric measure space
Abstract: In this manuscript, we study the nearly flat approximation of a conformally invariant gravitational theory in metric measure space (MMS). In addition, we investigate the vacuum solution of MMS and obtain its weak field limit in the spherically symmetric coordinates. We show that while it is already a vacuum solution, it can simulate dark matter when restricted to the framework of general relativity, i.e., a symmetry broken conformal frame. This is done by means of a density function which is an essential part of MMS. We derive an equation for the density function for a general profile of a rotation curve obtained from observations. Specifically, the density function corresponding to two well-known profiles PSS and NFW are provided.

Comment: This solution can study the dark matter, so it is possible to model it with EMRI?

• [2209.01038] Multi-scalar Gauss-Bonnet gravity: scalarized black holes beyond spontaneous scalarization
Abstract: Recently, a new nonlinear mechanism for black hole scalarization, different from the standard spontaneous scalarization, was demonstrated to exist for scalar Gauss-Bonnet theories in which no tachyonic instabilities can occur. Thus Schwarzschild black hole is linearly stable but instead nonlinear instability can kick-in. In the present paper we extend on this idea in the case of multi-scalar Gauss-Bonnet gravity with exponential coupling functions of third and fourth leading order in the scalar field. The main motivation comes from the fact that these theories admit hairy compact objects with zero scalar charge, thus zero scalar-dipole radiation, that automatically evades the binary pulsar constraints on the theory parameters. We demonstrate numerically the existence of scalarized black holes for both coupling functions and for all possible maximally symmetric scalar field target spaces. The thermodynamics and the stability of the obtained solution branches is also discussed.

Comment: New mechanism to form scalarized black hole solutions.

• [2209.00874] Gravitational waves and electromagnetic radiation from charged black hole binaries
Abstract: It is still an open issue if astrophysical black holes have electric charges or not. In this work, we analytically calculate gravitational and electromagnetic waveforms in the frequency domain for charged black hole binaries during the inspiral phase. In addition to the well-known waveforms, we also get a −11/6 power law gravitational wave component. The phase of waveforms for charged binary is fully derived. In the case of electromagnetic counterparts, we focus on the electromagnetic dipole radiation, but we include the quadrupole contribution to complete our discussion. We also obtain the chirp property of the electromagnetic waves. In the case of dipole radiation, the frequency-domain waves are proportional to , while appears in the quadrupole contribution. The frequency-domain waveforms can be used to estimate the charges of black holes in the current gravitational wave observations.

• [2209.01110] Recognizing the constitution of small bodies in extreme-mass-ratio inspirals by gravitational waves
Abstract: The extreme mass ratio inspirals(EMRIs) are promising gravitational wave(GW) sources for space-borne GW detectors. The signals of EMRIs usually have long timescales, ranging from several months to several years, and their detection requires accurate GW signal templates. In most waveform models, the compact objects in EMRIs are considered test particles, which do not consider the small bodies' spin, mass distribution, and tidal deformation. In this work, we simulate the GW signals of EMRIs by considering the compact objects' spin and mass quadrupole. We find that a compact object's spin can significantly influence the GW signals, and the tidal-induced and spin-induced quadrupoles matter only if the compact objects are white dwarfs, especially EMRIs of a higher symmetric mass ratio. We can distinguish white dwarfs from other compact objects in this case. The structures of black holes and neutron stars in EMRIs do not have detectable effects on GW signals. Furthermore, compared with the GW signals that use test particle approximation, the signal-to-noise ratios(SNRs) of GW signals that consider extended bodies decrease slightly, which hints that we can omit the spin and quadrupole of the compact object in the detection of EMRIs.
 

2022-0907

• [2209.02067] Doubling of physical states in the quantum scalar field theory for a remote observer in the Schwarzschild space-time
Abstract: We discuss the problem of canonical quantization of a free real massive scalar field in the Schwarzschild space-time. It is shown that a consistent procedure of canonical quantization of the field can be carried out without taking into account the black hole interior, so that in the resulting theory the canonical commutation relations are satisfied exactly, and the Hamiltonian has the standard form. However, unlike some papers, in which the expansion of the quantum field in spherical harmonics was used, here we use an expansion in scattering states for energies larger than the mass of the field. This reveals a strange property of the resulting quantum field theory - doubling of the quantum states, which look as having the same fixed momentum to an observer located far away from the black hole. This purely topological effect poses a question about the existence of black holes with event horizons.
 

