2022-1005

• [2210.01718] Detecting and Denoising Gravitational Wave Signals from Binary Black Holes using Deep Learning
Abstract: We present a convolutional neural network, designed in the auto-encoder configuration that can detect and denoise astrophysical gravitational waves from merging black hole binaries, orders of magnitude faster than the conventional matched-filtering based detection that is currently employed at advanced LIGO (aLIGO). The Neural-Net architecture is such that it learns from the sparse representation of data in the time-frequency domain and constructs a non-linear mapping function that maps this representation into two separate masks for signal and noise, facilitating the separation of the two, from raw data. This approach is the first of its kind to apply machine learning based gravitational wave detection/denoising in the 2D representation of gravitational wave data. We applied our formalism to the first gravitational wave event detected, GW150914, successfully recovering the signal at all three phases of coalescence at both detectors. This method is further tested on the gravitational wave data from the second observing run () of aLIGO, reproducing all binary black hole mergers detected in  at both the aLIGO detectors. The Neural-Net seems to have uncovered a pattern of 'ringing' after the ringdown phase of the coalescence, which is not a feature that is present in the conventional binary merger templates. This method can also interpolate and extrapolate between modeled templates and explore gravitational waves that are unmodeled and hence not present in the template bank of signals used in the matched-filtering detection pipelines. Faster and efficient detection schemes, such as this method, will be instrumental as ground based detectors reach their design sensitivity, likely to result in several hundreds of potential detections in a few months of observing runs.

• [2210.01133] Gravitational waves from extreme-mass-ratio systems in astrophysical environments
Abstract: We establish a generic, fully-relativistic formalism to study gravitational-wave emission by extreme-mass-ratio systems in spherically-symmetric, non-vacuum black-hole spacetimes. The potential applications to astrophysical setups range from black holes accreting baryonic matter to those within axionic clouds and dark matter environments, allowing to assess the impact of the galactic potential, of accretion, gravitational drag and halo feedback on the generation and propagation of gravitational-waves. We apply our methods to a black hole within a halo of matter. We find fluid modes imparted to the gravitational-wave signal (a clear evidence of the black hole fundamental mode instability) and the tantalizing possibility to infer galactic properties from gravitational-wave measurements by sensitive, low-frequency detectors.
 

2022-1006

• [2210.01909] Slowly Rotating Black Holes in 4D Gauss-Bonnet Gravity
Abstract: Since the recent derivation of a well-defined D -> 4 limit for 4D Gauss-Bonnet (4DGB) gravity, there has been considerable interest in testing it as an alternative to Einstein's general theory of relativity. In this paper, we construct slowly rotating black hole solutions for 4DGB gravity in asymptotically flat, de Sitter, and anti-de Sitter spacetimes. At leading order in the rotation parameter, exact solutions of the metric functions are derived and studied for all three of these cases. We compare how physical properties (innermost stable circular orbits, photon rings, black hole shadow, etc.) of the solutions are modified by varying coupling strengths of the 4DGB theory relative to standard Einstein gravity results. We find that a vanishing or negative cosmological constant in 4DGB gravity enforces a minimum mass on the black hole solutions, whereas a positive cosmological constant enforces both a minimum and maximum mass with a horizon root structure directly analogous to the Reissner-Nordstrom de Sitter spacetime. Besides this, many of the physical properties are similar to general relativity, with the greatest deviations typically being found in the low (near-minimal) mass regime.

Comments: to study the numerical method for black hole solutions

• [2210.02069] Quasi-Normal Modes from Bound States: The Numerical Approach
Abstract: It is known that the spectrum of quasi-normal modes of potential barriers is related to the spectrum of bound states of the corresponding potential wells. This property has been widely used to compute black hole quasi-normal modes, but it is limited to a few "approximate" potentials with certain transformation properties for which the spectrum of bound states must be known analytically. In this work we circumvent this limitation by proposing an approach that allows one to make use of potentials with similar transformation properties, but where the spectrum of bound states can also be computed numerically. Because the numerical calculation of bound states is usually more stable than the direct computation of the corresponding quasi-normal modes, the new approach is also interesting from a technical point of view. We apply the method to different potentials, including the Pöschl-Teller potential for which all steps can be understood analytically, as well as potentials for which we are not aware of analytic results but provide independent numerical results for comparison. As a canonical test, all potentials are chosen to match the Regge-Wheeler potential of axial perturbations of the Schwarzschild black hole. We find that the new approximate potentials are more suitable to approximate the exact quasi-normal modes than the Pöschl-Teller potential, particularly for the first overtone. We hope this work opens new perspectives to the computation of quasi-normal modes and finds further improvements and generalizations in the future.
 

2022-1007

• [2210.02634] Photon propagation in a material medium on a curved spacetime
Abstract: We consider a nonlinear dielectric medium surrounding a static, charged and spherically symmetric compact body which gravitational field is driven by General Relativity (GR). Considering the propagating waves on the dielectric medium, we describe the trajectory of light as geodesics on an effective geometry given by Hadamard's discontinuities. We analyze some consequences of the effective geometry in the propagation of light, with relation to the predictions of the background gravitational field, that includes corrections on the geometrical redshift and on the gravitational deflection of light. We show that the background electromagnetic field polarize the material medium, such that different polarizations of light are distinguished by different corrections on these quantities. As a consequence, we have two possible paths for the trajectory of light in such configuration, that coincide if we turn off the electromagnetic field or if the permittivity is constant. We show that the effective metric associated to the negative polarization, for a given dependence of the dielectric permittivity, is conformally flat.
 

