Narrow Results

This paper is motivated to study the weak gravitational lensing of the black hole in the context of Starobinsky–Bel–Robinsion gravity. We deduce the deflection angle of light in the weak field limits. For this purpose, we find the Gaussian curvature and apply the Gauss–Bonnet theorem. We also investigate the deflection angle at which light is deflected by a plasma medium. Furthermore, we analyze the graphical behavior of the deflection angle in the framework of SBR gravity. We also calculate the energy extracted from the black hole in SBR gravity, Einstein ring, and lens equation in the non-plasma and plasma framework. We conclude this study with a discussion of how light beams behave when they approach such a four-dimensional black hole by calculating the deflection angle.

We present three different theoretically foreseen, but unusual, astrophysical situations where the gravitational lens equation ends up being the same, thus producing a degeneracy problem. These situations are: (a) the case of gravitational lensing by exotic stresses (matter violating the weak energy condition and thus having a negative mass, particular cases of wormhole solutions can be used as an example), (b) scalar field gravitational lensing (i.e. when considering the appearance of a scalar charge in the lensing scenario), and (c) gravitational lensing in closed universes (with antipodes). The reasons that lead to this degeneracy in the lens equations, the possibility of actually encountering it in the real universe, and eventually the ways to break it, are discussed.

We investigate the spacetime of a local gauge string with a phenomenological energy momentum tensor, as prescribed by Vilenkin, in the presence of the cosmological term Λ. A set of solutions of full nonlinear Einstein's equations for interior region of such a string is presented.

We present simulations of interferometric Sunyaev-Zel'dovich effect (SZE) and optical weak lenisng observations for the forthcoming AMiBA experiment, aiming at searching for high-redshift clusters of galaxies. On the basis of simulated sky maps, we have derived theoretical halo number counts and redshift distributions of selected halo samples for an AMiBA SZE survey and a weak lensing follow-up survey. By utilizing the conditional number counts of weak lensing halos with the faint SZE detection, we show that a combined SZE and weak lensing survey can gain an additional fainter halo sample at a given false positive rate, which cannot be obtained from either survey alone.

We perform a set of ray-tracing simulations, using numerical cluster models, aiming at evaluating how the galaxy cluster efficiency for producing strong lensing events changes in different cosmological models with dark energy. The sample of investigated clusters for which we present our results here is composed by 7 dark matter halos. Each of them was simulated in 8 different cosmological models with constant and time-variable equation of state of dark energy. For all the clusters in the sample, we have measured the lensing cross sections for producing giant arcs, i.e. arcs having a minimum length-to-width ratio. We find that the lensing cross section for giant arcs is sensitive to the equation of state of quintessence. Indeed, the optical depth, which can be translated into a number of arcs by multiplying by the correct density of source galaxy on the sky, spans more than one order of magnitude among different cosmological models.

Four planets have recently been discovered by gravitational microlensing. The most recent of these discoveries is the lowest-mass planet known to exist around a normal star. The detection of planets in gravitational microlensing events was predicted over a decade ago. Microlensing is now a mature field of astrophysical research and the recent planet detections herald a new chapter in the hunt for low mass extra-solar planets. This paper reviews the basic theory of planetary microlensing, describes the experiments currently in operation for the detection and observation of microlensing events and compares the characteristics of the planetary systems found to date by microlensing. Some proposed schemes for improving the detection rate of planets via microlensing are also discussed.

We have investigated the gravitational lensing by two wormholes, viz., Janis–Newman–Winnicour (JNW) wormhole and Ellis wormhole. The deflection angles in the strong field limit are calculated and various lens parameters of two wormholes are compared. It is shown that the JNW wormhole exhibits the relativistic images, while the Ellis wormhole does not have any relativistic image due to the absence of its photon sphere.

Various TeVeS-inspired and f(R)-inspired theories of gravity have added an interesting twist to the search for dark matter and vacuum energy, modifying the landscape of astrophysics day by day. These theories can be together called a Non-uniform Dark Energy fluid (a Nu-Lambda fluid or a VΛ fluid); a common thread of these theories, according of an up-to-date summary by HZL¹, is a non-uniform vector field, describing an uneven vacuum energy fluid. The so-called "alternative" gravity theories are in fact in the standard GR gravity framework except that the cosmological "constant" is replaced by a nontrivial non-uniform vacuum energy, which couples the effects of Dark Matter and Dark Energy together by a single field. Built initially bottom-up rather than top-down as most gravity theories, TeVeS-inspired theories are healthily rooted on empirical facts. Here we attempt a review of some sanity checks of these fast-developing theories from galaxy rotation curves, solar system constraints, and gravitational lensing. We will also discuss some theoretical aspects of these theories related to the vacuum energy, and point out some analogies with electromagnetism and the Casimir effect.

