arXiv subjects

Ujjal Debnath

Publications and source records attributed to Ujjal Debnath.

At least 19 recordsLinked to original sources

arXiv2026

An Abel-Inversion Formalism for Spacetime Metric Reconstruction from Light Deflection

We develop an inverse lensing formalism for reconstructing the metric function of a static, spherically symmetric spacetime directly from the gravitational deflection angle of light. By formulating the inverse problem through an Abel transformation, we derive an integro-differential relation connecting the observable deflection profile to the underlying spacetime geometry. As a consistency check, we consider the case of vanishing deflection, $α(b)=0$, and recover $A(r)=1$, corresponding to flat Minkowski spacetime. We then apply the formalism to the gravitational deflection of light by the Sun using the observationally motivated leading-order expression $α(b)=\frac{2(1+γ)GM_{\odot}}{c^{2}b}.$The resulting metric function is shown to recover the Schwarzschild form in the weak-field limit $r\gg M$. We further consider the higher-order correction to the solar deflection angle and reconstruct the corresponding metric beyond the leading-order approximation. Interestingly, the $(M/r)^2$ term vanishes in the resulting weak-field expansion, yielding an improved approximation compared with the metric reconstructed from the leading-order deflection angle. Our results demonstrate that gravitational lensing observations can provide a direct route to reconstructing the underlying spacetime geometry without assuming a specific metric a priori. This formalism therefore, offers a novel framework for connecting observational light propagation with spacetime geometry.

gr-qc↗

arXiv2026

Thermodynamic stability of the new black hole solution in $f(R,G,T)$ gravity

In this literature, we derive an approximate black hole metric within the $ f(R, G, T) $ gravity framework, where the black hole is surrounded by an anisotropic fluid as the matter source. This approach allows us to study the influence of anisotropic pressures on the black hole structure. We then investigate the thermodynamic properties of the approximate solution, considering various types of matter fields. We analyze the thermodynamic quantities such as Wald's entropy, Hawking temperature, and specific heat for each case. After examining the specific heat, we find that the black holes are thermodynamically stable, with positive specific heat values indicating stability. This result suggests that the black holes under these conditions do not undergo thermal instability, ensuring their long-term viability.

gr-qc↗

arXiv2026

Charged Black Hole Solution in Hoyle Narlikar Gravity and its Thermodynamic Properties

In this paper, we derive a new class of analytic black hole solutions within the framework of Hoyle--Narlikar gravity, where the black hole is surrounded by an electric charge acting as the source. After obtaining the corresponding metric, we investigate the thermodynamic properties of the resulting black hole solutions, including the Hawking temperature and entropy. Subsequently, we compute the specific heat and Gibbs free energy in order to analyse the thermodynamic stability of the black hole in Hoyle--Narlikar gravity. An interesting result of our analysis is that the black hole becomes thermodynamically stable for large values of the horizon radius within this framework.

gr-qc↗

arXiv2026

Exploring the Observational Constraints and Cosmological Dynamics in f(Q,L_m) Gravity

We explore two scenarios of $f(Q,\mathcal{L}_m)$ gravity: linear and non-linear gravity models. The dynamical system analysis identifies two critical points for each of the proposed linear and nonlinear matter--geometry coupling models. These equilibrium points correspond to distinct phases of cosmic evolution. Depending on the model parameters, the resulting critical points successfully reproduce the observed sequence of cosmic evolution, from a decelerated matter-dominated Universe to the present epoch of accelerated expansion. The effective equation of state parameter ($ω_{\text{eff}}$) and the deceleration parameter ($q$) exhibit smooth transitions from decelerated to accelerated expansion, with transition epochs around $N_{\text{tr}} \approx -0.27$ for linear Model and $N_{\text{tr}} \approx -0.32$ for non-linear Model, consistent with late-time cosmic acceleration. Statistical constraints derived from CC+BAO, DESI DR II, and Pantheon$^+$ datasets provide best-fit values for the model parameters ($α, β, γ, H_0$), showing compatibility with current cosmological observations. The analysis employs Akaike (AIC) and Bayesian (BIC) information criteria to evaluate model performance. Our results demonstrate that $f(Q,\mathcal{L}_m)$ gravity provides a viable alternative framework for explaining late-time acceleration, with rich dynamical features that merit further exploration in view of upcoming high-precision surveys.

