General Relativity and Quantum Cosmology (gr-qc)

Abstract: We study the quasinormal mode (QNM) spectrum of an asymptotically AdS black hole with the Robin boundary condition at infinity. We consider the Schwarzshild-AdS$_4$ with the flat event horizon as the background spacetime and study its scalar field perturbation. Denoting leading coefficients of slow- and fast-decay modes of the scalar field at infinity as $\phi_1$ and $\phi_2$, respectively, we assume a linear relation between them as $\phi_2 = \cot(\theta/2) \phi_1$, where $\theta$ is a constant called the Robin parameter and periodic under $\theta\sim\theta+2\pi$. In a certain range of the Robin parameter, there is an instability driven by the boundary condition. We also find the holonomy in the QNM spectrum under the parametric cycle of the boundary condition: $\theta=0\to2\pi$. After the one-cycle, $n$-th overtone of the QNM moves to $(n-1)$-th overtone. The fundamental tone of the QNM is swept out to the infinity in the complex plane.

Abstract: In this work we establish in a rigorous manner, and a model independent way, the conditions for bypassing Bekenstein's no-scalar-hair theorem for static, spherically symmetric, and asymptotically flat black holes, while maintaining the validity of the energy conditions. Specifically, we argue that a hidden assumption in the theorem, namely the vanishing of the quantity $\mathcal{G} = \mathcal{E} + T^\theta\!_\theta$, where $\mathcal{E}$ is the energy density and $T^\theta\!_\theta$ the corresponding component of the energy-momentum tensor of the scalar field theory, can be relaxed. Indeed, if $\mathcal{G}$ is positive, as a consequence of the assumption on the validity of the energy conditions, then scalar hair is potentially allowed in the black hole's exterior, consistently with the gravitational equations and the generic properties of the (non-trivial) energy momentum tensor. As an explicit example, in which such a behaviour is realised, we discuss the well known model of a (3+1)-dimensional Schwarzschild black hole coupled to a spontaneously broken Yang-Mills $SU (2)$ gauge theory interacting with a Higgs scalar. We present a rather novel approach to obtain analytical black hole solutions for this system (in contrast to the numerical ones in the existing literature) by applying an appropriate perturbative treatment whereby the black hole configuration is derived as a result of backreaction of Higgs and gauge fields onto an initially fixed flat spacetime. The massive nature of the scalar and gauge fields in this example requires a special treatment, because of their asymptotic form, which we discuss in some detail.

Abstract: The problem of the possibility of observing a uniform sky glow through the throat of a Morris--Thorne wormhole by an observer located in another asymptotically flat space-time is considered. It is shown that an individual star has multiple images, and the image of a luminous sky has a complex structure and contains ring structures. The reasons for the emergence of such structures are considered. The distribution of radiation intensity in the image along the radial coordinate is constructed.In addition, an image of the Morris-Thorne wormhole was constructed against the background of uniform sky radiation in the observer's space. A comparison to observations can be made by producing a synthetic Morris-Thorne type wormhole image against a CMB background. This image has been constructed by a combination of the images for inner and outer areas.

Abstract: This paper considers the problem of searching for quiet, long-duration and broadband gravitational wave signals, such as stellar-mass binary black hole binaries, in mock LISA data. We propose a method that combines a semi-coherent likelihood with the use of a particle swarm optimizer capable of efficiently exploring a large parameter space. The semi-coherent analysis is used to widen the peak of the likelihood distribution over parameter space, congealing secondary peaks and thereby assisting in localizing the posterior bulk. An iterative strategy is proposed, using particle swarm methods to initially explore a wide, loosely-coherent likelihood and then progressively constraining the signal to smaller regions in parameter space by increasing the level of coherence. The properties of the semi-coherent likelihood are first demonstrated using the well-studied binary neutron star signal GW170817. As a proof of concept, the method is then successfully applied to a simplified search for a stellar-mass binary black hole in zero-noise LISA data. Finally, we conclude by discussing what remains to be done to develop this into a fully-capable search and how the method might also be adapted to tackle the EMRI search problem in LISA.

