arXiv subjects

Aritra Sanyal

Publications and source records attributed to Aritra Sanyal.

At least 19 recordsLinked to original sources

arXiv2026

The New Dark Matter Density Profile from JWST JADES Galaxies

We present the first dark matter density profile derived directly from James Webb Space Telescope JADES Data Release~3 NIRSpec observations of $N = 587$ galaxies spanning the cosmic noon epoch $1.5 \leq z \leq 3.5$. From each NIRSpec G235M/G395M spectrum we extract the H$α$ emission-line velocity dispersion $σ_{\rm ha}$, stack galaxies in four redshift bins of $Δz = 0.5$, and reconstruct representative group rotation curves within the General Theory of Relativity. Fitting the four-parameter modified exponential model and deriving the exact GTR energy density, we reduce the profile to a compact $[2/3]$ Padé approximant with all coefficients determined in closed form from JWST observations. The profile is cusp-free, fully analytic, and redshift-dependent. Energy conditions, causality, and orbital stability are all satisfied. Most strikingly, the central density $ρ(0)$ varies by less than $15\%$ across $z = 1.5$--$3.5$ despite a $35\%$ decline in the asymptotic rotation velocity, revealing a universal dark matter core saturation density at cosmic noon decoupled from baryonic evolution.

astro-ph.GA↗

arXiv2026

Charged galactic wormholes: shadow imaging, accretion disks, and charged-particle deflection

We investigate the gravitational deflection of charged massive particles by a charged galactic wormhole supported by the Sofue dark matter density profile \cite{r57}, $ρ(r)=ρ_0 e^{-r/r_0},$ where $ρ_0$ and $r_0$ denote the central dark matter density and the characteristic scale radius, respectively. This phenomenological profile models the dark matter distribution in galactic halos and acts as the matter source sustaining the wormhole geometry. Extending our previous investigation of light rays and neutral massive particles in this spacetime \cite{b1}, we study the combined effects of gravitational and electromagnetic interactions on the motion of charged massive particles. The deflection angle is computed independently using the Rindler--Ishak method based on the Jacobi metric and the Gauss--Bonnet theorem, enabling a systematic comparison between the two approaches. We find that the predictions of the two methods become nearly indistinguishable in the relativistic regime, whereas the differences at lower velocities arise from the velocity-dependent correction terms proportional to $(1-v^2)$ in the trajectory equation. We further perform backward relativistic ray tracing of optically thin accretion flows around the charged galactic wormhole to investigate its shadow and photon-ring structure. Although the shadow remains circular because of the spacetime's spherical symmetry, its radius, photon-ring structure, and intensity distribution exhibit a nontrivial dependence on the wormhole charge, providing potential observational signatures of charged galactic wormholes. These results provide new insights into the interplay between gravitational and electromagnetic interactions in charged wormhole spacetimes and their observational manifestations.

physics.gen-ph↗

arXiv2026

Evolving wormhole cosmology: modified Friedmann dynamics and observational constraints

We construct an evolving wormhole cosmological model from a Morris--Thorne metric with a separable, time-dependent shape function in a spatially flat Friedmann--Lemaitre--Robertson--Walker background, deriving the corrected wormhole energy-momentum tensor and a volume-averaging procedure that yields a modified Friedmann equation with a redshift-dependent wormhole correction term. We show that the shape-function index and throat-evolution exponent combine into a single macroscopic exponent $β$, and we demonstrate that the resulting wormhole equation of state is consistent with this geometric averaging. All traversability conditions are verified. Confronting the model with cosmic chronometer, DESI Data Release~2 BAO, and standard BAO data across seven dataset combinations, our findings show that all combinations are mutually consistent, with the joint fit giving a present-day expansion rate intermediate between local and CMB-inferred values, a deceleration-to-acceleration transition consistent with independent determinations, and an effective dark-energy equation of state close to, but distinguishable from, a pure cosmological constant. The evolving throat radius offers a novel, geometry-specific observable for discriminating wormhole spacetimes from phenomenological dark-energy parametrisations.

gr-qc↗

arXiv2026

A Dynamical-Photometric Phase Space for Spiral Galaxies: Probing the Local Coupling Between Light and Gravity