physics.gen-ph↗

arXiv2026

Influence of Generalized Ghost Dark Energy on Wormhole Geometry

This article presents a wormhole solution constructed from a generalized ghost dark energy (GGDE) source in general relativity. At first, a brief review of GGDE is presented along with the necessary mathematical frameworks. We employed the Markov Chain Monte Carlo (MCMC) technique to constrain the free parameters of the model using the CC+BAO, $Pantheon^+$, and their combined datasets. Furthermore, the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) were used to statistically assess the model's performance and determine its level of acceptance. Next, the basics of wormhole geometries along with the thin-shell formalism are explained in detail. After that, three different wormhole solutions associated with three different choices of the redshift function are presented ,and their various geometric properties as well as energy conditions are studied graphically. Finally, the stability of these thin--shell structures is examined analytically as well as graphically by studying the associated effective potential.

physics.gen-ph↗

arXiv2026

Cosmological Tests of $f(R,G,T)$ Dark Energy Model in FRW Universe

This research article presents a new cosmological model formulated within the $f(R,G,T)$ framework, focusing on the observational signatures and parameter constraints of the model. The Markov Chain Monte Carlo (MCMC) technique is employed to effectively explore the parameter space using data from 36 Cosmic Chronometers and 1701 Pantheon Plus data points. A comparative analysis is conducted between the proposed $f(R,G,T)$ model and the widely accepted $Λ$CDM model, considering various cosmological parameters, such as Deceleration, Snap, and Jerk. By evaluating these parameters, valuable insights into the dynamics and evolution of the universe within the context of the new model are obtained. Diagnostic tests including Statefinder and Om Diagnostic are performed to further investigate the behavior and consistency of the $f(R,G,T)$ model. These tests provide deeper insights into the properties of the model and its compatibility with observational data. The model is subjected to statistical analysis using Information Criteria to rigorously assess its goodness of fit to the data. This analysis helps determine the level of agreement between the $f(R,G,T)$ model and the observational data, establishing the viability and reliability of the proposed cosmological framework. The results highlight the potential of the $f(R,G,T)$ framework in understanding the fundamental aspects of the universe's evolution and dynamics. The comparative analysis with the $Λ$CDM model, along with the comprehensive diagnostic tests performed, demonstrates the efficacy and validity of the $f(R,G,T)$ model in explaining observed cosmological phenomena. These findings contribute to the ongoing pursuit of accurate and comprehensive models that provide a deeper understanding of the nature of our universe.

gr-qc↗

arXiv2026

$Λ(t)$CDM Model: Cosmological Implications and Dynamical System Analysis

We investigated a time-varying cosmological constant model using recent BAO measurements from DESI DR2, combined with Type Ia supernova samples (Pantheon$^{+}$, DES-Dovekie, and Union3) and CMB shift parameters, to constrain the $Λ(t)$CDM model parameters via Markov Chain Monte Carlo analysis. We find that the interaction term $Q(z)$ shows a sign change for all dataset combinations by crossing $Q(z)=0$, depending on the choice of the dataset: at low redshift $Q(z)<0$, indicating vacuum energy decaying into dark matter, while at high redshift $Q(z)>0$, corresponding to dark matter decaying into vacuum energy. The dynamical system analysis found three critical points, namely $P_1,P_2$, and $P_3$ respectively. The resulting critical points, determined by the underlying cosmological parameters, correspond to distinct epochs in cosmic evolution. Depending on the parameter combinations, these points characterize various cosmological phases, ranging from an accelerated stiff matter-dominated era to late-time accelerated expansion. The stability of each critical point is analyzed using linear stability theory, with the relevant physical constraints on the cosmological parameters duly incorporated throughout the analysis. For each dataset combinations, the $Λ(t)$CDM model predicts that $ω_0 > -1$, showing a preference for dynamical dark energy over the cosmological constant scenario with $ω_0 = -1$. Consequently, the model exhibits a transition phase in the range $N \equiv \log a(t) \approx -0.51$ to $-0.48$ and predicts $q_0$ in the range $-0.54$ to $-0.52$, with the precise transition point depending on the choice of dataset. Finally, the Bayesian evidence shows strong support for the $Λ(t)$CDM model over $Λ$CDM

