arXiv subjects

K. K. Nandi

Publications and source records attributed to K. K. Nandi.

At least 19 recordsLinked to original sources

arXiv2026

Galactic microlensing by backreacted massless wormholes

We study here a novel application of Kim \& Lee charged wormholes assuming them to be dark halo objects playing the role of lenses in the Galactic microlensing with source stars belonging to the Galactic Bulge and the Large Magellanic Cloud. First, we observe that both the backreacted scalar ($\alpha $) and electrically ($Q$) charged wormholes have the same zero ADM mass as has the background Ellis-Bronnikov wormhole having a special equation of state parameter $\gamma=-1$. In particular, we argue that, for $\alpha\neq 0$, the solution formally resembles, but can at best be sourcewise different from, that of the background wormhole. The charge ($Q\neq 0$) thus provides an extra degree of freedom that introduces a non-trivial redshift function $\Phi$ to the background, alters its throat radius to $r_{th}$, yet keeps the wormhole massless. Second, we focus on this electrically charged case and calculate the light deflection angle up to 4$^{th}$ PPN order, analyze the effect of $Q$ on the lensing observables such as the image positions, magnification, centroid and time delay of images of the source stars. Third, we analyze the probabilistic features such as optical depth and event rate estimated on the basis of the hypothesis that the wormhole lens could be bound or unbound to our Galaxy. Finally, we report an intriguing qualitative prediction that, compared to the Schwarzschild black hole, the Paczy\'{n}ski light curves of the electrically charged wormhole are much dimmer that also show characteristic gutters at the times the source enters and exits the Einstein ring. \textit{The gutters gradually come together as $Q$ approaches the extreme limit $r_{th}/\sqrt{2}$, at which the Einstein radius $R_{E}$ vanishes so that the source crosses it instantly.} It is speculated that re-analyzing past data on Galactic microlensing may betray the presence of charged wormholes.

gr-qc↗

arXiv2026

On the Kalb-Ramond modified Lorentz violating hairy black holes and Thorne's hoop conjecture

Recently, a class of static spherically symmetric power law corrected Lorentz violating (LV) Schwarzschild black holes in the Kalb-Ramond model have been derived and studied in the specific range of LV parameters ($0<\lambda \leq 2,\Upsilon \geq 0$) that correspond to energy condition preserving ($\rho >0$) source. On the other hand, there exist well known black holes that do not preserve the energy conditions. In this paper, we shall therefore relax energy conditions and numerically explore the horizon patterns of the enlarged class of LSMA black holes. Four generic types of LV corrected black holes emerge, which interestingly include the analogue of the \textit{braneworld} black hole ($\rho <0$) lending to $\Upsilon$ a new interpretation of "tidal charge" known as an imprint from the $5d$ bulk in the Randall-Sundrum scenario. We shall then show that Thorne's hoop conjecture, $\mathcal{H} \leq 1$, where $\mathcal{H}$ is the Hod function, consistently holds for three types and their generalizations. However, intriguingly, it turns out that, for the remaining type (viz., Schwarzschild-de Sitter and its generalizations), the hoop conjecture does \textit{not} hold. It is also shown that braneworld tidal charge black holes increases the LV correction to planetary perihelion advance in contrast to the decrease due to ordinary black holes thereby providing a qualitative distinction between them.

gr-qc↗

arXiv2026

Gravitational time advancement effect in Bumblebee gravity for Earth bound systems