We present a method for measuring higher-order weak lensing distortions of faint background galaxies, namely the weak gravitational flexion, by fully extending the Kaiser, Squires & Broadhurst method to include higher-order lensing image characteristics (HOLICs) introduced by Okura, Umetsu, & Futamase. Our HOLICs formalism allows accurate measurements of flexion from practical observational data in the presence of non-circular, anisotropic point spread function. We have applied our method to ground-based Subaru observations of the massive galaxy cluster A1689 at a redshift of z = 0.183. From the high-precision measurements of spin-1 first flexion, we obtain a high-resolution mass map in the central region of A1689. The reconstructed mass map shows a bimodal feature in the central 4′ × 4′ region of the cluster. The major, pronounced mass peak is associated with the brightest cluster galaxy and central cluster members, while the secondary peak is associated with a local concentration of bright galaxies. In Fourier space we separate the reconstructed mass distribution into cluster and subhalo components, from which we obtain projected subhalo masses associated with the primary and the secondary peaks to be M₁ = (2.2 ± 0.4) × 10¹³M_⊙/h, and M₂ = (1.1 ± 0.3) × 10¹³M_⊙/h, respectively.

Weak-lensing induced by clusters of galaxies can probe the total mass distribution out to the virial radius of the cluster, regardless of the nature of the mass or its dynamical state. To make a robust analysis, the cluster and background galaxy populations need to be separated. The E/S0 sequence of a cluster defines a boundary redward of which a reliable weak-lensing signal can be obtained from background galaxies, uncontaminated by the cluster. Below this limit, the signal is diluted by the proportion of unlensed cluster members. Employing deep Subaru and HST/ACS images of the massive cluster A1689, we use this dilution effect to carefully separate between the cluster members and the background, and thus derive the cluster light profile and luminosity functions to large radius. The light profile of A1689 is found to decline steadily to the limit of the data, r < 2 h⁻¹Mpc, with a constant slope, d log(L)/d log(r) = −1.12 ± 0.06. We derive a cluster luminosity function with a flat faint-end slope of α = −1.05 ± 0.07, nearly independent of radius and with no faint upturn to M_i′ < −12. The major advantage of this new approach is that no subtraction of far-field background counts is required.

We review some recent efforts in determining the absolute neutrino mass scale in cosmology. We illustrate in particular how distance measurements such as the baryon acoustic oscillations and the galaxy weak lensing can break the degeneracy between the neutrino mass and dark energy equation of state parameters.

Sun's luminosity in the visible changes at the 10^-3 level, following the 11 years period. This variation increases with energy, and in X-rays, which should not even be there, the amplitude varies up to ~ 10⁵ times stronger, making their mysterious origin since the discovery in 1938 even more puzzling, and inspiring. We suggest that the multifaceted mysterious solar cycle is due to some kind of dark matter streams hitting the Sun. Planetary gravitational lensing enhances (occasionally) slow moving flows of dark constituents toward the Sun, giving rise to the periodic behavior. Jupiter provides the driving oscillatory force, though its 11.8 years orbital period appears slightly decreased, just as 11 years, if the lensing impact of other planets is included. Then, the 11 years solar clock may help to decipher (overlooked) signatures from the dark sector in laboratory experiments or observations in space.

Using the strong field limit approach, the strong field gravitational lensing in a black hole with deficit solid angle (DSA) and surrounded by quintessence-like matter (QM) has been investigated. The results show that the DSA ${ϵ}^2$, the energy density of QM ${\rho }_0$ and the equation of state (EOS) parameter $w$ have some distinct effects on the strong field gravitational lensing. As ${ϵ}^2$ or ${\rho }_0$ increases, the deflection angle and the strong field limit coefficients all increase faster and faster. Moreover, the evolution of the main observables also has been studied, which shows that the curves at $w=-2/3$ are more steepy than those of $w=-1/3$. Compared with the Schwarzschild black hole, the black hole surrounded by QM has smaller relative magnitudes, and at $w=-1/3$ both the angular position and angular separation are slightly bigger than those of Schwarzschild black hole, but when $w=-2/3$, the angular position and the relative magnitudes all diminish significantly. Therefore, by studying the strong gravitational lensing, we can distinguish the black hole with a DSA and surrounded by QM from the Schwarzschild black hole and the effects of the DSA and QM on the strong gravitational lensing by black holes can be known better.

This paper reviews a phenomenological approach to the gravitational lensing by exotic objects such as the Ellis wormhole lens, where the exotic lens objects may follow a non-standard form of the equation of state or may obey a modified gravity theory. A gravitational lens model is proposed in the inverse powers of the distance, such that the Schwarzschild lens and exotic lenses can be described in a unified manner as a one parameter family. As observational implications, the magnification, shear, photo-centroid motion and time delay in this lens model are discussed.

