Nandan Roy

Recent DESI BAO measurements suggest dark energy may be dynamical rather thanstrictly constant. To investigate this, we introduce the δ-CDM framework, a controlleddeformation of ΛCDM in which deviations from a cosmological constant are encoded in aredshift-dependent function δ(z) defined by wde(z) = -1 + δ(z). We aim to reconstruct δ(z)from an underlying physical model. As a concrete example, we study thawing dark energyrealized by a scalar field. Within a unified setup, we include both quintessence and phantomfields via a switching parameter, so the formalism covers thawing quintessence and thawingphantom scenarios. We find that, within the thawing approximation, the reconstructed δ(z)is independent of the specific scalar-field realization. This illustrates that the effective fluiddescription in the δ-CDM framework can capture richer dark energy phenomenology. UsingPlanck CMB-SPA, DESI DR2 BAO, and Pantheon+ supernova data, we find this model isnot preferred over standard ΛCDM; hence current data do not statistically favor thawingdark energy. However, the ˜w0 ˜wa parametrization is still preferred over ΛCDM, hinting atmodels that allow the equation of state to cross the phantom divide rather than those thatforbid it. Overall, δ-CDM provides a way to implement minimal deviations from ΛCDMwhile remaining consistent with a broad class of physically motivated dark energy models.

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.

In this work, we investigate the Dirac–Born–Infeld chameleon scalar field model in light of the latest cosmological observations, analyzing the model both with the incorporation of the chameleon mechanism and without it. Results from current cosmological observations, such as Pantheon Plus, DES Y5, DESI DR2, and the compressed Planck likelihood, are used to constrain the model. We consider the AdS throat in the form of f (ϕ) = λ/ϕ4 and a potential V (ϕ) = m0 2ϕ 2 + m1 2 ϕ4 .One interesting finding from our analysis is that, without the chameleon mechanism, the mean value of m1 ≃ 0 indicates the self-interaction of the DBI field could potentially be negligible. Constraints on the potential parameters do not appear when considering the chameleon mechanism, but the warp parameter and chameleon coupling parameter are forced to satisfy η ≥ 0 and β ≤ 0. Different choices of β render the same background cosmological parameters. The equation of state wDE resides more in the quintessence region in the past. There is no phantom crossing in this model under our assumption of the warp throat and potential. By computing the ∆AIC relative to the ΛCDM model, we study statistical model comparison. The model as a candidate of dynamical dark energy is observationally viable at the cosmological background level.

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 $Λ$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.