2022-1010

• [2210.03657] Towards a more robust algorithm for computing the Kerr quasinormal mode frequencies
Abstract: Leaver's method has been the standard for computing the quasinormal mode (QNM) frequencies for a Kerr black hole (BH) for a few decades. We start with a spectral variant of Leaver's method introduced by Cook and Zalutskiy (arXiv: 1410.7698) and propose improvements in the form of computing the necessary derivatives analytically, rather than by numerical finite differencing. We also incorporate this derivative information into qnm, a Python package which finds the QNM frequencies via the spectral variant of Leaver's method. We confine ourselves to first derivatives only.
 

2022-1011

• [2210.03866] Symplectic mechanics of spinning particles in curved spacetime
Abstract: We provide a set of symplectic variables covering the phase space describing spinning particles in general relativity. The results are valid irrespective of the choice of spacetime background, orthonormal tetrad for defining the spin variables, and, most importantly, do not rely on spin supplementary conditions (or whether one has been imposed or not). We show that the symplectic variables have a natural physical interpretation, and give an example of what it can be used for in the context of spinning particles orbiting a Schwarzschild black hole. This paper presents a unified, covariant Hamiltonian formalism that will be used in subsequent works to study the integrability (or lack thereof) of such systems.

• [2210.04314] How general is the strong cosmic censorship bound for quasinormal modes?
Abstract: Hod's proposal claims that the least damped quasinormal mode of a black hole must have the imaginary part smaller than half of the surface gravity at the event horizon. The Strong Cosmic Censorship in General Relativity implies that this bound must be even weaker: half of the surface gravity at the Cauchy horizon. The appealing question is whether these bounds are limited by the Einstein theory only? Here we will present numerical evidence that once the black hole size is much smaller than then the radius of the cosmological horizon, both the Hod's proposal and the strong cosmic censorship bound for quasinormal modes are satisfied for general spherically symmetric black holes in an arbitrary metric theory of gravity. The low-lying quasinormal frequencies have the universal behavior in this regime and do not depend on the near-horizon geometry, but only on the asymptotic parameters: the value of the cosmological constant and black hole mass.
 

2022-1012

• [2210.04905] Matter traveling through a wormhole
Abstract: We revisit the numerical evolution of Ellis-Bronnikov-Morris-Thorne wormholes, which are constructed with a massless real ghost scalar field. For our simulations, we have developed a new code based on the standard 3+1 foliation of spacetime. We confirm that, for the massless symmetric wormhole, a pulse of regular scalar field causes the wormhole throat to collapse and form an apparent horizon, while a pulse of ghost scalar field can cause the wormhole throat to expand. As a new result, we show that it is possible for a pulse of regular matter to travel through the wormhole and then to send a light signal back before the wormhole collapses. We also evolve pulses of matter traveling through massive asymmetric wormholes, which has not previously been simulated.

• [2210.04927] Reflected Waves and Quantum Gravity
Abstract: In the context of canonical quantum gravity, we consider the effects of a non-standard expression for the gravitational wave function on the evolution of inflationary perturbations. Such an expression and its effects may be generated by a sudden variation in the (nearly constant) inflaton potential. The resulting primordial spectra, up to the leading order, are affected in the short and in the long wavelength regime, where an oscillatory behavior with a non-negligible amplitude is superimposed on the standard semiclassical result. Moreover, a novel, non-perturbative, approach is used to study the evolution. Finally, a simplified application is fully illustrated and commented.
 

2022-1014

• [2210.06500] Determining the spin of light primordial black holes with Hawking radiation
Abstract: We propose a method to determine the mass and spin of primordial black holes (PBHs) in the mass range  (Hawking temperatures ), based on measuring the energy of specific features in the photon Hawking emission spectrum, including both primary and secondary components. This is motivated by scenarios where PBHs in this mass range spin up as they evaporate, namely the string axiverse, where dimensionless spin parameters  can be achieved through the Hawking emission of hundreds or even thousands of light axion-like particles. Measuring the present PBH mass-spin distribution may thus be an important probe of physics beyond the Standard Model. Since the proposed method relies on the energy of the photons emitted by a given PBH, rather than on the associated flux, it is independent of the PBH-Earth distance and, as a byproduct, can also be used to infer the latter.
 