This paper is a novel application of the new effect of gravitational time advancement or \textit{negative} time delay, first predicted for static black holes (spin $a=0$), that can be regarded as complementary to the well known effect of positive Shapiro time delay. We shall extend the Shapiro time delay formalism up to third PPN order using the recently proposed spinning ($a\neq 0$) black hole solution of the Lorentz symmetry breaking (LSB) Bumblebee gravity that is believed to reveal signatures of quantum gravity at low energies. Adopting two practical examples of signal propagation along Earth-Moon and Earth-Satellite configurations, we shall calculate the influence of the Bumblebee parameter $\ell$ on time advancement using terms up to the second PPN order $\varpropto aM$ and $M^{2} $ as the Bumblebee solution is valid only upto first order in $a$. It is shown that there is a critical radial distance $r_{c}$ above the Earth, where the Shapiro delay vanishes, and beyond $r_{c}$ the delay becomes negative, i.e., time advancement begins to set in, leading to the intriguing consequence that the measured LLR distance to Moon or any Satellite becomes \textit{less} than the zeroth order Euclidean distance. It is shown that the LSB correction arises from the conical geometry of the massless Bumblebee spacetime leading to upper bounds on the correction to the zeroth order Euclidean time interval as $\delta \tau _{\text{LSB}}^{\text{Eucl}}<0.8\times 10^{-4}$ (ns) and to time advancement as $\Delta \tau _{\text{LSB}}^{\text{adv}}<-4.5\times 10^{-13}$ (ns), both estimates based on the bound on $\ell $ corresponding to the Cassini spacecraft experiment. We shall also briefly touch upon the feasibility of direct experimental detection of the advancement effect.

gr-qc↗

arXiv2026

Damour-Solodukhin Wormhole as a Black Hole Mimicker: The Role of Observers' Location

It has been recently argued that in semi-classical gravity, a minimal 2-sphere is not a horizon but a tiny throat of a wormhole, such as the Damour--Solodukhin wormhole (DSWH), with a free parameter $\lambda \neq 0$ separating it from a Schwarxzschild black hole (BH) ($\lambda =0$). As shown by DS, their horizonless WH can mimic many properties of a black hole (BH). Assuming that observing a BH mimicker is equivalent to observing a BH itself, we ask the question as to which identity of the object, a WH or a BH, an observer is likely to observe in a single experiment. To answer this, we introduce Tangherlini's new concept of indeterminacy in the gravitational field by portraying the field as a refractive medium. We then postulate that \textit{the identity of the observed object will depend on the probabilistic outcome of photon motion probing the object}. The probabilities will be described by Fresnel reflection ($R$) and transmission ($T$) coefficients derived by Tangherlini on the basis of a non-quantum statistical indeterminacy of photon motion in ordinary optical media. By adapting this approach to a gravitational "effective optical medium," we obtain two intriguing results: (i) The Fresnel coefficients at the DSWH throat are independent of mass $M$ but dependent solely on the parameter $\lambda \neq 0$. (ii) Depending on the location of the observer, what is a DSWH to one observer may appear as a BH to another observer for the same value of $\lambda \neq 0$.

physics.gen-ph↗

arXiv2026

On a Class of Harko-Kovacs-Lobo Wormholes

The Harko, Kov\'{a}cs, and Lobo wormhole (HKLWH) metric contains two free parameters: one is the wormhole throat $r_{0}$, and the other is a dimensionless deviation parameter $\gamma$ with values $0<\gamma <1$, the latter ensuring the needed violation of the null energy condition at the throat. In this paper, we study the energetics of the HKLWH and the influence of $\gamma$ on the tidal forces in the Lorentz-boosted frame. Finally, we apply\ a new concept, namely, the probabilistic identity of the object observed by different external observers in terms of the Fresnel coefficients derived by Tangherlini. The intriguing result is that observations can differ depending on the location of the observer, i.e., there is a nonzero probability that the HKLWH will be identified as a black hole even when $\gamma \neq 0$.

gr-qc↗

arXiv2026

Can a regular black hole be observationally distinguished from singular black holes as spinning lens partner in PSR-BH binaries?