The interplay between luminous matter distribution and the local gravitational field within disc galaxies encodes physical information beyond that captured by global scaling relations. We introduce a \emph{dynamical--photometric phase space} defined by the kinematic variable $X(R)=V(R)/R$ and the photometric variable $Y(R)=\mathrm{d}\ln I/\mathrm{d}\ln R$, placing the local gravitational scale and the logarithmic surface brightness gradient into direct pointwise correspondence at each galactocentric radius $R$. The quantity $X=V/R=ω$ represents the angular frequency of circular motion and acts as a probe of the local mean mass density, while $Y$ measures the radial steepness of the stellar light distribution. The baryon-dominated inner disc is characterized by large negative $Y$, whereas the dark-matter-dominated outer region approaches $Y\rightarrow0$. This two-regime behaviour is described by the smooth sigmoid relation $Y=[a\ln X+b]/(1+\exp[k(X-X_{\rm trans})])$, which reduces to the logarithmic coupling $Y=a\ln X+b$ in the baryonic zone. We apply this framework to 136 late-type galaxies from the SPARC database, spanning inclinations $20^{\circ}$--$89^{\circ}$, distances $1$--$130$\,Mpc, and five decades in stellar mass. The median coefficient of determination is $R^{2}=0.930$. Statistical validation includes eight independent tests together with 5-fold cross-validation. The transition parameter $X_{\rm trans}$ identifies the onset of dark-matter dominance, corresponding to a median transition radius $R_{\rm trans}=5.40$\,kpc across the sample.

gr-qc↗

arXiv2026

Accelerating expansion and isotropic sky-hemisphere consistency in Pantheon+ supernovae: a revised analysis in the dark energy debate

We perform four independent decompositions of the deceleration parameter $q_0$ using 1564 Type Ia supernovae (SNe Ia) from the Pantheon+ catalogue: by redshift bin, sky hemisphere, host galaxy mass, and supernova colour, correcting a coordinate error identified by Sah, Rameez & Sarkar (SRS26) in the sky-hemisphere direction used in our original analysis. Without any progenitor-age correction, the full sample yields $q_m=-0.490$, consistent with the $Λ$CDM expectation of $-0.55$. Applying the Son et al. (S25) progenitor-age correction shifts this to $q_m=-0.267$, remaining in the accelerating regime. The sky hemisphere test, using the corrected CMB dipole direction (RA$=167.80^\circ$, Dec$=-7.10^\circ$), shows consistent results between the CMB dipole ($q_m=-0.527$) and anti-dipole ($q_m=-0.464$) hemispheres, supporting isotropy but not the strong deceleration values reported previously. Our revised results are consistent with \lcdm{} and do not support either the original claim of near-zero baseline acceleration or the S25/SRS26 claim of a decelerating universe.

astro-ph.CO↗

arXiv2026

On the reconstruction of the Rotation Curve for Milky Way and its spacetime implications: a Machine Learning approach

We propose a machine learning-assisted analytical reconstruction of the Milky Way rotation curve and discuss its implications in a relativistic spacetime context. The rotation curve is reconstructed using 73 observational data points over the range 0.1-95.56 kpc, and we compare the performances of Ridge regression, LASSO regression, and feed-forward neural networks using a physically motivated functional basis. Ridge regression yields the most stable prediction with R2 = 0.9824 +/- 0.0064 and RMSE = 3.75 km/s, while retaining analytical interpretability. We embed the reconstructed velocity profile into a static, spherically symmetric spacetime, enabling the determination of the redshift function and the mass function through Einstein's equations. We verify that all energy conditions are satisfied, the sound speed remains subluminal, the circular orbits are stable, and the gravitational energy is negative, confirming the attractive nature of gravity. This framework provides a statistically validated, data-driven alternative to conventional dark matter halo models and establishes a direct connection between kinematical observables and relativistic spacetime geometry.

astro-ph.GA↗

arXiv2026

Interplay of Holographic and New Agegraphic Dark Energy in Cosmology: A hybrid dark energy model from a generalized length-time cut-off

The spatial and temporal infrared cutoffs are the source of the Holographic dark energy (HDE) and New Agegraphic dark energy (NADE) models, respectively. Inspired by the spacetime unification of space and time in General Relativity, we propose that a combined spacetime cutoff should govern the dark energy density, with HDE and NADE appearing as limiting cases when the spatial or temporal contribution predominates. In this connection, we explore a hybrid model of holographic and new Agegraphic dark energy, where the energy density of the resulting model is a combination of the two models. We consider an interacting and a non-interacting scenario between the hybrid dark energy model and cold dark matter. Cosmological implications of the model is studied via different cosmological parameters like the equation of state parameter, deceleration parameter, statefinder parameter, and Om-diagnostic. A stability check for the model has been performed using the squared speed of sound. Finally the parameter space of the model is constrained using observational data like Hubble data, BAO data and DESI data. We have also checked the Hubble tension for our hybrid model and found it to be substantially low in comparison to other models. From our analysis we see that the constructed hybrid dark energy model can describe the evolution of the universe successfully.