gr-qc↗

arXiv2025

Compact Objects in 4D Einstein Gauss Bonnet Gravity A Data Based Perspective

Cosmic evolution is the most sensational topic among researchers of modern cosmology. This article explores cosmic evolution in 4D Einstein Gauss Bonnet gravity, focusing on mass accretion of compact objects (black holes and wormholes) by dark energy. Three DE models CPL, JBP, and BA parameterizations are studied within 4D EGB gravity, with their Hubble parameters derived and compared against observational data (Cosmic Chronometers, Type Ia Supernovae, and Baryon Acoustic Oscillations). Bayesian analysis favors the CPL and BA models, with CPL providing the best fit. For black holes, mass accretion of CPL and JBP DE shows transitions between quintessence and phantom eras, while BA and $Λ$CDM strictly exhibit quintessence-like behavior, driving cosmic acceleration. In contrast, wormholes exhibit the opposite trend, favoring a phantom-dominated era for the BA and $Λ$CDM models. The study highlights the dynamic nature of DE in 4D EGB gravity and its role in cosmic expansion.

gr-qc↗

arXiv2025

SSFDE Model: Cosmological Implications and Dynamical System Analysis

In this paper, we consider an interacting scalar field dark energy model with an exponential potential and a dark sector coupling $ Q = 3γHρ_{dm} $, which has been observationally tested using recent baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument Data Release 2 , Unanchored Type Ia Supernovae, and the compressed CMB likelihood. We find that the Interacting model predicts a Hubble constant of $h = 0.659 \pm 0.0063$, deviating from the $Λ$CDM value by approximately $2.93σ$, while the Non-Interacting model shows a $3.78σ$ deviation. The positive coupling parameter ($ γ> 0 $) further suggests a transfer of energy from dark matter to dark energy. According to the Jeffreys scale, the Interacting model shows moderate evidence against the $Λ$CDM model, whereas the Non-Interacting model shows only inconclusive evidence. Further, we investigate both models through the lens of dynamical systems analysis. We formulate the cosmological evolution equations with a phenomenological interaction term and recast them into an autonomous system to study the qualitative behavior of cosmic expansion. Critical points of the system are identified and analyzed to study the corresponding cosmological dynamics. In the interacting model, we obtained five critical points, whereas in the non-interacting scenario, four distinct sets of critical points were identified. The obtained critical points, governed by cosmological parameters, represent distinct cosmic epochs, commencing from the early time stiff matter domination to late-time acceleration. Their stability is examined through linear stability analysis under appropriate physical constraints. The evolution of background cosmological parameters are also examined in terms of the dynamical system variable, and the obtained values align with observational results.

gr-qc↗

arXiv2025

Co-existence of alternative Generalized Chaplygin Gas and other Dark Energies in the Framework of Fractal Universe

We have explored the possibility of the co-existence of two forms of dark energy in the form of an alternative Generalized Chaplygin Gas (GCG) along with a scalar field of field theoretic or extra dimensional origin in the background of a fractal universe. The essential physical model parameters have been computed and their variations have been plotted. Fractal cosmology in an universe dominated by the alternative GCG (overcoming the drawbacks of the conventional form of GCG) has also been studied by computing the equation of state, deceleration and statefinder parameters. Their variations with the redshift have also been studied. We have tried to construct alternative cosmological models deviating from standard cosmology that can be put to observational testing.

