arXiv subjects

Ratul Mandal

Publications and source records attributed to Ratul Mandal.

5 recordsLinked to original sources

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 ($\omega_{\text{eff}}$) 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 ($\alpha, \beta, \gamma, H_0$), 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

$\Lambda(t)$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 $\Lambda(t)$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 $\Lambda(t)$CDM model predicts that $\omega_0 > -1$, showing a preference for dynamical dark energy over the cosmological constant scenario with $\omega_0 = -1$. 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 $\Lambda(t)$CDM model over $\Lambda$CDM

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 = 3\gamma H\rho_{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 $\Lambda$CDM value by approximately $2.93\sigma$, while the Non-Interacting model shows a $3.78\sigma$ deviation. The positive coupling parameter ($ \gamma > 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 $\Lambda$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

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 $f(Q) = Q e^{\frac{\beta Q_0}{Q}}$ 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

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 $f(R,G,T) = \alpha_1 G^{m} + \alpha_2 R^{\beta} - 2\alpha_3 \sqrt{-T},$ where $\alpha_i$ $ i = 1, 2, 3$), $ m $, and $\beta$ 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 $\Lambda$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↗