arXiv subjects

Nandan Roy

Publications and source records attributed to Nandan Roy.

At least 19 recordsLinked to original sources

arXiv2026

The role of anisotropy in $f(Q)$ gravity: insights from cosmological observations

We investigate the cosmological dynamics of Bianchi-I spacetime in symmetric teleparallel $f(Q)$ gravity through a dynamical system approach to analyse observational constraints. By reformulating the modified field equations into an autonomous system, we analyse two representative $f(Q)$ models and constrain their parameters using Pantheon Plus, DES Y5, DESI DR2, and compressed CMB data. The observational analysis yields consistent constraints across all dataset combinations and tightly bounds the anisotropic contribution, indicating that deviations from isotropy remain small. Both models reproduce the standard matter-dominated evolution and the observed late-time accelerated expansion while exhibiting distinct dark-energy dynamics. Model I undergoes a smooth phantom-divide crossing and approaches a de Sitter phase in the asymptotic future, whereas Model II evolves from an early phantom regime toward a cosmological-constant-like state around the present epoch, closely mimicking the late-time evolution of the $\Lambda$CDM model. These results indicate that anisotropic $f(Q)$ cosmology remains consistent with current background observations while admitting characteristic dark-energy evolution that may be testable with future cosmological surveys.

physics.gen-ph↗

arXiv2026

$\delta$-CDM: A Minimal Deformation of $\Lambda$CDM with Scalar Field Reconstruction

Recent DESI BAO observations provide intriguing hints that dark energy may be dynamical in nature. To investigate deviations of the dark energy equation of state (EoS) from $w = -1$, we introduce the $\delta$-CDM framework, a controlled deformation of $\Lambda$CDM in which deviations from a cosmological constant are parametrized by a redshift-dependent function $\delta(z)$, defined through $w_{\rm de}(z) = -1 + \delta(z)$. As an illustrative example, we reconstruct $\delta(z)$ using effective scalar field dynamics of thawing type, encompassing both quintessence and phantom regimes within a unified description. Notably, the reconstructed $\delta(z)$ is independent of the specific scalar field realization, ensuring theoretical robustness. Using Planck CMB-SPA data, DESI DR2 BAO measurements, and the Pantheon+ supernova sample within a Bayesian Markov Chain Monte Carlo analysis, we find that the $\tilde{w}_0\tilde{w}_a$ parametrization is preferred over this thawing-type realization of deviations from $w = -1$. Overall, the $\delta$-CDM framework provides a minimal yet flexible extension of $\Lambda$CDM, capable of capturing late-time dynamical features of dark energy.

astro-ph.CO↗

arXiv2026

Dynamical Systems in Cosmology: Reviewing An Alternative Approach

Dark energy is one of the deepest puzzles in modern cosmology, and mounting evidence suggests that it is not just a cosmological constant but a genuinely dynamical component. Although cosmology and dynamical systems theory emerged from different disciplines, dynamical systems methods have become essential tools to uncover the qualitative evolution of the universe. The equations governing homogeneous and isotropic cosmologies can be naturally written as systems of ordinary differential equations, making them an ideal arena for dynamical system analysis. This review begins with a sharp, streamlined introduction to the standard dynamical systems toolkit widely used in cosmology. We then move on to alternative formulations based on polar and hyperbolic variable transformations. These approaches unlock powerful new ways to probe a broad spectrum of scalar field dark energy models, to set and constrain initial conditions, and to analyze tracking behavior across wide classes of potentials. The review is self-contained, but consistently directs the reader to more specialized and in-depth treatments where needed.

astro-ph.CO↗

arXiv2026

CosmoDS: A Python toolkit for constraining cosmological models via dynamical systems analysis with Cobaya

We present a toolkit, CosmoDS, designed to study cosmological models at the background level using dynamical system analysis within the Cobaya framework. Dynamical system analysis is a powerful mathematical approach for studying nonlinear systems and is widely used in cosmology to investigate the stability and evolution of different cosmological models, particularly those involving dark energy. In this code, we provide a framework for constraining cosmological models using a dynamical system formulation. Most importantly, the toolkit is directly integrated with the Cobaya interface, allowing users to take advantage of the sophisticated statistical and inference tools already implemented in Cobaya for cosmological parameter estimation and model analysis.