Abstract: We developed a numerical methodology to compute the fully-relativistic propagation time of photons emitted by a pulsar in orbit around a massive compact object, like the supermassive black hole Sagittarius A* in the Galactic Center, whose gravitational field is described by a generic spherically symmetric space-time. Pulsars at the Galactic Center are usually regarded as the next major precision probe for theories of gravity, filling the current experimental gap between horizon-scale gravity tests and those at larger scales. We retain a completely general approach, which allows us to apply our code to the Schwarzschild space-time (by which we successfully validate our methodology) and to three different well-motivated alternatives to the standard black hole paradigm. The results of our calculations highlight departures spanning several orders of magnitudes in timing residuals, that are supposed to be detectable with future observing facilities like the Square Kilometer Array.

Abstract: We present the first empirical constraints on the polymer scale describing polymer quantized GWs propagating on a classical background. These constraints are determined from the polymer-induced deviation from the classically predicted propagation speed of GWs. We leverage posterior information on the propagation speed of GWs from two previously reported sources: 1) inter-detector arrival time delays for signals from the LIGO-Virgo Collaboration's first gravitational-wave transient catalog, GWTC1, and 2) from arrival time delays between GW signal GW170817 and its associated gamma-ray burst GRB170817A. For pure-GW constraints, we find relatively uninformative combined constraints of $\nu = 0.96\substack{+0.15 \\ -0.21} \times 10^{-53} \, \rm{kg}^{1/2}$ and $\mu = 0.94\substack{+0.75 \\ -0.20} \times 10^{-48} \, \rm{kg}^{1/2} \cdot s$ at the $90\%$ credible level for the two polymer quantization schemes, where $\nu$ and $\mu$ refer to polymer parameters associated to the polymer quantization schemes of propagating gravitational degrees of freedom. For constraints from GW170817/GRB170817A, we report much more stringent constraints of $\nu_{\mathrm{low}} =2.66\substack{+0.60 \\ -0.10}\times 10^{-56}$, $\nu_{\mathrm{high}} = 2.66\substack{+0.45 \\ -0.10}\times 10^{-56} $ and $\mu_{\mathrm{low}} = 2.84\substack{+0.64 \\ -0.11}\times 10^{-52}$, $\mu_{\mathrm{high}} = 2.76\substack{+0.46 \\ -0.11}\times 10^{-52}$ for both representations of polymer quantization and two choices of spin prior indicated by the subscript. Additionally, we explore the effect of varying the lag between emission of the GW and EM signals in the multimessenger case.

Abstract: We consider a test charged particle falling onto a Schwarzschild black hole and evaluate its electromagnetic field. The Regge-Wheeler equation is solved analytically by approximating the potential barrier with Dirac delta function and rectangular barrier. We show that for asymptotically large time measured by a distant observer the electromagnetic field approaches the spherically symmetric electrostatic field exponentially fast. This implies that in the region accessible to a distant observer the initial state of separated charge and Schwarzschild black hole becomes asymptotically indistinguishable from the Reisnner-Nordstr\"om solution. Implications of this result for models with plasma accretion on black holes are discussed.7 a

Abstract: To evaluate the probability of a gravitational-wave candidate originating from noise, GstLAL collects noise statistics from the data it analyzes. Gravitational-wave signals of astrophysical origin get added to the noise statistics, harming the sensitivity of the search. We present the Background Filter, a novel tool to prevent this by removing noise statistics that were collected from gravitational-wave candidates. To demonstrate its efficacy, we analyze one week of LIGO and Virgo O3 data, and show that it improves the sensitivity of the analysis by 20-40% in the high mass region, in the presence of 868 simulated gravitational-wave signals. With the upcoming fourth observing run of LIGO, Virgo, and KAGRA expected to yield a high rate of gravitational-wave detections, we expect the Background Filter to be an important tool for increasing the sensitivity of a GstLAL analysis.

