Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests gravity fundamental physics in the strong-field regime. We consider a wide range well-motivated deviations from classical General Relativity (GR) BH solutions, constrain them using Event Horizon Telescope (EHT) observations Sagittarius A$^*$ (Sgr A$^*$), connecting size bright ring emission to that underlying shadow exploiting high-precision measurements Sgr A$^*$'s mass-to-distance...

It is generally believed that the shadows of either a black hole or naked singularity arise due to photon spheres developing in these spacetimes. Here we propose new spherically symmetric spacetime which has no sphere, and show casts shadow absence sphere. We discuss some novel features this lightlike geodesics spacetime. compare here with cast by Schwarzschild first type Joshi-Malafarina-Narayan (JMN1) singularity, where for last two spacetimes formed presence seen, particular, size...

In this paper, we derive the solutions of orbit equations for a class naked singularity spacetimes and compare these with timelike orbits, that is, particle trajectories in Schwarzschild black hole spacetime. We find perihelion precession orbits hole. then discuss different distinguishable physical properties geometries, implications are discussed. Several interesting differences emerge from our results, including conclusion bound can precess opposite direction motion, which is not possible...

It is now known that the shadow not only property of a black hole, it can also be cast by other compact objects like naked singularities. However, there exist some novel features singularities which are elaborately discussed in recent articles. In earlier literature, shown singularity may admit negative precession bound timelike orbits cannot seen Schwarzschild and Kerr hole spacetimes. This distinguishable behavior orbit presence along with useful to distinguish between observationally....

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

Abstract In this paper, we construct the rotating Janis–Newman–Winicour (JNW) naked singularity spacetime using Newman–Janis Algorithm (NJA). We analyse NJA with and without complexification methods find that energy conditions do satisfied when skip step. study shadows cast by JNW compare them Kerr black hole. shadow of can be distinguished from While precession timelike bound orbits in spacetime, it have a negative (or opposite) precession, which is not present hole case. These novel...

Abstract The general theory of relativity predicts the relativistic effect in orbital motions S-stars which are orbiting around our Milky-way Galactic Center. post-Newtonian or higher-order approximated Schwarzschild black hole models have been used by GRAVITY and UCLA Center groups to carefully investigate S2 star’s periastron precession. In this paper, we scalar field on dynamics star. Hence, consider a spacetime, namely Janis-Newman-Winicour (JNW) spacetime is seeded minimally coupled,...

Abstract Astrometric observations of S-stars provide a unique opportunity to probe the nature Sagittarius-A* (Sgr-A*). In view this, it has become important understand and behavior timelike bound trajectories particles around massive central object. It is known now that whereas Schwarzschild black hole does not allow negative precession for S-stars, naked singularity spacetimes can admit positive as well orbits. this context, we study perihelion test particle in Kerr spacetime geometry....

By imposing suitable junction conditions on a spacelike hypersurface, we obtain two-parameter family of possible static configurations from gravitational collapse. These exemplify new class naked singularities. We show that these admit consistent description via two-fluid model, one which might be dust. then study lensing and accretion disk properties our solution point out differences with black hole scenarios. The distinctive features compared to the existing singularity solutions in...

We investigate here the locally naked singularity formed due to a spherically symmetric inhomogeneous collapsing cloud having nonzero isotropic pressure, in terms of its strength. A sufficient condition provided by Clarke and Krolak for it be Tipler strong has been used restrict parameters that represent nonlinear relation between physical radius radial coordinate outgoing null geodesic at singular center. Studying an end state requires information about dynamics collapse, which is unknown...

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 distinguish then these entities each other, in terms of further physical point out that holes always admit timelike bound orbits having positive perihelion precession. Also, while a singularity with photon sphere shadow, it could precession for such orbits, thereby mimicking hole. This indicates...

We construct a model for galactic dark matter that arises as solution of Einstein gravity, and is Bertrand space-time matched with an external Schwarzschild metric. This can explain rotation curves. Further, we study gravitational lensing in these space-times, particular consider rings, using the strong formalism Virbhadra Ellis. Our results are good agreement observational data, indicate under certain conditions, effects from may be similar to backgrounds.

We investigate here the local versus global visibility of a spacetime singularity formed due to gravitational collapse spherically symmetric dust cloud having nonzero velocity function. The conditions are investigated that ensure singularity, in sense outgoing null geodesics leave boundary matter future, whereas, past, these terminate at singularity. Explicit examples this effect constructed. require must be strong curvature Tipler, physical significance scenario considered. This may act as...

ABSTRACT The global visibility of a singularity as an end state the gravitational collapse spherically symmetric pressureless cloud is investigated. We show existence non-zero measured set parameters: total mass and initial mean density collapsing cloud, giving rise to physically strong globally visible for fixed velocity function. such indicates that stable under small perturbation in data causing its existence. This true marginally well non-marginally bound cases. possibility presence...

In this paper, we investigate the light trajectories and shadow properties in rotating version of null naked singularity (NNS) spacetime which is derived using Newman- Janis algorithm without complexification method. We discuss some geometrical causal structure Rotating Naked Singularity (RNS) spacetime. The gravitational lensing a analyzed, results are compared to those Kerr black hole. case hole, photon sphere exists for both prograde retrograde orbits, whereas RNS, only orbits. As result,...

Bertrand space-times (BSTs) are static, spherically symmetric solutions of Einstein's equations, that admit stable, closed orbits. Starting from the fact to a good approximation, stars in disc or halo regions typical galaxies move such orbits, we propose that, under certain physical assumptions, dark matter distribution some low surface brightness can seed particular class BSTs. In Newtonian limit, it is shown for flat rotation curves, our proposal leads an analytic prediction...

Abstract We investigate here the final state of gravitational collapse a non-spherical and non-marginally bound dust cloud as modeled by Szekeres spacetime. show that directionally globally naked singularity can be formed in this case near collapsing boundary not at its geometric center, is typically for spherical collapse. This strong curvature sense Tipler criterion on strength. The null geodesics escaping from would less scattered certain directions since close to cloud, current scenario....

In the present paper, we study thermal properties of geometrically thin accretion disks surrounding null naked singularity (NNS) and first type Joshi-Malafarina-Narayan (JMN1) spacetimes compare results with disk around equally massive Schwarzschild black hole. First, examine circular orbits in these spacetimes. The emitted flux, radiation spectra, efficiency, temperature distribution on surface are then investigated. efficiency conversion accreting matter into is found to be substantially...

We discuss the equilibrium conditions of gravitational collapse a spherically symmetric matter cloud. analyze spinor structure general collapsing spacetime and redefine by using Cartan scalars. qualitatively investigate configuration two-fluid system consisting dustlike fluid with negative equation state. use our results to certain cosmological scenarios where dark energy can cluster inside overdense regions together reaches stable configuration. compare outcomes work existing virialization...

This paper explores the evolution of overdense region dark matter in presence a nonminimally coupled scalar field which is used to model quintessence and phantomlike energy. We focus on algebraic coupling, where interaction Lagrangian independent derivatives field. To make our more relativistic, like minimal coupling scenario we studied earlier, consider spacetime structure that internally closed Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) externally generalized Vaidya spacetime. allows...

Abstract We investigate the global causal structure of end state a spherically symmetric marginally bound Lemaitre–Tolman–Bondi (LTB) (Lemaitre 1933 Ann. Soc. Sci. Brux. A 53 51, Tolman 1934 Proc. Natl Acad. USA 20 169–76, Bondi 1947 Mon. Not. Astron. 107 410) collapsing cloud (which is well studied in general relativity) framework modified gravity having generalized Lagrangian <?CDATA $R+\alpha R^2$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">...