gr-qc↗

arXiv2025

Strong gravitational lensing by black hole in F(R) Euler Heisenberg Gravity's Rainbow

We investigate gravitational lensing in the strong-field regime for a black hole in F(R) Euler Heisenberg Gravity with Rainbow gravity modifications. This black hole spacetime is characterized by the Euler Heisenberg parameter, F(R) parameters, Rainbow functions, the black hole charge Q, and mass M . We numerically compute the strong deflection angle and its associated coefficients, and explore their astrophysical implications for supermassive black holes in different galaxies. Our findings show that increasing the Euler Heisenberg parameter enhances key lensing observables such as the photon sphere radius, critical impact parameter, angular position, relative magnification, Einstein ring radius, and time delay for a fixed Q . Conversely, increasing Q decreases these parameters while keeping other quantities fixed. Additionally, a higher Euler Heisenberg parameter reduces the deflection angle and angular separation S , whereas increasing Q causes both to increase. Our study reveals that black holes in this modified gravity framework can act as strong gravitational lenses, producing deflection angles that surpass those of Reissner Nordstrom and Schwarzschild black holes under specific conditions. These results highlight the unique topological features of modified charged black holes and suggest their potential as astrophysical candidates, offering new insights into their observational signatures. Furthermore, we analyze the sensitivity of the lensing predictions with respect to changes in the functional forms of the Rainbow functions. The combined effects of F(R) gravity, Euler Heisenberg electrodynamics, and Rainbow gravity introduce novel qualitative features not present in the individual models.

astro-ph.GA↗

arXiv2025

Exploring the dynamics of coincident f(Q) gravity in the presence of DBI-essence scalar field

In theoretical cosmology, symmetric teleparallel gravity or $f(Q)$ gravity based on nonmetricity tensor Q has become an interesting alternative to General relativity in recent years. The present research paper contains a rigorous dynamical system analysis of coincident f (Q) gravity in the presence of a generalized DBI essence scalar field. We have considered two different models of coincident f(Q) gravity, such as power law model $f(Q) = Q + nQ^m$ and exponential model $f(Q) = Q e^{\frac{βQ_0}{Q}}$ respectively, where n, m, \b{eta} are constant parameter and Q is the nonmetricity component. In this study, the generalized DBI essence scalar field acts as an additional dark energy component. After obtaining the field equation for the corresponding cosmological model, we employed several dynamical variables to form the dynamical system. The critical points of these dynamical systems are influenced by cosmological parameters and associated with particular epochs in the cosmological timeline. For different combinations of cosmological parameters, the critical points exhibit different cosmological eras, starting from the accelerated stiff matter era to late-time acceleration phenomena. The stability criteria of each critical point are studied by using linear stability theory, and the physical constraints on the cosmological parameters are also considered during this analysis. Furthermore, the current values of energy densities, deceleration parameters, and equation of state parameters obtained from the evolution diagram are compatible with observational data.

gr-qc↗

arXiv2025

Possible Formation of Traversable Wormholes and Their Thermodynamic Analysis in $\mathcal{F}(Q,\mathcal{L}_{m},\mathcal{T})$ Gravity

In this work, we investigate static and spherically symmetric traversable wormhole solutions within the framework of the extended symmetric teleparallel gravity, specifically the $\mathcal{F}(Q,\mathcal{L}_{m},\mathcal{T})$ gravity theory, where $Q$, $\mathcal{L}_{m}$, and $\mathcal{T}$ are the respective representations of the non-metricity scalar, the matter Lagrangian, and the trace of the energy-momentum tensor. By employing a specific redshift function and deriving the shape function through the Karmarkar condition, we examine the fundamental geometric features required for a viable wormhole structure. The analysis confirms the satisfaction of key conditions such as the throat condition, flaring-out condition, and asymptotic flatness. A detailed study of energy conditions for various values of model parameters reveals that the null energy condition and averaged null energy condition are violated near the throat, indicating the presence of exotic matter. Additionally, thermodynamic quantities such as temperature, pressure, specific heat, work density, and energy flux are analyzed, all of which support the thermal and equilibrium stability of the wormhole. Our findings demonstrate that even in extended theories like $\mathcal{F}(Q,\mathcal{L}_{m},\mathcal{T})$ gravity, exotic matter remains essential for sustaining traversable wormholes. This work lays the foundation for further investigations into their stability under dynamical perturbations and potential astrophysical implications.