Abstract: We investigate the effects of the Einstein cubic gravity (ECG) on regular black hole solutions driven by nonlinear electrodynamics (NLE) sources. The ECG tends to form a naked singularity at the origin for a high ECG coupling constant. Assuming that ECG provides only perturbative corrections to the regular magnetic charged solutions, we found modified regular solutions with a de Sitter-like core whose cosmological constant depends on the magnetic charge and the ECG coupling constant. The thermodynamic stability is investigated by means of the Hawking temperature and the heat capacity. In fact, for a small charge and ECG coupling, the Hawking temperature is regularized, leaving a thermodynamic stable remnant for a small $r_h \neq 0$. The heat capacity reveals that the ECG regular black hole undergoes a phase transition between an unstable into a stable configuration.

Abstract: We present a new avenue to black hole evaporation using a heat-kernel approach analogous as for the Schwinger effect. Applying this method to an uncharged massless scalar field in a Schwarzschild spacetime, we show that spacetime curvature takes a similar role as the electric field strength in the Schwinger effect. We interpret our results as local pair production in a gravitational field and derive a radial production profile. The resulting emission peaks near the unstable photon orbit. Comparing the particle number and energy flux to the Hawking case, we find both effects to be of similar order. However, our pair production mechanism itself does not explicitly make use of the presence of a black hole event horizon.

Abstract: X-ray astronomy provides information regarding the electromagnetic emission of active galactic nuclei and X-ray binaries. These events provide details regarding the astrophysical environment of black holes and stars, and help us understand gamma-ray bursts. They produce estimates for the maximum mass of neutron stars and eventually will contribute to the discovery of their equation of state. Thus, it is crucial to study these configurations to increase the yield of X-ray astronomy when combined with multimessenger gravitational-wave astrophysics and black hole shadows. Unfortunately, an exact solution of the field equations does not exist for neutron stars. Nevertheless, there exist a variety of approximate compact objects that may characterize massive or neutron stars. The most studied approximation is the Hartle-Thorne metric that represents slowly-rotating compact objects, like massive stars, white dwarfs and neutron stars. Recent investigations of photon orbits and shadows of such metric revealed that it exhibits chaos close to resonances. Here, we thoroughly investigate particle orbits around the Hartle-Thorne spacetime. We perform an exhaustive analysis of bound motion, by varying all parameters involved in the system. We demonstrate that chaotic regions, known as Birkhoff islands, form around resonances, where the ratio of the radial and polar frequency of geodesics, known as the rotation number, is shared throughout the island. This leads to the formation of plateaus in rotation curves during the most prominent $2/3$ resonance, which designate nonintegrability. We measure their width and show how each parameter affects it. The nonintegrability of Hartle-Thorne metric may affect quasiperiodic oscillations of low-mass X-ray binaries, when chaos is taken into account, and improve estimates of mass, angular momentum and multipole moments of astrophysical compact objects.

Abstract: We analyze the ringdown phase of the first detected black-hole merger, GW150914, using a simulation-based inference pipeline based on masked autoregressive flows. We obtain approximate marginal posterior distributions for the ringdown parameters, namely the mass, spin, and the amplitude and phases of the dominant mode and its first overtone. Thanks to the locally amortized nature of our method, we are able to calibrate our posteriors with injected simulations, producing posterior regions with guaranteed (i.e. exact) frequentist coverage of the true values. For GW150914, our calibrated posteriors provide only mild evidence (~ 2 sigma) for the presence of an overtone, even if the ringdown is assumed to start at the peak of the amplitude.

Abstract: We formulate a bi-Connection Theory of Gravity whose Gravitational action consists of a recently defined mutual curvature scalar. Namely, we build a gravitational theory consisting of one metric and two affine connections, in a Metric-Affine Gravity setup. Consequently, coupling the two connections on an equal footing with matter, we show that the geometry of the resulting theory is, quite intriguingly, that of Statistical Manifold. This ultimately indicates a remarkable mathematical correspondence between Gravity and Information Geometry.