astro-ph.CO↗

arXiv2026

Chiellini-Integrable Cosmologies with Phantom Divide Crossing

We investigate exact cosmological solutions with a massive scalar field minimally coupled to the Einstein-Hilbert action in General Relativity. For an extended Higgs-like scalar self-interaction, we find that the resulting field equations belong to the damped Ermakov-Painlev\'e II class and construct novel analytical solutions within the framework of the Chiellini integrability condition. We analyze whether the expanding branch of the solutions can describe a late-time cosmic acceleration, using a combined statistical analysis of BAO, CMB, cosmic chronometer and Pantheon+SHOES supernova datasets. A crucial outcome of this exercise is the analytical emergence of a smooth phantom divide crossing in the dark energy equation of state, achieved without introducing any pathological instabilities. The reconstruction yields a present-day Hubble parameter $H_0 \gtrsim 70 \,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, with a reduced tension relative to the $\Lambda$CDM cosmology. The results indicate that Chiellini-integrable scalar cosmologies are capable of providing a robust and analytically controlled framework for modeling late-time cosmic acceleration and phantom divide crossing, offering a viable alternative to phenomenological dark-energy parametrizations.

gr-qc↗

arXiv2026

Quintom Dark Energy: Future Attractor and Phantom Crossing in Light of DESI DR2 Observation

We study the late-time cosmological dynamics of a two-field dark energy model consisting of a canonical quintessence scalar field and a phantom scalar field in a spatially flat FLRW universe. The fields are minimally coupled to gravity and uncoupled at the level of the potential, with the quintessence sector governed by an exponential potential and the phantom sector by an inverse power-law potential. By reformulating the background equations as a five-dimensional autonomous dynamical system, we identify and analyze the fixed points and their stability properties, revealing stable late-time attractors corresponding to phantom-dominated accelerated expansion. We confront the model with observations through a Bayesian parameter estimation performed using the \textsc{Cobaya} framework, employing several combinations of recent cosmological data sets, including Pantheon+ supernovae, compressed cosmic microwave background distance priors, DESI DR2 baryon acoustic oscillation measurements, and DES Year-5 supernova data. The observational constraints favor a dynamical dark energy sector moderately and are consistent with deviations from a cosmological constant at the present epoch. The regions of parameter space preferred by the data are compatible with the stable accelerating solutions identified in the dynamical analysis, establishing a direct connection between phase-space stability and observational viability. A notable feature of the model is that the effective dark energy equation of state undergoes phantom divide crossing in a gradual and asymptotic manner, rather than as a sharp transition.

astro-ph.CO↗

arXiv2025

Is Phantom Barrier Crossing Inevitable? A Cosmographic Analysis

Recent findings from the Dark Energy Spectroscopic Instrument (DESI), analyzed together with supernova observations and CMB measurements, provide statistically significant indications (at the 2-5$\sigma$ level) of a time-varying dark energy component alongwith a possible phantom-to-quintessence transition in the recent past. In this letter, we investigate the evolution of dark energy using a model-independent cosmographic approach and explore the possibility of phantom barrier crossing. By mapping the differential equation defining jerk parameter into an anharmonic oscillator, we derive an analytical expression for the dark energy equation of state (EoS), which, remarkably, depends on a single parameter. Using DESI-DR2 BAO data, supernova data, and a compressed Planck likelihood, we constrain the cosmological parameters and find deviations from a cosmological constant at late times. Unlike the CPL parametrization, our results show no phantom barrier crossing, highlighting the power of kinematic reconstructions in probing the nature of dark energy. Furthermore, using a dynamical system approach, we demonstrate that $w_{DE}=-1$ acts as a bifurcation point with degenerate stable fixed points and therefore prevents solutions from crossing this barrier from either side.

astro-ph.CO↗

arXiv2025

Constraints on DBI dark energy with chameleon mechanism

In this work, we investigate the Dirac--Born--Infeld (DBI) scalar field model with and without the inclusion of the chameleon mechanism in light of the latest cosmological observations. We constrain the model using data from Pantheon Plus, DES Y5, DESI DR2, and the compressed Planck likelihood. We consider an AdS throat of the form $f(\phi) = \lambda / \phi^4$ and a potential $V(\phi) = m_0^2 \phi^2 + m_1^2 \phi^4$. Our analysis shows that, for both cases, the mean value $m_1 \simeq 0$ suggests that the DBI field may lack significant self-interaction, with only an upper bound on $m_1$. The warp parameter is constrained to $\eta \geq 0$, while the chameleon coupling satisfies $\beta \leq 0$. No crossing of the phantom divide is observed under the assumed form of the warp factor and potential. We perform a statistical model comparison using the $\Delta \mathrm{AIC}$ relative to the $\Lambda$CDM model. Although the DBI model provides a slightly better fit to the data in terms of $\Delta \chi^2$, the improvement is negligible. Consequently, the DBI model is mildly disfavored for both the cases compared to the $\Lambda$CDM model.

