arXiv subjects

Saibal Ray

Publications and source records attributed to Saibal Ray.

At least 19 recordsLinked to original sources

arXiv2026

Testing $f(Q)$ Gravity with DESI DR2 and Strong-Lensing Time Delays

Symmetric teleparallel gravity provides an alternative description of gravitation in which non-metricity replaces curvature and torsion. Its extension through $f(Q)$ gravity offers a different geometric description of the late-time expansion of the Universe and its accelerated phase. In this work, we investigate two $f(Q)$ models, a normalized power-law model and a square-root exponential model, and test their ability to describe the late-time expansion history. We constrain the model parameters through Markov chain Monte Carlo analyses using Cosmic Chronometer measurements, DESI DR2 baryon acoustic oscillations, strong-lensing time-delay observations, and three Type Ia supernova compilations, Pantheon$^+$, Union 3.0, and DES Y5. We compare both models with the flat $\Lambda$CDM model using the minimum $\chi^2$, Akaike information criterion, and Bayesian information criterion. The square-root exponential model provides a better statistical fit than $\Lambda$CDM for the combinations of Cosmic Chronometer, DESI DR2, and strong-lensing time-delay data with Pantheon$^+$ and Union 3.0, with improvements in both the goodness of fit and information criteria. The normalized power-law model remains statistically competitive with $\Lambda$CDM for the supernova-inclusive combinations, although the information criteria do not favor its additional parameter. We also determine the transition redshift from cosmic deceleration to acceleration for both models, obtaining consistent values across the different dataset combinations. The transition redshifts agree with observational estimates of the cosmic acceleration epoch. Overall, our results support $f(Q)$ gravity as a viable alternative to $\Lambda$CDM for explaining the late-time accelerated expansion of the Universe without requiring a cosmological constant.

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 $\Lambda$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

Observationally Constrained Cosmological model in $f(Q,\mathcal{L}_{m})$ Gravity with $H(z)$ parameterization

In the present work, we explore an observationally constrained cosmological model in the framework of $f(Q,\mathcal{L}_{m})$ gravity, where $Q$ denotes the non-metricity scalar and $\mathcal{L}_{m}$ represents the matter Lagrangian density. To derive the modified Friedmann field equations, we consider a flat FLRW space-time. We have considered a specific parameterization of the Hubble parameter $H(z)$ to explore the cosmic evolution, which successfully describes the shift of the cosmos from its initial decelerated expansion period to the current accelerated scenario. The free model parameters are constrained using recent observational datasets including Cosmic Chronometers (CC), Pantheon+SH0ES, Union 3.0, DESI-BAO, and CMB distance priors using MCMC approach through the $\chi^2$-minimization process. The derived results indicate that the present model remains consistent with recent cosmological observations. We note that the deceleration parameter exhibits a signature flipping behavior at transition redshift $z_t \approx 0.643$, confirming the transition from matter-dominated deceleration to dark-energy-driven acceleration. The equation of state (EOS) parameter remains in the quintessence region and exhibits an asymptotical approach to the $\Lambda$CDM limit at late times. Moreover, the estimated cosmic age can be found as $13.724^{+0.087}_{-0.048}$ Gyr, which agrees well with recent observational estimations. The statefinder and Om diagnostics support the quintessence nature of the model. At the same time, the examination of energy conditions reveals that two specific energy conditions, viz. Null Energy Condition (NEC) and Dominant Energy Condition (DEC) are fulfilled, while the Strong Energy Condition (SEC) is violated, validating the accelerated expansion of the universe.

gr-qc↗

arXiv2025

Transitioning late-time cosmology with the Hubble parameterization

We investigate a late-time cosmological model for a homogeneous and isotropic space-time in the Rastall theory. We explore the observational constraints on the Hubble parameter by using the latest cosmological datasets such as cosmic microwave background radiation (Planck), baryon acoustic oscillations (DESI) and Type Ia Supernovae (Union 3.0). As a result, we explicitly demonstrate that the specific redshift transition occurs, namely, there happens a phase shift in the evolution of the universe from the initial deceleration era to the current accelerating phase of the cosmological scenario. Furthermore, we show that with the latest dataset of DESI-BAO clubbed with CC, CMB, and Union 3.0, the current value of the Hubble parameter is estimated as $H_0 = 66.945 \pm 1.094$, which can be compatible with the available observations.

gr-qc↗

arXiv2025

Observational constraints on f(Q,T) gravity from the mass-radius relation and stability of compact stars

