The presence of dark matter around a black hole remarkably affects its spacetime. We consider the effects of dark matter on the shadow of a new solution to the Einstein equations that describes a rotating black hole in the background of perfect dark matter fluid (PFDM), along with its extension to nonzero cosmological constant Λ. Working in Boyer-Lindquist coordinates, we consider the effects of the PFDM parameter α on the shadow cast by a black hole with respect to an observer at position (ro, θo). By applying the Gauss-Bonnet theorem to the optical geometry we find that notable distortions from a Kerr black hole can occur. We describe their dependence on α and Λ.

In this paper, we first propose a universal coupling between the gravity and matter in the framework of the Hořava-Lifshitz theory of gravity with an extra U(1) symmetry for both the projectable and non-projectable cases. Then, using this universal coupling we study the post-Newtonian approximations and obtain the parameterized post-Newtonian (PPN) parameters in terms of the coupling constants of the theory. Contrary to the previous works in which only two PPN parameters were calculated, we obtain all PPN parameters. We then, for the first time in either projectable or non-projectable case, find that all the solar system tests carried out so far are satisfied in a large region of the parameters space. In particular, the same results obtained in general relativity can be easily realized here. A remarkable feature is that the solar system tests impose no constraint on the parameter λ appearing in the kinetic part of the action. As a result, the solar system tests, when combined with the condition for avoidance of strong coupling, do not lead to an upper bound on the energy scale M * that suppresses higher dimensional operators in the theory. This is in sharp contrast to other versions of the HL theory. This is a reminiscent of Lifshitz scalars [6] in condensed matter physics, hence the theory is often referred to as the Hořava-Lifshitz (HL) gravity. Clearly, such a scaling breaks explicitly the Lorentz symmetry and thus 4dimensional diffeomorphism invariance. Hořava assumed that it is broken only down to(1.2) the so-called foliation-preserving diffeomorphism, denoted often by Diff(M, F ).Once the general covariance is broken, it immediately results in a proliferation of independent coupling constants [2, 7, 8], which could potentially limit the predictive power of the theory. To reduce the number of independent coupling constants, Hořava introduced two independent conditions, the projectability and the detailed balance [2]. The former requires that the lapse function N be a function of t only, N = N (t), while the latter requires that the gravitational potential should be obtained from a superpotential W g , given by an integral of the gravitational Chern-Simons term ω 3 (Γ) over a 3dimensional space, W g ∼ Σ ω 3 (Γ). With these two conditions, the general action contains only five independent coupling constants.The HL theory has soon attracted a lot of attention, and been found that the projectability condition leads to several undesirable properties, including infrared instability [2,9] and strong coupling [7, 10-12], although they

Early diagnosis is the most important determinant of oral and oropharyngeal squamous cell carcinoma (OPSCC) outcomes, yet most of these cancers are detected late, when outcomes are poor. Typically, nonspecialists such as dentists screen for oral cancer risk, and then they refer high-risk patients to specialists for biopsy-based diagnosis. Because the clinical appearance of oral mucosal lesions is not an adequate indicator of their diagnosis, status, or risk level, this initial triage process is inaccurate, with poor sensitivity and specificity. The objective of this study is to provide an overview of emerging optical imaging modalities and novel artificial intelligence–based approaches, as well as to evaluate their individual and combined utility and implications for improving oral cancer detection and outcomes. The principles of image-based approaches to detecting oral cancer are placed within the context of clinical needs and parameters. A brief overview of artificial intelligence approaches and algorithms is presented, and studies that use these 2 approaches singly and together are cited and evaluated. In recent years, a range of novel imaging modalities has been investigated for their applicability to improving oral cancer outcomes, yet none of them have found widespread adoption or significantly affected clinical practice or outcomes. Artificial intelligence approaches are beginning to have considerable impact in improving diagnostic accuracy in some fields of medicine, but to date, only limited studies apply to oral cancer. These studies demonstrate that artificial intelligence approaches combined with imaging can have considerable impact on oral cancer outcomes, with applications ranging from low-cost screening with smartphone-based probes to algorithm-guided detection of oral lesion heterogeneity and margins using optical coherence tomography. Combined imaging and artificial intelligence approaches can improve oral cancer outcomes through improved detection and diagnosis.

Scalar-tensor gravity, with the screening mechanisms to avoid the severe constraints of the fifth force in the Solar System, can be described with a unified theoretical framework, the so-called screened modified gravity (SMG). Within this framework, in this paper we calculate the waveforms of gravitational-waves (GWs) emitted by inspiral compact binaries, which include four polarization modes, the plus h+, cross h×, breathing h b , and longitudinal hL modes. The scalar polarizations h b and hL are both caused by the scalar field of SMG, and satisfy a simple linear relation. With the stationary phase approximations, we obtain their Fourier transforms, and derive the correction terms in the amplitude, phase, and polarizations of GWs, relative to the corresponding results in general relativity. The corresponding parametrized post-Einsteinian parameters in the general SMG are also identified. Imposing the noise level of the ground-based Einstein Telescope, we find that GW detections from inspiral compact binaries composed of a neutron star and a black hole can place stringent constraints on the sensitivities of neutron stars, and the bound is applicable to any SMG theory. Finally, we apply these results to some specific theories of SMG, including chameleon, symmetron, dilaton and f (R).