To answer the question posed in the title, we consider a novel diagnostic, viz., the difference in the times of arrival (TOA) at the observer of two light rays that simultaneously emanate from a source behind a spinning lens and pass by either side of the lens to reach the observer. This is completely different from the usual Shapiro gravitational time delay, where only one onward light ray is reflected back to the observer. The TOA essentially samples the frame dragging caused by the spinning lens, apart from other lens parameters. Assuming a charged \textit{regular} Ay\'{o}n-Beato and Garc\'{\i}a black hole as the spinning lens partner in some typical astrophysical PSR-BH binaries, which provide the best laboratory for testing the TOA effect, we theoretically study how the prediction depends on the gyromagnetic ratio $\left(Q/M\right)$ and how it compares with those when the role of spinning lens partner is played by the centrally \textit{singular} Kerr-Newman and Kerr black holes. The numerical estimates for two illustrative binary lens systems show $\mu$sec level delay at the zeroth order, which should be measurable. However, the TOA predictions under thin-lens approximation are shown to differ only at third or higher orders of smallness indicating that the regular and singular black holes \textit{cannot} be observationally distinguished despite significant qualitative differences existing among them.

gr-qc↗

arXiv2026

Testing generalized spacetimes for black holes using the Hod function representation of the hoop conjecture

The hoop conjecture, due to Thorne, is a fundamental aspect of black holes in classical general relativity. Recently, generalized classes of regular spherically symmetric static black holes with arbitrary exponents coupled to nonlinear electrodynamics have been constructed in the literature. The conjecture in those spacetimes could be violated if only the asymptotic mass $M_{\infty}$ is used. To avoid such violations, Hod earlier suggested the appropriate mass function and stated the conjecture in terms of what we call the Hod function. The conjecture can then be applied to any given static spacetime to test whether or not it represents black holes. It is shown here that the conjecture is protected in the above constructed class of generalized spacetimes thus supporting them as black holes. However, it is argued that there are factors, including violation of the conjecture, that militate against the proposed \textit{new} class of solutions to be qualifying as black holes. Finally, we exemplify that the Hod mass $M(r\leq R)$ in the conjecture is exactly the \textit{matter} counterpart of the Misner-Sharp \textit{geometrical} quasilocal mass $m(r\leq R)$ of general relativity. Thus any conclusion based on Hod function is strictly a conclusion of general relativity.

gr-qc↗

arXiv2026

Comparing accretion disk profiles of Bogush-Galt'sov naked singularity and Kerr black hole

It is well known that the Einstein-scalar system of general relativity can in principle yield non-unique exact spinning naked singularities, which lead to unique Kerr black hole when the scalar field is switched off. It is a challenging task to observationally distinguish these two types of objects. Since accretion process could be a viable diagnostic for this distinction, the purpose of the present work is to explore whether there could be features in the accretion profiles distinguishing the singularity from a Kerr black hole. Here we study the Novikov-Thorne thin accretion to a \textit{new} spinning naked singularity with a scalar charge $\sigma$ recently reported by Bogush and Gal'tsov (BG). Our study reveals that: (1) The conversion efficiency $\epsilon$ of the BG naked singularity is \textit{independent} of $\sigma$ and (2) The maxima of emissivity profiles for the BG singularity tend to shift towards the inner disk ISCO boundary $r=r_{ms}$ and peak at a value significantly larger than those of a Kerr black hole with the increase of $a$, $\sigma$ and relative shrinking of $\sqrt{-g}$. All these effects are \textit{quantitatively} tabulated, which reveal, for instance, that the flux from the naked singularity could be as high as $10^{5}$ times larger than that of a Kerr black hole. Since these distinguishing features are known to be shared also by other models of naked singularity, it is tempting to speculate that such behavior could be hallmark of naked singularities.