2022-0909

• [2209.03740] Self-force correction to the deflection angle in black-hole scattering: a scalar charge toy model
Abstract: Using self-force methods, we consider the hyperbolic-type scattering of a pointlike particle carrying a scalar charge  off a Schwarzschild black hole. For given initial velocity and impact parameter, back-reaction from the scalar field modifies the scattering angle by an amount , which we calculate numerically for a large sample of orbits (neglecting the gravitational self-force). Our results probe both strong-field and field-weak scenarios, and in the latter case we find a good agreement with post-Minkowskian expressions. The scalar-field self-force has a component tangent to the four-velocity that exchanges particle's mass with scalar-field energy, and we also compute this mass exchange as a function along the orbit. The expressions we derive for the scattering angle (in terms of certain integrals of the self-force along the orbit) can be used to obtain the gravitational self-force correction to the angle in the physical problem of a binary black hole with a large mass ratio. We discuss the remaining steps necessary to achieve this goal.

• [2209.03351] Neutron stars as extreme laboratories for gravity tests
Abstract: Neutron stars are versatile in their application to studying various important aspects of fundamental physics, in particular strong-field gravity tests and the equation of state for super-dense nuclear matter at low temperatures. However, in many cases these two objectives are degenerate to each other. We discuss how pulsar timing and gravitational waves provide accurate measurements of neutron star systems and how to effectively break the degeneracy using tools like universal relations. We also present perspectives on future opportunities and challenges in the field of neutron star physics.
 

2022-0912

• [2209.04060] Priorities in gravitational waveform modelling for future space-borne detectors: vacuum accuracy or environment?
Abstract: In preparation for future space-borne gravitational-wave (GW) detectors, should the modelling effort focus on high-precision vacuum templates or on the the astrophysical environment of the sources? We perform a systematic comparison of the phase contributions caused by 1) known environmental effects in both gaseous and stellar matter backgrounds, or 2) high-order post-Newtonian terms in the evolution of mHz GW sources. We use the accuracy of currently available analytical waveform models as a benchmark and find the following trends: the largest unmodelled contributions are likely environmental for binaries lighter than , where z is the redshift. Binaries heavier than  do not require more accurate waveforms due to low signal-to-noise ratios (SNRs). For high-SNR sources, environmental influences are relevant at low redshift, while high-order vacuum templates are required at . Led by these findings, we argue that including environmental effects in waveform models should be prioritised in order to maximize the science yield of mHz detectors.
 

2022-0922

• [2209.09980] Event horizons are tunable factories of quantum entanglement
Abstract: That event horizons generate quantum correlations via the Hawking effect is well known. We argue, however, that the creation of entanglement can be modulated as desired, by appropriately illuminating the horizon. We adapt techniques from quantum information theory to quantify the entanglement produced during the Hawking process and show that, while ambient thermal noise (e.g., CMB radiation) degrades it, the use of squeezed inputs can boost the non-separability between the interior and exterior regions in a controlled manner. We further apply our ideas to analog event horizons concocted in the laboratory and insist that the ability to tune the generation of entanglement offers a promising route towards detecting quantum signatures of the elusive Hawking effect.
 

2202-0927

• [2209.12291] Regularized Stable Kerr Black Hole: Cosmic Censorships, Shadow and Quasi-Normal Modes
Abstract: Black hole solutions in general relativity come with pathologies such as singularity and mass inflation instability, which are believed to be cured by a yet-to-be-found quantum theory of gravity. Without such consistent description, one may model theory-agnostic phenomenological black holes that bypass the aforesaid issues. These so-called regular black holes are extensively studied in the literature using parameterized modifications over the black hole solutions of general relativity. However, since there exist several ways to model such black holes, it is important to study the consistency and viability of these solutions from both theoretical and observational perspectives. In this work, we consider a recently proposed model of regularized stable rotating black holes having two extra parameters in addition to the mass and spin of a Kerr solution. We start by computing their quasi-normal modes under scalar perturbation and investigate the impact of those additional parameters on black hole stability. In the second part, we study the shadow structures of these regularized black holes and obtain stringent bounds on the parameter space requiring consistency with Event Horizon Telescope observations of  and  shadows.
 