gr-qc↗

arXiv2026

Ricci-Cubic Holographic Dark Energy: Confronting Observations, Stability and the Cosmic Coincidence Problem

In this work, we constrain the parameter space of the Ricci-Cubic Holographic Dark Energy (RCHDE) model using several observational datasets, including Hubble parameter measurements, cosmic chronometer (CC) data, Baryon Acoustic Oscillation (BAO) data, and recent DESI observations. The RCHDE model is constructed from a cubic curvature invariant formed through cubic contractions of the Ricci and Riemann tensors. To estimate the model parameters, we employ the Markov Chain Monte Carlo (MCMC) sampling technique within a Bayesian inference framework. The resulting likelihood contours provide both marginalized and joint posterior distributions of the model parameters. The best-fit cosmological evolution predicted by the RCHDE model is reconstructed and compared with observational $H(z)$ measurements as well as with the standard $Λ$CDM cosmological model. The best-fit value obtained in our model exhibits a moderate Hubble tension of approximately $2.3σ$ with respect to the reference value for $Λ$CDM. While this indicates a noticeable discrepancy, it remains significantly lower than the $\sim 5σ$ tension typically reported between early- and late-Universe measurements, suggesting a partial alleviation of the tension. In addition to the statistical parameter estimation, we perform an enhanced machine learning analysis using observational Hubble parameter data. We have done a comparative stability analysis between different holographic dark energy models using the squared speed of sound, where it is seen that the RCHDE model does not have any upper hand over its counterparts. Finally, the cosmic coincidence problem is tested to compare the efficiency of the RCHDE model in comparison to other models. It is found that the RCHDE model produced a significant alleviation to the cosmic coincidence problem, outshining its counterparts.

astro-ph.CO↗

arXiv2026

Exact Gravastar Solution

Astrophysical black holes arise as exact solutions of the Einstein field equations. Therefore, any alternative, such as a gravastar, must satisfy the same level of mathematical rigor and internal consistency. A physically viable gravastar model should not rely on approximations or ad hoc matching of regions, but instead provide a single, exact, and self-consistent solution of the Einstein field equations throughout the entire spacetime. In this work, we propose an exact solution to the Einstein field equations in the context of gravitational vacuum stars (gravastars), originally introduced by Mazur and Mottola. This framework presents an alternative end state of gravitational collapse, leading to the formation of a compact object distinct from a classical black hole. Our model is constructed by dividing the gravastar into three regions, each described by exact solutions of the Einstein field equations. We analyze the key physical properties of the resulting configuration and examine its theoretical consistency and astrophysical viability. This study provides a clear and systematic assessment of gravastars as potential alternatives to black holes.

gr-qc↗

arXiv2026

Revisiting The Gravitational Mirroring In Presence of Compact Objects

We propose a novel concept of astrophysical mirroring in the schwarzschild framework, which emerges as a direct consequence of gravitational lensing effects occurring in the immediate vicinity of extremely dense massive objects within spacetime. Through rigorous theoretical calculations and numerical ray-tracing analysis, we demonstrate that sufficiently compact astrophysical objects possess the capability to induce such extreme curvature in spacetime that the resulting gravitational field can bend light rays to extraordinary degrees, creating what we term a "reflection image" or mirror-like appearance of the source in distant regions of space. We discuss the theoretical framework as well as the observational consequences of this phenomenon.

gr-qc↗

arXiv2026

Cosmic Hysteresis in Reconstructed $f(T)$ Bounce Models A Torsion-Based Thermodynamic Perspective

