arXiv subjects

Rajibul Shaikh

Publications and source records attributed to Rajibul Shaikh.

At least 19 recordsLinked to original sources

arXiv2026

General relativistic limit of f(R) gravity in the horizon scale of black hole

The environment of the Galactic Centre (GC) black hole, Sgr A* gives us new opportunities to test black hole physics and deviation from General Relativity. In this analytical study we calculate null geodesics around the GC black hole for the recently developed stationary, axisymmetric and vacuum metric of f(R) gravity theory. We study the impact of scalaron degree of freedom of f(R) gravity theory on size and shape of the black hole shadow. A minimum bound of $10^{-17}$ eV for scalaron mass has been obtained by using $1 \sigma$ upper bounds on deviation parameter measured by the Keck telescope and the VLT. Scalarons lighter than this bound are found to increase shadow size beyond that measured by the Event Horizon Telescope. We calculate shadow displacement and asymmetry and infer that $10^{-16}$ eV scalarons which produce exact Kerr sized shadow yield 6\% departure from Kerr quadrupole. From asymmetry we infer preservation of black hole no-hair theorem for such scalaron mass and further examine possibility of violation of the theorem. The same mass scale is found to reproduce the PPN parameter ($\gamma$) constrained in the weak field limit of the solar system. Gravitational identifiers, the Kretschmann scalar ($\kappa$) and gravitational potential ($\phi$) have been used to infer scalaron masses in the regime of S-stars near Sgr A* which are found to be consistent with the limits obtained by using shadow scales. We ensure existence of an appropriate general relativistic limit of f(R) gravity scalaron mass in the horizon scale of the black hole.

gr-qc↗

arXiv2024

Homogeneous spacetime with shear viscosity

We study the homogeneous and anisotropic evolution of Bianchi type-I spacetime driven by perfect fluid with shear viscosity. We obtain exact solutions by considering the simplest form of the equation of state wherein the pressure and the shear stress are proportional to the energy density individually. A special case of our general solutions represent Bianchi type-VII cosmology. We analyse the singularity structure of the solutions and its connection with various energy conditions. We find that the initial singularity can be removed only for the Bianchi type-VII. We also analyse the late-time behaviour of the solutions and find that, compared to the usual Friedmann universe, the spacetime expands less rapidly and the energy density drops faster.

gr-qc↗

arXiv2023

Perfect fluid with shear viscosity and spacetime evolution

We investigate the anisotropic evolution of spacetime driven by perfect fluid with off-diagonal shear-viscosity components. We consider the simplest form of the equation of state for fluid, for which the pressure and the shear stress are proportional to the energy density individually. At late times, compared with the usual Friedmann universe, we find that the spacetime expands less rapidly as the energy density drops faster due to the transfer to the shear stress. Very interestingly, for some ranges of the equation-of-state parameters, we find that the initial big-bang singularity can be removed.

gr-qc↗

arXiv2023

Strong gravitational lensing by wormholes

We study strong gravitational lensing by a class of static wormhole geometries. Analytical approaches to the same are developed, and the results differ substantially from strong lensing by black holes, first reported by Bozza. We consider two distinct situations, one in which the observer and the source are on the same side of the wormhole throat, and the other in which they are on opposite sides. Distinctive features in our study arise from the fact that photon and antiphoton spheres might be present on both sides of the wormhole throat, and that the throat might itself act as a photon sphere. We show that strong gravitational lensing thus opens up a rich variety of possibilities of relativistic image formation, some of which are novel, and are qualitatively distinct from black hole lensing. These can serve as clear indicators of exotic wormhole geometries.

gr-qc↗

arXiv2023

Testing black hole mimickers with the Event Horizon Telescope image of Sagittarius A$^*$

The Event Horizon Telescope (EHT) has recently observed the image and shadow of the supermassive compact object Sagittarius A$^*$ (Sgr A$^*$). According to the EHT collaboration, the observed image is consistent with the expected appearance of a Kerr black hole. However, it is well-known that some non-Kerr objects may mimic many of the properties of the Kerr black hole, and hence, their shadows might be consistent with the observed shadow of Sgr A$^*$. In this work, we consider two black hole mimickers and study their shadows. The first mimicker is a rotating generalisation of the recently proposed static, spherically symmetric black-bounce spacetime by Simpson and Visser where the central Schwarzschild singularity is replaced by a minimal surface. The second one is the $γ$-metric which is a static, axially symmetric singular solution of the vacuum Einstein's equations without an event horizon. We put constraint on the parameters of these black hole mimickers by comparing their shadows with the one observed for Sgr A$^*$.