2202-0928

• [2209.13387] Extreme-mass-ratio burst detection with TianQin
Abstract: The capture of compact objects by massive black holes in galaxies or dwarf galaxies will generate short gravitational wave signals, called extreme-mass-ratio bursts (EMRBs), before evolving into extreme-mass-ratio inspirals. Their detection will provide an investigation of the black hole properties and shed light on astronomy and astrophysics. In this work, we investigate the detection number of the TianQin observatory on EMRBs. Our result shows that TianQin can detect tens of EMRBs events during its mission lifetime. For those detected events, we use the Fisher information matrix to quantify these uncertainties in the inference of their parameters. We consider the possible network of TianQin+LISA and study how a network can improve parameter estimation. The result shows that, for most sources, the CO mass, the MBH mass, and the MBH spin can be determined with an accuracy of the order  and the sky localization can be determined with an accuracy of 10 square degrees. We further explore the gravitational wave background generated by those unsolved EMRBs and conclude that it is about  times weaker than TianQin's sensitivity and thus it can be ignored.
 

2022-0929

• [2209.13829] Flux-balance laws in scalar self-force theory
Abstract: The motion of a radiating point particle can be represented by a series of geodesics whose "constants" of motion evolve slowly with time. The evolution of these constants of motion can be determined directly from the self-force equations of motion. In the presence of spacetime symmetries, the situation simplifies: there exist not only constants of motion conjugate to these symmetries, but also conserved currents whose fluxes can be used to determine their evolution. Such a relationship between point-particle motion and fluxes of conserved currents is a flux-balance law. However, there exist constants of motion that are not related to spacetime symmetries, the most notable example of which is the Carter constant in the Kerr spacetime. In this paper, we first present a new approach to flux-balance laws for spacetime symmetries, using the techniques of symplectic currents and symmetry operators, which can also generate more general conserved currents. We then derive flux-balance laws for all constants of motion in the Kerr spacetime, using the fact that the background, geodesic motion is integrable. For simplicity, we restrict derivations in this paper to the scalar self-force problem, although the generalization to the gravitational case is straightforward.

2022-0930

• [2209.14873] Light Rings around Five Dimensional Stationary Black Holes and Naked Singularities
Abstract: The existence of light rings in a spacetime is closely related to the existence of black hole horizons and observables such as the ringdown and the shadow. Black holes, compared to nonvacuum ultracompact objects, have rather unique environments. To this aim, recently Cunha et al. topological arguments, independent of the underlying gravity theory, were developed to prove the existence of unstable light rings outside the Killing horizon of four dimensional asymptotically flat, stationary, axisymmetric, non-extremal black holes. Here we extend these arguments to five-dimensional stationary black holes. Generically in five dimensions, there are two possible conserved angular momenta, hence the four-dimensional discussion does not extend verbatim to five dimensions; nevertheless, we prove that there is a light ring for each rotation sense for a stationary black hole. We give the static and the Myers-Perry rotating black holes as examples. We also show that when the horizon of the black hole disappears and the singularity becomes naked, only one of the light rings survives; a similar phenomenon also occurs in four dimensions which might allow testing the Cosmic Censorship hypothesis.

• [2209.14527] Spinorial Wheeler-DeWitt wave functions inside a horizon
Abstract: We revisit the solution of the Wheeler-DeWitt (WDW) equation inside the horizon of spherical black holes and planar topological black holes in arbitrary dimensions. For these systems, the solutions of the equations are found to have the same form. Therefore, Yeom's Annihilation-to-nothing interpretation can be applied to each case. We have introduced the Dirac-type WDW equations into quantum cosmology in a recent paper, so we also apply our formulation to the quantum theory of the interior of the black hole in order to obtain the solution of the spinorial wave function. The shape of the wave packet of the spinorial WDW wave function indicates that the variation of Yeom's interpretation holds in this scheme.