We investigate the emergence of cosmic hysteresis in cyclic and bouncing cosmologies within the framework of reconstructed $f(T)$ gravity. In contrast to curvature-based modifications of General Relativity, teleparallel gravity attributes gravitation to spacetime torsion encoded in the torsion scalar $T$. By reconstructing viable $f(T)$ functions corresponding to analytically prescribed nonsingular bouncing scale factors and coupling the geometry to a minimally interacting canonical scalar field, we demonstrate that asymmetric scalar field dynamics between expansion and contraction phases give rise to a non-vanishing thermodynamic work integral $\oint p_ϕ\, dV$ over complete cycles. This hysteresis manifests as closed loops in the $(w_ϕ,a)$ plane, signifying thermodynamic memory and irreversibility. We derive the modified Friedmann equations, establish exact bounce and turnaround conditions, and discuss the implications of torsion-induced hysteresis for the cosmological arrow of time. Our results confirm that cosmic hysteresis is a generic feature of cyclic universes in modified gravity, extending beyond curvature-based theories.

gr-qc↗

arXiv2025

Some New Types of Well-Behaved Polynomial Redshift Parametrization of Dark Energy Equation of State

In this paper, we explore a new type of smooth and well-behaved polynomial redshift function that can avoid a future singularity. Using this function, we have proposed different redshift parametrizations of the dark energy equation of state, drawing motivation from different polynomial functions like conventional polynomial, Legendre polynomial, Laguerre polynomial, Chebyshev polynomial and Fibonacci polynomial. The main feature of these parametrizations is their well-behaved nature throughout the evolution of the universe, which was a matter of concern in most of the previous polynomial parametrizations of the dark energy equation of state (EoS). This form of parametrization may be considered as an extension of those forms with no divergence at any redshift value. A comprehensive observational data analysis is performed with the Hubble, BAO and DESI datasets to constrain the parameter space of the models. Confidence contours showing joint and marginalized posterior distribution with different combinations of datasets are generated using a Markov Chain Monte Carlo approach. We see that our improved parametrizations enable us to derive more stringent restrictions on the current dark energy EoS and its derivative, which improves performance. Finally, a machine learning analysis is performed using some suitable algorithms like ELR, PILR, ANN, SVR, ERFR and GBR to compare the models. Among all the tested polynomial bases, the Legendre basis demonstrated superior performance with the lowest test RMSE and reduced $χ^{2}$ value under the Modified Differential Evolution theoretical model, indicating exceptional physical accuracy and numerical stability.

gr-qc↗

arXiv2025

Testing the Generalized Second Law in $(2+1)$-Dimensional Cosmology: Holographic Entropy Bounds and Observational Constraints

We investigate the validity of the Generalized Second Law (GSL) of thermodynamics in a $(2+1)$-dimensional holographic cosmological model with a negative cosmological constant. Adopting a horizon thermodynamics framework, we examine two prominent entropy bounds, the Fischler--Susskind (FS) bound and the Hubble Entropy (HE) bound, in both expanding and contracting universes, including the effects of quantum entropy corrections. Our theoretical analysis shows that the FS bound is intrinsically incompatible with the GSL in contracting $(2+1)$-dimensional universes, regardless of spatial curvature or exotic matter content, and that this incompatibility persists even when quantum corrections are considered. In contrast, the HE bound is consistent with the GSL in expanding universes under classical conditions and can also be reconciled in certain contracting scenarios when quantum effects are included. To complement the theoretical study, we perform a Markov Chain Monte Carlo (MCMC) analysis using recent Baryon Acoustic Oscillations (BAO), Cosmic Chronometer (CC), and Hubble parameter datasets to constrain the model parameters. The best-fit results reveal good cross-dataset consistency, with the cosmological constant parameter $ψ$ remaining stable across all probes. These findings identify the HE bound as a more robust candidate for holographic constraints in lower-dimensional cosmology, while demonstrating the limitations of the FS bound. Our results not only clarify the status of the GSL in $(2+1)$-dimensional settings but also provide a framework for testing entropy bounds with future high-precision cosmological data.

gr-qc↗

arXiv2025

Constraints on multi-fluid cosmology in modified Gauss-Bonnet gravity models with different observational data sets

In the present work, we incorporate redshift-space distortion measurement to investigate the growth of large scale structure within the framework of multi-fluid cosmology in the context of modified Gauss-Bonnet gravity. Using three different modified Gauss-Bonnet gravity models, we compare the predictions of modified Gauss-Bonnet gravity expansion history-through the Friedmann equation with Hubble and BAO data sets and constrain models parameters. Within the context of multi-fluid cosmology in modified Gauss-Bonnet gravity, we obtain the structure growth equation. This equation is then combined with Sigma_8 to get f_Sigma_8 predictions-which is compared with redshift-space distortion data to constrain models parameters to obtain best-fit values including Sigma_8. This involves performing a Markov Chain Monte Carlo (MCMC) analysis for these specific forms of modified Gauss-Bonnet models.