gr-qc↗

arXiv2023

A rotating modified JNW spacetime as a Kerr black hole mimicker

The Event Horizon Telescope has recently observed the images and shadows of the compact objects M87$^*$ and Sgr A$^*$ at the centres of the galaxies Messier 87 and Milky Way. This has opened up a new window in observational astronomy to probe and test gravity and fundamental physics in the strong-field regime. In this paper, we consider a rotating version of a modified Janis-Newman-Winicour metric, study its shadow, and constrain the metric parameters using the observed shadows of M87$^*$ and Sgr A$^*$. Depending on parameter values, the spacetime metric represents either a naked singularity or a wormhole. We find that the naked singularity case is not consistent with observations, as it casts a shadow which is much smaller than the observed ones. On the other hand, the shadow formed by the wormhole branch, depending on the parameter values, is consistent with the observations. We put constraints on the wormhole throat radius by comparing the shadow with the observed ones of M87$^*$ and Sgr A$^*$.

gr-qc↗

arXiv2021

Shadows of Lorentzian traversable wormholes

The prospect of identifying wormholes by investigating the shadows of wormholes constitute a foremost source of insight into the evolution of compact objects and it is one of the essential problems in contemporary astrophysics. The nature of the compact objects (wormholes) plays a crucial role on shadow effect, which actually arises during the strong gravitational lensing. Current Event Horizon Telescope observations have inspired scientists to study and to construct the shadow images of the wormholes. In this work, we explore the shadow cast by a certain class of rotating wormhole. To search this, we first compose the null geodesics and study the effects of the parameters on the photon orbit. We have exposed the form and size of the wormhole shadow and have found that it is slanted as well as can be altered depending on the different parameters present in the wormhole spacetime. We also constrain the size and the spin of the wormhole using the results from M87* observation, by investigating the average diameter of the wormhole as well as deviation from circularity with respect to the wormhole throat size. In a future observation, this type of study may help to indicate the presence of a wormhole in a galactic region.

gr-qc↗

arXiv2021

Shadows in conformally related gravity theories

Null geodesics are invariant under a conformal transformation, and thus it might seem natural to assume that the observables corresponding to the shadow of a space-time are also conformally invariant. Here, we argue instead, that since the Arnowitt-Deser-Misner mass and the active gravitational mass of an asymptotically flat space-time are not, in general, invariant under such conformal transformations, the shadow radius for photon motion in a space-time would be quantitatively different, when viewed from two different conformally related frames, although the expression for the shadow radius is similar. We then use this fact to propose a novel method to constrain the relevant parameters in a gravity theory conformally related to general relativity. As examples of our method, we constrain the parameter space in Brans-Dicke theory, and a class of brane-world gravity models, by using the recent observational data of M$87^{*}$ by the Event Horizon Telescope.

gr-qc↗

arXiv2021

Thin accretion disks around traversable wormholes

In this paper, we aim to investigate various physical properties and characteristics of radiation emerging from the surface of accretion disks, in a rotating traversable axially symmetric wormhole spacetime of the Teo class. We have studied the marginally stable orbits and accretion efficiency graphically, corresponding to different values of dimensionless spin parameter $J/M^2$ ranging from 0.2 to 1.5 and some values of the throat radius $r_0$, in comparison to the Kerr black hole with the same parameter values, and also tabulated the results. The energy flux radiated by the accretion disk $F(r)$, the temperature distribution $T(r)$ and the emission spectra $νL(ν)$ is plotted, corresponding to varying values of the dimensionless spin parameter $J/M^2$ and throat radius $r_0$. Also, the critical frequency at which the luminosity attains its maximum value, for various values of the angular momentum of the wormhole $J/M^2$ and $r_0$ is tabulated. Lastly, we have employed ray-tracing technique, to produce the intensity map of the image of an accretion disk, as observed by an asymptotic observer, under two conditions: firstly when the disk is on the same side as the observer and we have also compared those with the images of an accretion disk in case of Kerr black hole with same parameters. Secondly, the images have been provided when the disk and observer are on opposite sides of the throat. This study may help to detect and distinguish wormhole geometries from other compact objects.