gr-qc↗

arXiv2026

Terrestrial Sagnac delay in scalar-tensor-vector-gravity

The scalar-tensor-vector-gravity (STVG), a prototype of modified gravity developed by Moffat, can correctly explain galaxy rotation curves, cluster dynamics, Bullet Cluster phenomena and cosmological data without invoking the observationally elusive general relativistic (GR) dark matter. Further, recent observations of neutron star masses are shown to defy some GR predictions, whereas STVG turns out to be more consistent with those observations. These successes indicate that STVG could be a potential candidate for a new theory of gravity. However, an important question concerns the possible range of values of the STVG dimensionless parameter $\alpha$ imposed by various physical scenarios. In the literature, the range $0.03<\alpha <2.47$ corresponding to different central source masses has been suggested. We show here that the $\alpha$ can be considerably constrained into the range $0<\alpha<10^{-5}$ assuming that the updated GPS fluctuation does not exceed the $\alpha$-dependent correction to the terrestrial Sagnac delay.

gr-qc↗

arXiv2026

Accretion Flow onto Ellis-Bronnikov Wormhole

Study of accretion onto wormholes is rather rare compared to that onto black holes. In this paper, we consider accretion flow of cosmological dark energy modeled by barotropic fluid onto the celebrated Ellis--Bronnikov wormhole (EBWH) built by Einstein minimally coupled scalar field $\phi$, violating the null energy condition. The accreting fluid is assumed to be phantom, quintessence, dust and stiff matter. We begin by first pointing out a mathematical novelty showing how the EBWH can lead to the Schwarzschild black hole under a complex Wick rotation. Then, we analyze the profiles of fluid radial velocity, density and the rate of mass variation of the EBWH due to accretion and compare the profiles with those of the Schwarzschild black hole. We also analyze accretion to the massless EBWH that has zero ADM mass but has what we call nonzero Wheelerian mass (``mass without mass''), composed of the non-trivial scalar field, that shows gravitational effects. Our conclusion is that the mass of SBH due to phantom and non-phantom accretion increases consistently with known results, while, in contrast, the mass of EBWH decreases. Accretion to massless EBWH (i.e., to nonzero Wheelerian mass) shares the same patterns as those of the massive EBWH; hence there is no way to distinguish massive and massless cases by means of accretion flow. The contrasting mass variations due to phantom accretion could be a reflection of the distinct topology of the central objects.

gr-qc↗

arXiv2026

Times of arrival (TOA) of signals in the Kerr-MOG black hole

Modified gravity (MOG) theories are alternatives to general relativity (GR) that arose primarily from the need to explain the observed galactic flat rotation curves without invoking the elusive dark matter hypothesized by GR. A well known MOG is the Scalar-Tensor-Vector-Gravity (STVG) developed by Moffat, who has also found a spinning solution called the Kerr-MOG black hole (BH) characterized by the spin $a$ and MOG parameter $\alpha$, the latter determining the strength of the gravitational vector forces. We consider the static-MOG metric ($a=0$) to first understand how the nature of geometry drastically changes depending on different sectors of $\alpha $. Then we study the influence of $\alpha $ in each sector on a new astrophysical diagnostic caused by \textit{frame dragging}, viz., the difference $\Delta t$ in the times of arrival (TOA) at the observer of signals emanating from a variable pulsar (PSR) passing behind a Kerr-MOG lens in a PSR-BH binary system. The study generalizes the zeroth order Laguna-Wolszczan formula up to third PPN order in $\left(1/r\right)$ using thin-lens approximation, which reveals how $\Delta t$ is influenced both by $a$ and $\alpha$. The magnitude and sign of $\alpha $ indicate deviations from GR ($\alpha =0$) and future measurements may constrain $\alpha$ provided a suitable binary is identified.

gr-qc↗

arXiv2026

Observable strong field effects of extra spacetime dimension in the braneworld black hole