In this work, we study the quasinormal modes of Schwarzschild and Schwarzschild (Anti-) de Sitter black holes by a matrix method. The proposed method involves discretizing the master field equation and expressing it in form of a homogeneous system of linear algebraic equations. The resulting homogeneous matrix equation furnishes a non-standard eigenvalue problem, which can then be solved numerically to obtain the quasinormal frequencies.A key feature of the present approach is that the discretization of the wave function and its derivatives are made to be independent of any specific metric through coordinate transformation. In many cases, it can be carried out beforehand which in turn improves the efficiency and facilitates the numerical implementation. We also analyze the precision and efficiency of the present method as well as compare the results to those obtained by different approaches. * Electronic address:

In this paper, we first show that the definition of the universal horizons studied recently in the khronometric theory of gravity can be straightforwardly generalized to other theories that violate the Lorentz symmetry, by simply considering the khronon as a probe field and playing the same role as a Killing vector field. As an application, we study static charged (D + 1)-dimensional spacetimes in the framework of the healthy (non-projectable) Horava-Lifshitz (HL) gravity in the infrared limit, and find various solutions. Some of them represent Lifshitz space-times with hyperscaling violations, and some have black hole structures. In the latter universal horizons always exist inside the Killing horizons. The surface gravity on them can be either larger or smaller than the surface gravity on the Killing horizons, depending on the space-times considered. Although such black holes are found only in the infrared, we argue that black holes with universal horizons also exist in the full theory of the HL gravity. A simple example is the Schwarzschild solution written in the Painleve-Gullstrand coordinates, which is also a solution of the full theory of the HL gravity and has a universal horizon located inside the Schwarzschild Killing horizon.

In this paper, we study the existence of universal horizons in a given static spacetime, and find that the test khronon field can be solved explicitly when its velocity becomes infinitely large, at which point the universal horizon coincides with the sound horizon of the khronon. Choosing the timelike coordinate aligned with the khronon, the static metric takes a simple form, from which it can be seen clearly that the metric is free of singularity at the Killing horizon, but becomes singular at the universal horizon. Applying such developed formulas to three well-known black hole solutions, the Schwarzschild, Schwarzschild anti-de Sitter, and Reissner-Nordstr\"om, we find that in all these solutions universal horizons exist and are always inside the Killing horizons. In particular, in the Eddington-Finkelstein and Painleve-Gullstrand coordinates, in which the metrics are not singular when crossing both of the Killing and universal horizons, the peeling-off behavior of the khronon is found only at the universal horizons, whereby we show that the values of surface gravity of the universal horizons calculated from the peeling-off behavior of the khronon match with those obtained from the covariant definition given recently by Cropp, Liberati, Mohd and Visser.Comment: revtex4, 17 figures. Version appeared in Phys. Rev. D91, 024047 (2015

In this letter, a matrix method is employed to study the scalar quasinormal modes of Kerr as well as Kerr-Sen black holes. Discretization is applied to transfer the scalar perturbation equation into a matrix form eigenvalue problem, where the resulting radial and angular equations are derived by the method of separation of variables. The eigenvalues, quasinormal frequencies ω and angular quantum numbers λ, are then obtained by numerically solving the resultant homogeneous matrix equation. This work shows that the present approach is an accurate as well as efficient method for investigating quasinormal modes. Black holes constitute an intriguing topic in astrophysics and theoretical physics, where the gravitational force is so strong that nothing can escape from inside of its event horizon. The study of the properties of black holes might lead to insightful perspectives on quantum gravity. The observation of many astronomical phenomena, as, for instance, gravitational lensing, become more accessible when associated to very compact stellar objects such as black holes. Among others, one of the most important tools in the study of black holes is the analysis of its quasinormal mode (QNM) oscillations, which describe the late time dynamics of black holes or black hole binaries, and therefore provide valuable information on the inherent properties of the black hole spacetime as well as its stability. Recently, such signal was observed in the LIGO's first direct detection of gravitational wave [1,2]. KeywordsGenerally, the QNM problem can be reformulated in terms of a Schrödinger-type equation. Due to mathematical difficulties, an exact analytic solution is not always attainable. Therefore, semi-analytical approximate and numerical methods have been proposed to calculate the quasinormal frequency (QNF) [3][4][5][6][7], for example, the Pöschl-Teller potential method [8], continued fractions method [9,10], the Horowitz-Hubeny method (HH) for anti-de Sitter spacetime [11], the WKB approximation [12][13][14], the finite difference method [15][16][17][18][19][20] and the asymptotic iteration method [21][22][23][24] among others [25][26][27].In this letter, we make use of a matrix method [28] to calculate the scalar QNF's for rotating Kerr and Kerr-Sen black hole spacetimes. By using the method of separation of variables, the radial and angular parts of the linearized perturbation equation of the scalar fields are given by [29,30] (whereHere, a ∈ [0,] gives the angular momentum per unit mass. When b = 0, it is the Kerr-Sen black hole case, which reduces to the Kerr black hole spacetime at b = 0. For the case of Kerr black hole, in order to compare our results with those from the continuous fraction method, the mass of the black hole is taken to be M = 1/2. On the other hand, for the case of Kerr-Sen black hole, the mass M = (2b + r 0 + r i )/2 and angular momentum a = √ r i r 0 can be expressed in terms of the event horizon r 0 and the inner horizon r i . m represents the magnetic quantum number and u = cos θ ∈ [−1, 1]. It...