In this investigation we examine the astrophysical consequences of the influence of pressure anisotropy on the physical properties of observed pulsars within the background of $f(Q,T)$ gravity by choosing a specific form $f(Q, T)=\psi_1\, Q + \psi_2 T$, where $\psi_1$ and $\psi_2$ are the model parameters. Initially, we solve the modified field equations for anisotropic stellar configurations by assuming the physically valid metric potential along with anisotropic function for the distribution of the interior matter. We test the derived gravitational model subject to various stability conditions to confirm physically existence of compact stars within the $f(Q,T)$ gravity context. We analyze thoroughly the influence of anisotropy on the effective density, pressure and mass-radius relation of the stars. The present inspection of the model implies that the current gravitational models are non-singular and able to justify for the occurrence of observed pulsars with masses exceeding 2 $M_{\odot}$ as well as masses fall in the {\em mass gap} regime, in particular merger events like GW190814. The predicted radii for the observed stars of different masses fall within the range \{10.5 km, 14.5 km\} for $\psi_1\leq 1.05$ whereas the radius of PSR J074+6620 is predicted to fall within \{13.09 km, 14.66 km\} which is in agreement with the predicted radii range \{11.79 km, 15.01 km\} as can be found in the recent literature.

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 $p = (\zeta - 1)(\rho + \rho_0)$, where $\zeta$ and $\rho_0$ 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

Anisotropic cosmology using observational datasets: exploring via machine learning approaches

In the current study, we present the observational data constraints on the parameters space for an anisotropic cosmological model of Bianchi I type spacetime in general relativity (GR). For the analysis, we consider observational datasets of Cosmic Chronometers (CC), Baryon Acoustic Oscillation (BAO), and Cosmic Microwave Background Radiation (CMBR) peak parameters. The Markov chain Monte Carlo (MCMC) technique is utilized to constrain the best-fit values of the model parameters. For this purpose, we use the publicly available Python code from CosmoMC and have developed the contour plots with different constraint limits. For the joint dataset of CC, BAO, and CMBR, the parameter's best-fit values for the derived model are estimated as $ H_0 = 69.9\pm 1.4$ km/s/Mpc, $ \Omega_{m0}=0.277^{+0.017}_{-0.015}$, $ \Omega_{\Lambda 0} = 0.722^{+0.015}_{-0.017}$, and $\Omega_{\sigma 0} = 0.0009\pm0.0001$. To estimate $H(z)$, we explore machine learning (ML) techniques like linear regression, Artificial Neural Network (ANN), and polynomial regression and thereafter analyze the results with the theoretically developed $H(z)$ for the proposed model. Among these ML techniques, the polynomial regression exceeds the performance compared to other techniques. Further, we also note that larger dataset provides a better understanding of the cosmological scenario in terms of ML view point.

gr-qc↗

arXiv2025

Relativistic massive compact stars supported by decoupled matter: Implications for mass-radius bounds

The merger of binary neutron stars (BNSs) is a remarkable astrophysical event where all four fundamental forces interplay dynamically across multiple stages, producing a rich spectrum of multi-messenger signals. These observations present a significant multiphysics modeling challenge but also offer a unique opportunity to probe the nature of gravity and the strong nuclear interaction under extreme conditions. The landmark detection of GW170817 provided essential constraints on the properties of non-rotating neutron stars (NSs), including their maximum mass (M_{max}) and radius distribution, thereby informing the equation of state (EOS) of cold, dense nuclear matter. While the inspiral phase of such events has been extensively studied, the post-merger signal holds even greater potential to reveal the behavior of matter at supranuclear densities, particularly in scenarios involving a transition to deconfined quark matter. Motivated by the recent gravitational wave event GW190814 (2.5-2.67 M_{\odot}), we revisit the modeling of high-mass compact stars to investigate their internal structure via a generalized polytropic EOS. This framework incorporates a modified energy density profile and is coupled with the TOV equations. We explore mass-radius (M-R) relationships within both GR and the minimal geometric deformation (MGD) approach. Specifically, we constrain the radii of four massive compact objects PSR J1614-2230 (1.97^{+0.04}_{-0.04}\,M_{\odot}), PSR J0952-0607 (2.35^{+0.17}_{-0.17}\,M_\odot), GW190814 (2.5-2.67\,M_\odot), and GW200210 (2.83^{+0.47}_{-0.42}\,M_\odot) and demonstrate that our theoretical M-R curves are consistent with observational data. These findings provide meaningful constraints on the EOS and underscore the potential of alternative gravity models to accommodate ultra-massive compact stars within a physically consistent framework.