astro-ph.CO↗

arXiv2025

Constraint on Momentum-coupled Dark Energy using DESI DR2

In this work, we study two scalar field driven dark energy models characterized by the axion potential and the inverse power law potential, each coupled to dark matter through a momentum exchange interaction. By formulating the dynamics as an autonomous system, we identify the equilibrium points and analyze their stability. To constrain these models, we utilize observational data from Pantheon Plus Type Ia Supernovae, DES Y5, DESI DR2 BAO, and Planck 2018 CMB compressed likelihood, employing Markov Chain Monte Carlo (MCMC) methods. Both potentials exhibit weak to strong preference over the $\Lambda$CDM model, with a particularly strong preference for the momentum-coupled scenario when Supernova data are included in the analysis. Furthermore, we find the coupling parameter to be negative, with no lower bound, for both potentials. This suggests that momentum-exchange coupling between the dark sectors cannot be ruled out. From the stability analysis, we observe that for both potentials, the late-time attractor corresponds to a dark energy dominated phase, and the scalar field can behave as a stiff fluid during the early epoch.

astro-ph.CO↗

arXiv2024

Quintessence scalar field and cosmological constant: Dynamics of a multi-component dark energy model

This study explores the dynamics and phase-space behavior of a multi-component dark energy model, where the dark sector consists of a minimally coupled canonical scalar field and the cosmological constant, using a dynamical system analysis setup for various types of potential for which a general parameterization of the scalar field potentials has been considered. Several fixed points with different cosmological behaviors have been identified. A detailed stability analysis has been done and possible late-time attractors have been found. For this multi-component dark energy model, the late-time attractors are either fully dominated by the cosmological constant or represent a scenario where a combination of the scalar field and the cosmological constant dominates the universe. In this type of model, there is a possibility that the scalar field can become dynamical quite early compared to the standard era of dark energy domination. However, our analysis indicates that this early time contribution of the scalar field occurs deep in the matter-dominated era, not before the recombination era.

gr-qc↗

arXiv2024

$G_{3}$ -- interacting scalar tensor dark energy

We study the effect of adding an interaction in the $G_3$ term of Horndeski theory, where the propagation of gravitational waves are not modified. We derive the background and perturbation equations of motion from the action. We also derive the no-ghost and Laplacian instability conditions for tensor modes and scalar mode propagation. Then we study the evolution of the matter perturbation in the quasi-static approximation. We find that the gravitational couplings to the baryonic and cold dark matter over density are modified in this theory. We introduce a concrete model of the free function in the theory and study the background and linear perturbation dynamics. We then use the genetic algorithm to test the model. We compare the $H(z)$ function of the model and the $H(z)$ curve predicted by the genetic algorithm, using the $H(z)$ data. For the perturbation sector we compute the $f\sigma_{8}$ observable for the model and compare it with the predicted function from the genetic algorithm from the $f\sigma_{8}$ data.

gr-qc↗

arXiv2024

Dynamical dark energy in the light of DESI 2024 data

The latest findings from the DESI (Dark Energy Spectroscopic Instrument) data release 1 (DR1) [1], combined with data from the cosmic microwave background and supernovae, suggest a preference for dynamical dark energy over the cosmological constant. This study has considered the Chevallier-Polarski- Linder (CPL) parameterization for the dark energy equation of state (EoS) and has indicated a possible phantom barrier crossing in the recent past. Despite CPL being the most commonly used parameterization, recent research has pointed out issues with its prior selection and parameter degeneracies. In this paper, we propose an alternative two-parameter parameterization of the dark energy equation of state (EoS). At higher redshifts, it behaves like the cosmological constant. At redshifts z < 1, this parameterization closely approximates the CPL form but deviates from it at lower redshifts. Our findings also indicate that the current value of the EoS of dark energy resembles quintessence, with evidence of a recent crossing of the phantom barrier, supporting the conclusions in [1]. Furthermore, our model significantly reduces the Hubble tension to about 2.8{\sigma} when compared to Hubble Space Telescope and SH0ES data [2], and to 1.6{\sigma} with standardized TRGB and Type Ia supernova data [3]. Bayesian model selection using Bayes factors and Akaike Information Criteria (ACI), shows a strong preference for our parameterization over the {\Lambda} CDM model, aligned with the DESI2024 results and favoring dynamical dark energy.