Inspired by the string theory, the braneworld picture introduces extra dimensions beyond the four that may have observable non-trivial effects in short distance (strong field) gravity experiments. A case in point is the Randall-Sundrum braneworld picture that projects the $5d$ bulk Weyl tensor onto the $3d$ brane providing a stress tensor in the effective Einstein field equations on the brane. Dadhich, Maartens, Papadopoulos and Rezania (DMPR) derived an exact braneworld black hole solution of the brane vacuum field equations. The solution formally resembles that of Reissner-Nordstr\"{o}m but is physically different from it since the "tidal charge" $\Upsilon$ in the solution is not the electric charge but an imprint from the fifth dimension allowing both signs in the power law modification $\pm \frac{\Upsilon ^{2}}{r^{2}}$ to the Schwarzschild metric $\left( \Upsilon = 0\right)$. The corresponding black holes are designated as DMPR$\pm$. We study here the effect of $\Upsilon$ on strong field lensing observables and compare in the eikonal limit the ring down quasinormal mode (QNM) frequencies of DMPR$-$ with those of DMPR$+$, the two variants of tidal charge modified Schwarzschild black hole ($\Upsilon = 0$). It turns out that the tidal charge can significantly modify the Schwarzschild lensing observables and QNM frequencies. In particular, we find that the Pretorius-Khurana critical exponent $\gamma$ of circular null orbits in the DMPR$-$ black hole has a lower value than that for the Schwarzschild black hole, which indicates a stronger Lyapunov instability suggesting that the accretion disks of DMPR$-$ black holes would appear brighter. The case of the SgrA* black hole is considered for a possible constraint on $\Upsilon$ from the EHT observation of its shadow size.

gr-qc↗

arXiv2023

Strong field lensing by Damour-Solodukhin wormhole

We investigate the strong field lensing observables for the Damour-Solodukhin wormhole and examine how small the values of the deviation parameter $λ$ need be for reproducing the observables for the Schwarzschild black hole. While the extremely tiny values of $λ$ indicated by the matter accretion or Hawking evaporation are not disputed, it turns out that $λ$ could actually assume values considerably higher than those tiny values and still reproduce black hole lensing signatures. The lensing observations thus provide a surprising counterexample to the intuitive expectation that all experiments ought to lead to the mimicking of black holes for the same range of values of $λ$.

gr-qc↗

arXiv2023

Modified gravity black hole lensing observables in weak and strong field of gravity

We extend a recent work on weak field first order light deflection in the MOdified Gravity (MOG) by comprehensively analyzing the actual observables in gravitational lensing both in the weak and strong field regime. The static spherically symmetric black hole (BH) obtained by Moffat is what we call here the Schwarzschild-MOG (abbreviated as SMOG) containing repulsive Yukawa-like force characterized by the MOG parameter $α>0$ diminishing gravitational attraction. We point out a remarkable feature of SMOG, viz., it resembles a regular \textit{brane-world} BH in the range $-1<α<0$ giving rise to a negative "tidal charge" $Q$ ($=\frac{1}{4}\frac{α}{1+α}$) interpreted as an imprint from the $5D$ bulk with an imaginary source charge $q$ in the brane. The Yukawa-like force of MOG is attractive in the brane-world range enhancing gravitational attraction. For $-\infty <α<-1$, the SMOG represents a naked singularity. Specifically, we shall investigate the effect of $α$ or Yukawa-type forces on the weak (up to third PPN order) and strong field lensing observables. For illustration, we consider the supermassive BH SgrA* with $α=0.055$ for the weak field to quantify the deviation of observables from GR but in general we leave $α$ unrestricted both in sign and magnitude so that future accurate lensing measurements, which are quite challenging, may constrain $α$.

gr-qc↗

arXiv2023

Lensing observables: Massless dyonic vis-à-vis Ellis wormhole

Stable massless wormholes are theoretically interesting in their own right as well as for astrophysical applications, especially as galactic halo objects. Therefore, the study of gravitational lensing observables for such objects is of importance, and we do here by applying the parametric post-Newtonian method of Keeton and Petters to massless dyonic charged wormholes of the Einstein-Maxwell-Dilaton field theory and to the massless Ellis wormhole of the Einstein minimally coupled scalar field theory. The paper exemplifies how the lensing signatures of two different solutions belonging to two different theories could be qualitatively similar from the observational point of view. Quantitative differences appear depending on the parameter values. Surprisingly, there appears an unexpected divergence in the correction to differential time delay, which seems to call for a review of its original derivation.