gr-qc↗

arXiv2021

Shadows and thin accretion disk images of the $γ$-metric

The $γ$-metric is a static, axially-symmetric singular solution of the vacuum Einstein's equations without an event horizon. This is a two-parameter family of solutions, generic values of one of which (called $γ$) measure the deviation from spherical symmetry. Here, we first study the shadow cast by this geometry, in order to constrain the $γ$-metric from observations. We find that for $γ< 1/2$, there are, in principle, no shadows cast. On the other hand, shadows cast for all values of $γ\geq 1/2$ are consistent with observations of M$87^*$ by the Event Horizon Telescope. We also study images of thin accretion disks in the $γ$-metric background. In situations where the $γ$-metric possesses light rings, these qualitatively mimic Schwarzschild black holes with the same ADM mass, while in the absence of such rings, they are drastically different from the black hole case.

gr-qc↗

arXiv2021

A stellar constraint on Eddington-inspired Born-Infeld gravity from cataclysmic variable binaries

Eddington-inspired Born-Infeld gravity is an important modification of Einstein's general relativity, which can give rise to non-singular cosmologies at the classical level, and avoid the end-stage singularity in a gravitational collapse process. In the Newtonian limit, this theory gives rise to a modified Poisson's equation, as a consequence of which stellar observables acquire model dependent corrections, compared to the ones computed in the low energy limit of general relativity. This can in turn be used to establish astrophysical constraints on the theory. Here, we obtain such a constraint using observational data from cataclysmic variable binaries. In particular, we consider the tidal disruption limit of the secondary star by a white dwarf primary. The Roche lobe filling condition of this secondary star is used to compute stellar observables in the modified gravity theory in a numerical scheme. These are then contrasted with the values obtained by using available data on these objects, via a Monte Carlo error progression method. This way, we are able to constrain the theory within $5σ$ confidence level.

gr-qc↗

arXiv2021

Constraining alternatives to the Kerr black hole

The recent observation of the shadow of the supermassive compact object M87$^*$ by the Event Horizon Telescope (EHT) collaboration has opened up a new window to probe the strong gravity regime. In this paper, we study shadows cast by two viable alternatives to the Kerr black hole, and compare them with the shadow of M87$^{*}$. The first alternative is a horizonless compact object (HCO) having radius $r_0$ and exterior Kerr geometry. The second one is a rotating generalisation of the recently obtained one parameter ($r_0$) static metric by Simpson and Visser. This latter metric, constructed by using the Newman-Janis algorithm, is a special case of a parametrised rotating non-Kerr geometry obtained by Johannsen. Here, we constrain the parameter $r_0$ of these alternatives using the results from M87$^{*}$ observation. We find that, for the mass, inclination angle and the angular diameter of the shadow of M87$^{*}$ reported by the EHT collaboration, the maximum value of the parameter $r_0$ must be in the range $2.54r_{+}\leq r_{0,max}\leq 3.51r_{+}$ for the dimensionless spin range $0.5\leq a_{*}\leq 0.94$, with $r_{+}$ being the outer horizon radius of the Kerr black hole at the corresponding spin value. We conclude that these black hole alternatives having $r_0$ below this maximum range (i.e. $r_0\leq r_{0,max}$) is consistent with the size and deviation from circularity of the observed shadow of M87$^{*}$.

gr-qc↗

arXiv2020

Shadow of nulllike and timelike naked singularities without photon spheres

In this paper, we derive general conditions for a shadow to occur, without a photon sphere in a spacetime, caused by central nulllike or timelike naked singularities. Using these conditions, we propose classes of spacetimes which have nulllike and timelike naked singularities at the center, and that fulfill these `shadow without photon sphere' conditions. Considering additional asymptotically flat conditions, we show that, for a fixed Schwarzschild mass, the timelike naked singularities can cast a shadow of a size that is equal to, greater or smaller, than the size of a black hole shadow. On the other hand, the size of shadow of a nulllike naked singularity is always less than that of a black hole. Such novel features of shadows of nulllike and timelike naked singularities in the absence of photon spheres may help us to distinguish between black holes and naked singularities observationally.