2022-1018

• [2210.09254] Black hole merger simulations in wave dark matter environments
Abstract: The interaction of binary black hole mergers with their environments can be studied using numerical relativity simulations. These start only a short finite time before merger, at which point appropriate initial conditions must be imposed. A key task is therefore to identify the configuration that is appropriate for the binary and its environment at this stage of the evolution. In this work we study the behaviour of wave dark matter around equal mass black hole binaries, finding that there is a preferred, quasi-stationary profile that persists and grows over multiple orbits, in contrast to heavier mass dark matter where any overdensity tends to be dispersed by the binary motion. Whilst different initial configurations converge to the preferred quasi-stationary one after several orbits, unwanted transient oscillations are generated in the process, which may impact on the signal in short simulation runs. We also point out that naively superimposing the matter onto a circular binary results in artificially eccentric orbits due to the matter backreaction, which is an effect of the initial conditions and not a signature of dark matter. We discuss the further work required so that comparison of waveforms obtained with environments to vacuum cases can be done in a meaningful way.
 

2022-1019

• [2210.09357] Gravitational-wave imprints of compact and galactic-scale environments in extreme-mass-ratio binaries
Abstract: Circumambient and galactic-scale environments are intermittently present around black holes, especially those residing in active galactic nuclei. As supermassive black holes impart energy on their host galaxy, so the galactic environment affects the geodesic dynamics of solar-mass objects around supermassive black holes and subsequently the gravitational waves emitted from such non-vacuum extreme-mass-ratio binaries. Only recently an exact general-relativistic solution has been found that describes a Schwarzschild black hole immersed in a dark matter halo profile of the Hernquist type. We perform an extensive geodesic analysis of test particles delving in such non-vacuum spacetimes and compare our results with those obtained in vacuum Schwarzschild spacetime, as well as their dominant gravitational-wave emission. Our findings indicate that the radial and polar oscillation frequency ratios, which indicate resonances, descend deeper into the extreme gravity regime as the compactness of the halo increases. This translates to a gravitational redshift of non-vacuum geodesics and their resulting waveforms with respect to the vacuum ones; a phenomenon which has also been observed for ringdown signals in these setups. For compact environments, we find that the apsidal precession of orbits is strongly affected due to the gravitational pull of dark matter; the orbit's axis can rotate in the opposite direction as that of the orbital motion, leading to a retrograde precession drift that depends on the halo's mass, as opposed to the typical prograde precession transpiring in vacuum and galactic-scale environments. Gravitational waves in retrograde-to-prograde orbital alterations demonstrate transient frequency phenomena around a critical non-precessing turning point, thus they may serve as a `smoking gun' for the presence of dense dark matter environments around supermassive black holes.

2022-1020

• [2210.10587] Rechargeable black hole battery
Abstract: This paper proposes a classical physical process to use a Schwarzschild black hole as a rechargeable battery. In its one cycle the black hole does not loss energy but can at most transform 25% of input mass into available electric energy in a controllable and slow way. We build the theory about the dynamics of black hole in cavity to study the discharging process. Under "quasi-static approximation", we study various properties of this battery, including the internal resistance, efficiency of discharging, maximum output power, cycle life and totally available energy. Differing from that usual Penrose process or superradiance can extract appreciable energy only from extremely rotational or charged black hole, the black hole here is non-rotational and only needs very tiny amounts of charge but in its life it can supply available electric energy much larger than its initial mass.
 

2022-1027

• [2210.12898] Periodic analogues of the Kerr solutions: a numerical study
Abstract: In recent years black hole configurations with non standard topology or with non-standard asymptotic have gained considerable attention. In this article we carry out numerical investigations aimed to find periodic coaxial configurations of co-rotating 3+1 vacuum black holes, for which existence and uniqueness has not yet been theoretically proven. The aimed configurations would extend Myers/Korotkin-Nicolai's family of non-rotating (static) coaxial arrays of black holes. We find that numerical solutions with a given value for the area A and for the angular momentum J of the horizons appear to exist only when the separation between consecutive horizons is larger than a certain critical value that depends only on A and |J|. We also establish that the solutions have the same Lewis's cylindrical asymptotic as Stockum's infinite rotating cylinders. Below the mentioned critical value the rotational energy appears to be too big to sustain a global equilibrium and a singularity shows up at a finite distance from the bulk. This phenomenon is a relative of Stockum's asymptotic's collapse, manifesting when the angular momentum (per unit of axial length) reaches a critical value compared to the mass (per unit of axial length), and that results from a transition in the Lewis's class of the cylindrical exterior solution. This remarkable phenomenon seems to be unexplored in the context of coaxial arrays of black holes. Ergospheres and other global properties are also presented in detail.

• [2210.12641] Quasinormal ringing and echoes of black bounces in a cloud of strings
Abstract: In the string theory, the fundamental blocks of nature are not particles but one-dimensional strings. Therefore, a generalization of this idea is to think of it as a cloud of strings. Rodrigues et al. embedded the black bounces spacetime into string cloud, which demonstrates that the existence of string cloud makes the Bardeen black hole singular, while the black bounces spacetime remains regular [Phys. Rev. D 106, 084016 (2022)]. In this work, we study the quasinormal modes of black bounces spacetime surrounded by a cloud of strings to explore what gravitational effects are caused by string cloud. The quasinormal ringing of the regular black hole and traversable wormhole with string cloud are presented. Our results demonstrate that the black bounce spacetime with strings cloud is characterized by gravitational wave echoes as it transitions from regular black holes to wormholes.