gr-qc↗

arXiv2025

f(R, $G$, T) Gravity: Cosmological Implications and Dynamical System Analysis

We consider the cosmological implications of a four-dimensional extension of the Gauss-Bonnet $f(G)$ gravity, where $G$ is the Gauss-Bonnet topological invariant, in which the Einstein-Hilbert action is replaced by an arbitrary function $f(R,G,T)$ of G, of the Ricci scalar $R$, and of the trace $T$ of the matter energy-momentum tensor. By construction, the extended Gauss-Bonnet type action involves a non-minimal coupling between matter and geometry. The field equations of the model are obtained by varying the action with respect to the metric. The generalized Friedmann equations, describing the cosmological evolution in the flat Friedmann-Lemaitre-Robertson-Walker geometry, are also presented in their general form. We investigate the cosmological evolution of the Universe in the generalized Einstein-Gauss-Bonnet theiry for a specific choice of the Lagrangian density, as given by $f(R,G,T) = α_1 G^{m} + α_2 R^β - 2α_3 \sqrt{-T},$ where $α_i$ $ i = 1, 2, 3$), $ m $, and $β$ are model parameters. First, the theoretical predictions of the model are compared with a set of observational data (Cosmic Chronometers, Type IA Supernovae, Baryon Acoustic Oscillations) via an MCMC analysis, which allows us to obtain constraints on the model parameters. A comparison with the predictions of the $Λ$CDM system is also performed. Next, the generalized Friedmann equations are reformulated as a dynamical system, and the properties of its critical points are studied by using the Lyapunov linear stability analysis. This investigation allows for the reconstruction of the Universe's history in this model, from the early inflationary era to the late accelerating phase. The statefinder diagnostic parameters for the model are also considered from the dynamical system perspective.

gr-qc↗

arXiv2025

The New Black Hole Solution with Anisotropic Fluid in f(R,Lm, T) Gravity: Thermodynamics

In this work, we derive a new class of analytic black hole solutions within the framework of f(R,Lm, T) gravity, where the black hole is surrounded by an anisotropic fluid acting as the matter source. We consider both linear and nonlinear forms of the function f(R,Lm, T), enabling a detailed exploration of how anisotropic pressures and different f(R,Lm, T) influence the spacetime structure. Furthermore, we derive the conditions on the coupling parameters (\b{eta}1, \b{eta}2, \b{eta}3) under which the energy conditions are satisfied. Utilising these constraints, we then investigate the thermodynamic behaviour of the resulting black hole solutions in the presence of various matter fields, namely, dust, radiation, and a quintessence field, each characterised by a distinct equation-of-state parameter. An important outcome of this study is that the results obtained deviate from those predicted by standard General Relativity. It is also observed that these deviations depend explicitly on the interaction between the matter Lagrangian Lm and the trace T of the energy-momentum tensor.

gr-qc↗

arXiv2025

Reconstructions of Einstein-Aether Gravity from Barrow Agegraphic and New Barrow Agegraphic Dark Energy models: Examinations and Observational Limits