gr-qc↗

arXiv2020

Constraining modified gravity from tidal phenomena in binary stars

In beyond-Horndeski theories of gravity, the Vainshtein screening mechanism might only be partially effective inside stellar objects. This results in a modification of the pressure balance equation inside stars, often characterized by a single parameter ($Υ$) in isotropic systems. We show how to constrain such theories of modified gravity, using tidal effects. We study such effects in cataclysmic variable star binaries and numerically obtain limits on the critical masses of the donor stars, below which they are tidally disrupted, by modeling them in beyond-Horndeski theories. This is contrasted with values of the donor masses, obtained using existing observational data, by a Monte Carlo error progression method. A best fit scenario of the two yields a parametric constraint in the theories that we consider, within the approximations used. Here, we obtain the allowed range $ 0 \le Υ\le 0.47 $.

astro-ph.SR↗

arXiv2020

Perihelion Precession and Shadows near Blackholes and Naked Singularities

It is now known that, apart from black holes, some naked singularities can also cast shadows which provide their possible observable signatures. We examine the relevant question here as to how to distinguish then these entities from each other, in terms of further physical signatures. We point out that black holes always admit timelike bound orbits having positive perihelion precession. Also, while a naked singularity with a photon sphere can cast a shadow, it could also admit positive perihelion precession for such orbits, thereby mimicking a black hole. This indicates that compact objects with photon spheres (shadows) always admit positive perihelion precession of timelike bound orbits around them. On the other hand, a naked singularity without a photon sphere could admit both positive and negative perihelion precession but need not have a shadow. In this paper, we construct a spacetime configuration which has a central naked singularity but no photon sphere, and it can give both shadow and a negative perihelion precession. Our results imply that, whereas the presence of a shadow and a positive perihelion precession implies either a black hole or a naked singularity, the presence of a shadow and a negative perihelion precession simultaneously would imply a naked singularity only. We discuss our results in the context of stellar motions (motion of the `S' stars) around the Sgr-A* galactic center.

gr-qc↗

arXiv2019

Tidal disruption near black holes and their mimickers

Black holes and wormholes are solutions of Einstein's field equations, both of which, from afar, look like a central mass. We show here that although at large distances both behave like Newtonian objects, close to the event horizon or to the throat, black holes and wormholes have different tidal effects on stars, due to their respective geometries. We quantify this difference by a numerical procedure in the Schwarzschild black hole and the exponential wormhole backgrounds, and compare the peak fallback rates of tidal debris in these geometries. The tidal disruption rates in these backgrounds are also computed. It is shown that these quantities are a few times higher for wormholes, compared to the black hole cases.

astro-ph.HE↗

arXiv2019

Observational signatures of wormholes with thin accretion disks

We numerically construct images of thin accretion disks in rotating wormhole backgrounds, for the Kerr-like and the Teo class of wormholes. Our construction is illustrated by two methods, a semi-analytic scheme where separated null geodesic equations obtained by analytically integrating the second order equations once are used, as well as by a numerical ray-tracing method utilizing a fourth order Runge-Kutta algorithm. Our result shows dramatic differences between accretion disk images in wormhole backgrounds, compared to black hole ones, specifically because a wormhole can in principle have accretion disks on both sides of its throat. We establish the nature of the images if the observer and the disk are on two opposite sides of the throat, and show that these can provide conclusive observational evidence of wormhole geometries.

gr-qc↗

arXiv2019

Can we distinguish black holes from naked singularities by the images of their accretion disks?

We study here images of thin accretion disks around black holes and two classes of naked singularity spacetimes and compare these scenarios. The naked singularity models which have photon spheres have single accretion disk with its inner edge lying outside the photon sphere. The images and shadows created by these models mimic those of black holes. It follows, therefore, that further and more detailed analysis of the images and shadows structure in such case is needed to confirm or otherwise the existence of an event horizon for the compact objects such as the galactic centers. However, naked singularity models which do not have any photon spheres can have either double disks or a single disk extending up to the singularity. The images obtained from such models significantly differ from those of black holes. Moreover, the images of the two classes of naked singularities in this latter case, differ also from one another, thereby allowing them to be distinguish from one another through the observation of the images.

gr-qc↗