We study signals of the weak and strong deflection gravitational lensings by an Extended Uncertainty Principle (EUP) black hole, which is based on a modified Heisenberg relation with an additional correction of position-uncertainty. Gravitational lensing observables, including positions, magnifications and differential time delays between lensed images, are obtained in both scenarios and analyzed for the supermassive black holes (SMBHs) in the Galactic Center (Sgr A*) and M87. We find that, for Sgr A*, measurements on the separation between the primary and secondary images in the weak deflection lensing and the apparent size of the photon sphere in the strong deflection lensing are two feasible ways to constrain EUP, imposing comparable lower bounds on the fundamental scale of EUP as $\sim$10¹⁰ m. For the SMBH in M87, measurements on strong deflection lensing observables are only available and they can give a much bigger lower bound as $\sim$10¹³ m. These results might provide hints for probing EUP black holes by gravitational lensings.

Vacuum Brans-Dicke (BD) theory continues to receive widespread attention since it is consistent with solar and cosmological experiments. The theory can be self-consistently described in two frames, the Jordan frame (JF) and the conformally rescaled Einstein frame (EF), the transformations providing an easy passage from one frame to the other at the level of actions and solutions. While coordinate transformations do not change curvature properties, conformal transformations do change them leading to corresponding changes in the numerical values of observables. A previous article by Bhadra et al.${}^{10}$ did exemplify this change between JF and EF using the diagnostic of second-order light deflection. This important work leaves room for further improvements on two points, which we do here. First, the measurement of second-order effect faced technically unsurmountable difficulties even around the Sun, hence actually abandoned. Second, the comparison of quantitative values between JF and EF should be based on a common value of$\omega$ connecting the two frames. Keeping these in mind, we investigate a technically easier diagnostic, viz., the weak field lensing (WFL) and compare the quantitative changes at common $\omega$ to show that the two frames can indeed be distinguished by lensing experiments. Specifically, the predictions of light deflection, image position, total magnification and magnification factor are computed in the EF and compared with those recently obtained (by Gao et al.${}^{22}$) directly in the JF BD class I solution. The use of the value of BD coupling constant $\left|\omega \right|=50,000$, suggested by the Cassini spacecraft solar experiment, reveals that an exceptionally high degree of accuracy is needed to experimentally rule out one or the other frame by means of WFL measurements.

In this paper, we calculate the weak deflection angle by Casimir wormhole and its shadow. To do so, we derive the Gaussian optical curvature and use the Gauss–Bonnet theorem (GBT). Then we find the deflection angle by Casimir wormhole in weak field limits. Moreover, we obtain the weak deflection angle in the presence of plasma medium and see the effect of the plasma medium on the weak deflection angle. Moreover, we study a shadow of Casimir wormhole and we plot and discuss them. We show the shadow of Casimir wormhole’s behavior when changing the value of a.

We present a detailed study of gravitational lensing around a rotating Bañados–Teitelboim–Zanelli (BTZ) black hole in (2 + 1)-dimensional gravity. The study of orbits for massless test particle around this BH spacetime is performed to describe the nature of cosmological constant in lower dimensions. We study the effect of cosmological constant on the photon orbit in view of other critical parameters. The bending angle of light is studied in view of different values of cosmological constant for direct and retrograde motion of test particles. It is being observed that the bending angle slightly decreases as the value of cosmological constant increases in the negative region.

In this paper, by using a recently found black hole solution in the framework of the Poincaré gauge theory of gravity, we study gravitational lensing for a system where the lens is a static spherically symmetric black hole. By analyzing the equations of motion for light rays in a spacetime with torsion, we derive the deflection angle as the light emitted from a source pass through near the black hole and numerically solve the resulting integral. We also study the effects of torsion on the position of images. The results show that the presence of torsion slightly alters both the deflection angle and position of images in this setup.

In the article we review observational features of black holes where an influence of a gravitational field is dominant and we must use strong gravitational field approach for GR. Recent X-ray observations of microquasars and Seyfert galaxies reveal broad emission lines in their spectra, which can arise in the innermost parts of accretion disks. Simulations indicate that at low inclination angle the line is measured by a distant observer as characteristic two-peak profile. However, at high inclination angles (> 85°) two additional peaks arise. This phenomenon was discovered by Matt et al. (1993) using the Schwarzschild black hole metric to analyze such an effect. They assumed that the effect is applicable to a Kerr metric far beyond the range of parameters that they exploited. We check and confirm their hypothesis about such a structure of the spectral line shape for the Kerr metric case. We discuss how analysis of the iron spectral line shapes could give an information about an upper limit of magnetic field near black hole horizon.