We present a comprehensive investigation exploring the theoretical framework of Einstein-Aether gravity theory when combined with two modified cosmological paradigms: the Barrow Agegraphic Dark Energy (BADE) and its newer variant, the New Barrow Agegraphic Dark Energy (NBADE). Our study focuses on reconstructing the functional form of the Einstein-Aether Lagrangian component $F(K)$ from these phenomenological dark energy models. Model parameters are constrained using a Markov Chain Monte Carlo (MCMC) approach based on multiple datasets, including cosmic chronometers (CC), Baryon Acoustic Oscillations (BAO), and the Pantheon+SH0ES compilation. Using best-fit parameters, we analyze various cosmological diagnostics: Hubble and deceleration parameter evolution, dark energy equation of state $ω_{DE}$, density parameter trajectories, $ω'_{DE}$--$ω_{DE}$ phase space behavior, statefinder diagnostics $(r,s^*)$ and $(r,q)$, and Om(z) trajectories. Both models exhibit late-time acceleration, with the dark energy sector showing a quintessence-like nature in the current epoch and evolving toward a phantom regime in the future. Stability analysis based on the squared sound speed $v_s^2$ highlights partial epoch-dependent stability. While our results demonstrate reasonable agreement with observational data and reveal physically plausible dynamics, the models do not yet offer a fundamentally superior alternative to other dark energy reconstructions. Nonetheless, their behavior under modified entropy assumptions and their flexibility in dynamical diagnostics provide a useful framework for probing non-standard extensions of Einstein-Aether gravity and dark energy phenomenology.

gr-qc↗

arXiv2025

How parameter constraining can influence the mass accretion process of a Black Hole in the Generalized Rastall Gravity Theory ?

Black holes, one of the greatest enigmas of our Universe, are challenging to decipher. This work is dedicated to observing the changes in the mass of a non-singular black hole with the evolution of the Universe in the generalized Rastall gravity framework, considering the effects of parameter constraining. We examine two recently developed dynamical dark-energy equation of state parameterization models: Chaudhary-Bouali-Debnath-Roy-Mustafa-type~(CBDRM)~parameterization and Chaudhary-Debnath-Mustafa-Maurya-Atamurotov-type~(CDMMA)~parameterization. Starting with the concept and fundamental equations of the generalized Rastall gravity theory, we introduce the two models along with their equations of state, energy density equations, and corresponding Hubble parameter equations. We then constrain the required parameters using Monte Carlo Markov chain (MCMC) analyses to ensure the accuracy and reliability of our study. Next, we discuss the non-singular black holes from the perspective of generalized Rastall gravity theory and some of their essential properties. Finally, we pursue the primary goal of our work: analyzing the mass accretion process. We derive the mass equation for both models in terms of the redshift function, represent the results graphically, and compare them with the standard $Λ$CDM model of the Universe. Our findings indicate that the accretion of both CBDRM~and~CDMMA dark energy parameterizations, considering constrained parameter values, leads to an increase in the mass of the black hole during the Universe's evolution within the generalized Rastall gravity theory, revealing the true nature of dark energy.

gr-qc↗

arXiv2025

On the Field Theoretical Description of an Alternative Model to Generalized Chaplygin Gas and its Thermodynamic Behaviour

This paper investigates a newly proposed fluid description of dark energy within the framework of the late-time accelerated expansion of the universe. Our primary objective is to explore the theoretical foundation of the proposed equation of state by establishing its correspondence with well-known scalar field models such as quintessence, k-essence, and DBI-essence. Through this correspondence, we reconstruct key field parameters, including the scalar field $ϕ$ and scalar potential $V(ϕ)$, and analyze their evolutionary behavior across cosmic time. The study also evaluates the model's physical consistency and cosmological implications by examining fundamental energy conditions - Null Energy Condition (NEC), Dominant Energy Condition (DEC), and Strong Energy Condition (SEC). Furthermore, we conduct a comprehensive stability analysis to ensure the robustness of the model and investigate its thermodynamic properties, including possible phase transitions using entropy and Gibbs free energy. To assess the observational viability of the model, we compare its predictions against recent datasets, including Cosmic Chronometers (CC), Baryon Acoustic Oscillation (BAO), and Supernova Type Ia from the Pantheon+SH0ES compilation and Union 2.1, as well as recent DESI and DESY5 data. Our analysis demonstrates that the proposed fluid model aligns well with observational constraints, reproduces the late-time acceleration of the universe, and offers a compelling alternative to the standard $Λ$-CDM model while maintaining consistency with current data.

gr-qc↗