gr-qc↗

arXiv2021

Anisotropic compact stars: Constraining model parameters to account for physical features of tidal Love numbers

In this paper, we develop a new class of models for a compact star with anisotropic stresses inside the matter distribution. By assuming a linear equation of state for the anisotropic matter composition of the star we solve the Einstein field equations. In our approach, for the interior solutions we use a particular form of the ansatz for the metric function $g_{rr}$. The exterior solution is assumed as Schwarzschild metric and is joined with the interior metric obtained across the boundary of the star. These matching of the metrices along with the condition of the vanishing radial pressure at the boundary lead us to determine the model parameters. The physical acceptability of the solutions has verified by making use of the current estimated data available from the pulsar 4U1608-52. Thereafter, assuming anisotropy due to tidal effects we calculate the Love numbers from our model and compare the results with the observed compact stars, viz. KS 1731- 260,4U 1608- 52,4U 1724- 207,4U 1820- 30,SAX J1748.9-2021 and EXO 1745-268. The overall situation confirms physical viability of the proposed approach,which can shed new light on the interior of the compact relativistic objects.

gr-qc↗

arXiv2020

Can accretion properties distinguish between a naked singularity, wormhole and black hole?

We first advance a mathematical novelty that the three geometrically and topologically distinct objects mentioned in the title can be exactly obtained from the Jordan frame vacuum Brans I solution by a combination of coordinate transformations, trigonometric identities and complex Wick rotation. Next, we study their respective accretion properties using the Page-Thorne model which studies accretion properties exclusively for $r\geq r_{\text{ms}}$ (the minimally stable radius of particle orbits), while the radii of singularity/ throat/ horizon $r<r_{\text{ms}}$. Also, its Page-Thorne efficiency $ε$ is found to increase with decreasing $r_{\text{ms}}$ and also yields $ε=0.0572$ for Schwarzschild black hole (SBH). But in the singular limit $r\rightarrow r_{s}$ (radius of singularity), we have $ε\rightarrow 1$ giving rise to $100 \%$ efficiency in agreement with the efficiency of the naked singularity constructed in [10]. We show that the differential accretion luminosity $\frac{d\mathcal{L}_{\infty}}{d\ln{r}}$ of Buchdahl naked singularity (BNS) is always substantially larger than that of SBH, while Eddington luminosity at infinity $L_{\text{Edd}}^{\infty}$ for BNS could be arbitrarily large at $r\rightarrow r_{s}$ due to the scalar field $ϕ$ that is defined in $(r_{s}, \infty)$. It is concluded that BNS accretion profiles can still be higher than those of regular objects in the universe.

gr-qc↗

arXiv2020

On the Hoop conjecture in Einstein gravity coupled to nonlinear electrodynamics

The famous hoop conjecture by Thorne has been claimed to be\ violated in curved spacetimes coupled to linear electrodynamics. Hod \cite{Hod:2018} has recently refuted this claim by clarifying the status and validity of the conjecture appropriately interpreting the gravitational mass parameter $M$. However, it turns out that partial violations of the conjecture might seemingly occur also in the well known regular curved spacetimes of gravity coupled to \textit{nonlinear electrodynamic}s. Using the interpretation of $M$ in a generic form accommodating nonlinear electrodynamic coupling, we illustrate a novel extension that the hoop conjecture is \textit{not} violated even in such curved spacetimes. We introduce a Hod function summarizing the hoop conjecture and find that it surprisingly encapsulates the transition regimes between "horizon and no horizon" across the critical values determined essentially by the concerned curved geometries.

gr-qc↗