astro-ph.CO↗

arXiv2024

Interacting dark sector: a dynamical system perspective

We investigate the interaction between the dark sectors from the point of view of a dynamical system analysis. A general setup for interacting dark energy models that incorporates both quintessence and phantom fields through a switch parameter, allowing an interaction in the dark sectors, has been considered. In the first part of our analysis, we have not assumed any specific form of the interaction, and in the second part, we invoked examples in a general framework of the interaction. The potentials of the scalar field are classified into two broad classes of potentials: exponential and non-exponential. We identify the potential late-time attractors of the system, which have a complete dark energy domination. From our analysis, it is evident there could be an interaction between the dark sector. The interaction, if any, weakens over time. We find for the quintessence field the transfer of energy from dark matter to dark energy can flip the direction, and on the contrary, for the phantom field, it is only from dark matter to dark energy.

gr-qc↗

arXiv2023

Tracker behaviour of quintom dark energy and the Hubble tension

We study the dynamics of the quintom dark energy model using state-of-the-art cosmological observations. The set of equations has been converted into an autonomous system using suitable transformations of the variables. We have discussed the fixed points of the model and the general phase-space behavior, in particular, in finding the existence of the tracker solutions for this model. The observations suggest that at late times the phantom field should dominate the dark energy sector with an approximately 15% share to the quintessence counterpart, and with both fields tracking the background at early times. A Bayesian model comparison with LambdaCDM has also been done by computing the Bayes factor and a positive preference has been obtained for the quintom model. Although not fully resolved, the Hubble tension can be reduced to 2.6{\sigma} when compared with the value of H0 reported in [1] and to 1.6{\sigma} when compared with that of [2].

astro-ph.CO↗

arXiv2023

Dynamical Analysis of the Dirac-Born-Infeld type of Tachyon field minimally coupled with barotropic fluid using EOS parametrization of a field

In this paper, we present a dynamical system analysis of the tachyon dark energy model by parametrization of the equation of state (EoS) of the dark energy. The choice of parametrization can constrain the form of the field potential, and as a result, the theory can be directly constrained from the observation without assuming a particular form of the potential.

gr-qc↗

arXiv2023

Exploring the Possibility of Interacting Quintessence Model as an Alternative to the $\Lambda$CDM Model

This study examines interacting quintessence dark energy models and their observational constraints for a general parameterization of the quintessence potential, which encompasses a broad range of popular potentials. Four different forms of interactions are considered. The analysis is done by expressing the system as a set of autonomous equations for each interaction. The Bayesian Model Comparison has been used to compare these models with the standard Lambda Cold Dark Matter ({\Lambda}CDM) model. Our analysis shows positive and moderate evidence for the interacting models over the {\Lambda}CDM model.

astro-ph.CO↗

arXiv2022

Dynamical systems analysis of tachyon dark energy model from a new perspective

In this work we present a new scheme to study the tachyon dark energy model using dynamical systems analysis by considering parametrization of the equation of state(EoS) of the dark energy. Both the canonical and phantom field dynamics are investigated. In our method we do not require any explicit form of the tachyon potential. Instead of the potential we start with an approximate form of the EoS of the tachyon field. This EoS is phenomenologically motivated and contains some dimensionless parameters. Using our method we can construct the dynamical system which gives rise to the time evolution of the universe. We have considered two different parametrizations of the EoS and studied the phase space dynamics in details. Our analysis shows Taylor series parametrization of the EoS has serious cosmological limitations. Our proposal is generic in nature and can be applied to other scalar field dark energy models.

gr-qc↗

arXiv2022

Quintessence or Phantom: Study of scalar field dark energy models through a general parametrization of the Hubble parameter

In this work we propose a simple general parametrization scheme of the Hubble parameter for the scalar field dark energy models. In our approach it is possible to incorporate both the quintessence and phantom scalar field in a single analytical scheme and write down relevant cosmological parameters which are independent of the nature of the scalar field. A general condition for the phantom barrier crossing has also been obtained. To test this approach, a well behaved parametrization of the normalized Hubble parameter has been considered and a wide variety of observational data like CMB data, Supernovae data, BAO data etc. has been used to constraint the various cosmological parameters. It has been found that data prefer the present value of the equation of state of the dark energy to be in the phantom domain. One interesting outcome of this analysis is that although the current value of the dark energy equation of state is phantom in nature, a phantom crossing of the EOS has taken place in the recent past. We have also carried out the Bayesian model comparison between $ΛCDM$ model and the proposed model which indicates that this model is favored by data as compared to $ΛCDM$ model.

astro-ph.CO↗