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 $\Lambda$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↗

arXiv2025

Cosmological model in the framework of $f(R,\mathcal{L}_{m})$ gravity with quadratic equation of state parameter

In this study, we have explored a transitioning cosmological model of universe's expansion in $f(R,\mathcal{L}_{m})$ gravity. The quadratic type of equation of state parameter in the form $\omega=-1 + \alpha (1 + z) + \beta (1 + z)^2$, where $\alpha$ and $\beta$ are constants, is considered to determine the explicit solution of field equations and derive Hubble parameter in term of redshift $z$. The model parameters are estimated taking observational datasets of BAO, Pantheon, and CC using MCMC analysis. Some dynamical properties like EOS parameter, energy density, pressure, and deceleration parameter are described. The cosmographic parameter like statefinders ($r,s$), jerk parameter ($j$) etc are also thoroughly explained. The energy conditions are also examined to validate the viability of the proposed model. We observe a transition redshift at $z_{t} = 0.942^{+0.112}_{-0.164}$ with the present value of deceleration parameter $q_0 = -0.4815^{+0.0362}_{-0.0096}$.

gr-qc↗

arXiv2025

Joint Analysis of Constraints on f(R) Parametrization from Recent Cosmological Observations

In this study, we present constraints on the parameters of three well-known $f(R)$ gravity models, viz. (i) Hu-Sawicki, (ii) Starobinsky, and (iii) ArcTanh by using a joint analysis of recent cosmological observations. We perform analytical approximations for the Hubble parameter, $H(z)$, and cosmological distances in terms of the Hubble constant $(H_0)$, matter density $(\Omega_{m0})$, and a deviation parameter $b$ for each model. {Our analysis combines early and late-universe cosmological data from five cosmological observations:} (a) Hubble parameter measurements (Cosmic Chronometers), (b) Type Ia Supernovae (Union 3.0), (c) Baryon Acoustic Oscillations (DESI-2025), (d) Gamma-Ray Bursts (GRBs) and (e) Cosmic Microwave Background (CMB). We first optimize the models using each dataset independently, and subsequently, we perform a comprehensive joint analysis combining all four datasets. Our results show that the Hu-Sawicki and ArcTanh models do not deviate significantly from the $\Lambda$CDM model at 95% confidence level for individual datasets and remain consistent at 99% confidence level in the joint analysis. In contrast, the Starobinsky model shows a strong deviation and appears as a viable alternative to $\Lambda$CDM. We also constrain the transition redshift parameter ($z_t$), and check that the obtained value agrees with the values inferred from both early-time measurement (Planck) and late-time data from Type Ia Supernovae. These results support the potential support of $f(R)$ gravity to explain the late-time cosmic acceleration effectively. Finally, a statistical model comparison using $\chi^2_{\text{min}}$, AIC, and BIC indicates that all three $f(R)$ models are favored over $\Lambda$CDM, with the Starobinsky model receiving very strong support.

astro-ph.CO↗

arXiv2025

Role of gravitational decoupling on theoretical insights of relativistic massive compact stars in the mass gap

Advancements in theoretical simulations of mass gap objects, particularly those resulting from neutron star mergers and massive pulsars, play a crucial role in addressing the challenges of measuring neutron star radii. In the light of this, we have conducted a comprehensive investigation of compact objects (CSs), revealing that while the distribution of black hole masses varies based on formation mechanisms, they frequently cluster around specific values. For instance, the masses observed in GW190814 $(23.2^{+1.1}_{-1.0} \, M_{\odot})$ and GW200210 $(24.1^{+7.5}_{-4.6} M_{\odot})$ exemplify this clustering. We employed the gravitational decoupling approach within the framework of standard general relativity and thus focusing on the strange star model. This model highlights the effects of deformation adjusted by the decoupling constant and the bag function. By analyzing the mass-radius limits of mass gap objects from neutron star mergers and massive pulsars, we can effectively constrain the free parameters in our model, allowing us to predict the radii and moments of inertia for these objects. The mass-radius ($M-R$) and mass-inertia ($M-I$) profiles demonstrate the robustness of our models. It is shown that as the decoupling constant $\beta$ increases from 0 to 0.1 and the bag constant $\mathcal{B}_g$ decreases from 70 $MeV/fm^3$ to 55 $MeV/fm^3$, the maximum mass reaches $M_{max} = 2.87 \, M_\odot$ with a radius of 11.20 km. In contrast, for $\beta = 0$, the maximum mass is $M_{max} = 2.48 \, M_\odot$ with a radius of 10.69 km. Similarly, it has been exhibited that as $\beta$ decreases to 0, the maximum mass peaks at $M_{max} = 2.95 M_\odot$ for $\mathcal{B}_g = 55 MeV/fm^3$ with a radius of 11.32 km. These results not only exceed the observed masses of CSs but also correlate with recent findings from gravitational wave events like GW190814 and GW200210.