gr-qc↗

arXiv2025

Cosmic Hysteresis in Reconstructed $f(R)$ Bounce Models: A Thermodynamic Study

We study the emergence of cosmic hysteresis in cyclic bouncing universes within the framework of analytically reconstructed $f(R)$ gravity. Using exact bouncing scale factor solutions of exponential and power-law forms, we reconstruct the corresponding $f(R)$ models and investigate the thermodynamic behavior of a minimally coupled scalar field in these geometries. The pressure evolution during expansion and contraction phases is shown to be asymmetric, leading to a non-vanishing thermodynamic work integral over each cycle, defined by $\oint p_ϕ\, dV$. We identify closed hysteresis loops in the equation-of-state space and quantify the net energy transfer per cycle. Our results reveal that such reconstructed $f(R)$ models generically support irreversible evolution, demonstrating a natural emergence of the thermodynamic arrow of time. These findings provide new insight into the dissipative features of modified gravity and the long-term dynamics of cyclic cosmological scenarios.

gr-qc↗

arXiv2025

Observational constraints on holography in $(2 + 1)$-dimensional cosmology with a generalized equation of state

In this study we explore the cosmic holographic principle, as proposed by Fischler and Susskind~\cite{Fischler}, within the framework of $(2 + 1)$-dimensional cosmological models. A generalized equation of state is employed, given by $p = (ζ- 1)(ρ+ ρ_0)$, where $ζ$ and $ρ_0$ are treated as two free parameters. The analysis confirms the validity of the holographic principle in all flat and open universes. However, for a $(2 + 1)$-dimensional closed universe, we apply the method proposed by Kaloper and Linde~\cite{Kaloper}, and observe that the holographic principle is generally not satisfied. Furthermore, we examine the stability of the proposed model using the Markov chain Monte Carlo (MCMC) method, and estimate the best-fit values for the model parameters based on observational Hubble data sets.

physics.gen-ph↗

arXiv2025

Cosmic Hysteresis in $f(R)$ Gravity: A Thermodynamic Perspective on Cyclic Bouncing Universes

We investigate the emergence of cosmic hysteresis in cyclic bouncing cosmologies within the framework of $f(R)$ modified gravity theories. Building upon previous studies that explored hysteresis phenomena in braneworld and Einstein-Gauss-Bonnet gravity, we analyze how scalar field dynamics coupled to a modified gravitational background generates asymmetric pressure evolution during expansion and contraction phases. This asymmetry manifests as a non-vanishing work integral over complete cosmic cycles, $\oint p\,dV \neq 0$, which we interpret as a fundamental thermodynamic signature of irreversible cyclic evolution. We derive the modified Friedmann equations for a representative $f(R) = R + αR^2$ model, establish analytical conditions for bounce and turnaround events, and quantify the thermodynamic work performed during each cycle through comprehensive numerical analysis. Our findings demonstrate that hysteresis effects are generic features of scalar-tensor dynamics in higher-curvature gravity theories and can drive cumulative evolution of the cosmic scale factor across multiple cycles, potentially providing new insights into the resolution of cosmological singularities and the thermodynamic arrow of time in modified gravity frameworks.

gr-qc↗

arXiv2025

Observational analysis of bulk viscous modified Chaplygin gas in (2+1)-dimensional universe using MCMC

This paper investigates regarding cosmological implications of a bulk viscous modified Chaplygin gas (MCG) in (2+1)-dimensional Friedmann-Robertson-Walker spacetime, incorporating both theoretical analysis and observational constraints. We derive analytical solutions for both viscous and non-viscous cases, revealing distinct behavior in energy density evolution, Hubble parameter dynamics, and deceleration parameter transitions. A comprehensive perturbation analysis illustrates how bulk viscosity dampens the structure growth oscillations, addressing a key challenge faced by Chaplygin gas models in higher dimensions. Using Markov chain Monte Carlo (MCMC) techniques with Hubble parameter and Pantheon supernova datasets, we impose constraints on our model parameters, obtaining $H_0 = 67.90$ km s$^{-1}$ Mpc$^{-1}$, showing remarkable consistency with Planck $Λ$CDM estimations despite the dimensional reduction. Our findings suggest that lower-dimensional viscous cosmology captures essential features of cosmic evolution while providing valuable theoretical insights into the interplay between dissipative effects and exotic equations of state.

gr-qc↗