astro-ph.HE↗

arXiv2024

Thermodynamics of a Non-canonical $f(\bar{R},\bar{T})$ gravity

This work comprises a study of the thermodynamic behavior of modified $f(\bar{R},\bar{T})$ gravity, which had been developed based on a non-canonical theory known as K-essence theory. In this development, we use the Dirac-Born-Infeld (DBI) type of non-standard Lagrangian. We develop a modified first law and generalized second law of thermodynamics (GSLT) within the modified $f(\bar{R},\bar{T})$ gravity, where we consider the background metric to be the usual Friedmann-Lema$\hat{\text{i}}$tre-Robertson-Walker (FLRW) type. A graphical analysis of surface gravity has been performed for the modified FLRW metric via the $f(\bar{R},\bar{T})$ theory, which is different from the usual FLRW gravity through the usual $f(R,T)$ gravity. Exponential and power law scale factors are used to analyze cosmic surface gravity. Through the investigation of the modified GSLT, using the relation of scale factor with the scalar field, we have seen that during the initial phase of the universe, the entropy's rate of change may be either negative or positive, contingent upon the value of the curvature constant. The negativity of the entropy change indicates that the modified GSLT is not feasible in that particular area for a particular curvature constant. These traits suggest that during the inflationary period, entropy might have been either negative or positive. It has also been seen that entropy saturates every curvature value at different time ranges, which indicates the heat death of the universe.

gr-qc↗

arXiv2024

Finslerian extension of an anisotropic strange star in the domain of modified gravity

In this article, we apply the Finsler spacetime to develop the Einstein field equations in the extension of modified geometry. Following Finsler geometry, which is focused on the tangent bundle with a scalar function, a scalar equation should be the field equation that defines this structure. This spacetime maintains the required causality properties on the generalized Lorentzian metric manifold. The matter field is coupled with the Finsler geometry to produce the complete action. In this work, we use modified gravity to develop the Einstein field equations from the variational principle. Developed Einstein field equations are employed on the strange stellar system to improve the study. The interior of the system is made of a strange quark, maintained by the MIT Bag equation of state. In addition, the modified Tolman-Oppenheimer-Volkov (TOV) equation is formulated. In particular, the anisotropic stress attains the maximum at the surface. The mass-central density variation justifies the stability of the system.

gr-qc↗

arXiv2024

Constraining anisotropic universe under $f(R,T)$ theory of gravity

We try to find the possibility of a Bianchi V universe in the modified gravitational field theory of $f(R,T)$. We have considered a Lagrangian model in the connection between the trace of the energy-momentum tensor $T$ and the Ricci scalar $R$. In order to solve the field equations a power law for the scaling factor was also considered. To make a comparison of the model parameters with the observational data, we put constraints on the model under the datasets of the Hubble parameter, Baryon Acoustic Oscillations, Pantheon, joint datasets of Hubble parameter + Pantheon, and collective datasets of the Hubble parameter + Baryon Acoustic Oscillations + Pantheon. The outcomes for the Hubble parameter in the present epoch are reasonably acceptable, especially since our estimation of this $H_0$ is remarkably consistent with various recent Planck Collaboration studies that utilize the $\Lambda$-CDM model.

gr-qc↗

arXiv2024

Research status of the Mendeleev Periodic Table: a bibliometric analysis

In this paper, we present a bibliometric analysis of the Mendeleev Periodic Table. We have conducted a comprehensive analysis of the Scopus-based database using the keyword "Mendeleev Periodic Table". Our findings suggest that the Mendeleev Periodic Table is an influential topic in the field of Inorganic as well as Organic Chemistry. Future researchers may focus on expanding our analysis to include other bibliometric indicators to gain a more comprehensive understanding of the impact of the Mendeleev Periodic Table in chemistry-based scientific investigations and even in the field of astrochemistry.

cs.DL↗

arXiv2024

Late time phantom characteristic of the model in $f(R,T)$ gravity with quadratic curvature term

We propose a novel cosmological framework within the $f(R,T)$ type modified gravity theory, incorporating a non-minimally coupled with the higher order of the Ricci scalar ($R$) as well as the trace of the energy-momentum tensor ($T$). Therefore, our well-motivated chosen $f(R,T)$ expression is $ R + R^m + 2 \lambda T^n$, where $\lambda$, $m$, and $n$ are arbitrary constants. Taking a constant jerk parameter ($j$), we derive expressions for the deceleration parameter ($q$) and the Hubble parameter ($H$) as functions of the redshift $z$. We constrained our model with the recent Observational Hubble Dataset (OHD), $Pantheon$, and $ Pantheon $ + OHD datasets by using the analysis of Markov Chain Monte Carlo (MCMC). Our model shows early deceleration followed by late-time acceleration, with the transition occurring in the redshift range $1.10 \leq z_{tr} \leq 1.15$. Our findings suggest that this higher-order model of $f(R,T)$ gravity theory can efficiently provide a dark energy model for addressing the current scenario of cosmic acceleration.

gr-qc↗