arXiv subjects

Sonej Alam

Publications and source records attributed to Sonej Alam.

8 recordsLinked to original sources

arXiv2026

Thermodynamic reconstruction in observationally constrained dynamical dark energy models

We perform an observational and thermodynamic analysis of four cosmological models $\Lambda$CDM, CPL, MPL, and the three-parameter MmAH parametrization. All dynamical models improve the fit relative to $\Lambda$CDM, with $\Delta\chi^2 \sim 6$-$6.5$, with MPL providing the best fit, while MmAH remains viable. Although Bayesian evidence mildly favors $\Lambda$CDM due to its lower complexity, the dynamical dark energy models remain competitive alternatives given current observations. We then reconstruct thermodynamic quantities within observationally constrained cosmological models, extending previous theoretical studies. The bestfit heat capacity reconstruction indicates that the divergence associated with a second order thermodynamic phase transition coincides with the deceleration-acceleration transition only in $\Lambda$CDM, whereas for the dynamical dark energy models it occurs at distinct redshifts, suggesting that this coincidence is not universal. The generalized second law is satisfied for all models over $0 \le z \le 1$, while the Hessian analysis reveals a transient instability at the phase transition; however, the late time thermodynamic stability is model dependent, with CPL and MPL remaining stable and $\Lambda$CDM and MmAH failing to satisfy both stability conditions simultaneously. These results show that thermodynamic properties reconstructed within observationally constrained cosmological models provide a complementary probe of dark energy, with the thermodynamic phase transition emerging as an intrinsically model dependent phenomenon.

astro-ph.CO↗

arXiv2026

Gaussian process reconstruction of scalar field dynamics from recent cosmological data

We reconstruct the late-time expansion history and dark energy dynamics using available cosmological data. We consider dark energy as an effective minimally coupled canonical scalar field without assuming a specific form for its potential. For reconstruction, we use Gaussian Process (GP) regression with a joint dataset consisting of 32 cosmic chronometer measurements (CC32), DESI DR2 baryon acoustic oscillation data, three Type Ia supernova compilations (Pantheon+, Union3, and DES Y5), and compressed CMB distance priors. From the reconstructed Hubble parameter and its derivatives, we obtain the scalar field kinetic and potential energy densities, the equation of state $w(z)$, and the dimensionless slope parameter $|\lambda(z)|$ of the scalar field potential. The reconstructed potential is nearly constant at late times, with $\tilde{V}(0) \simeq 0.68$--$0.70\rho_{c,0}$, close to the dark-energy density in flat $\Lambda$CDM. The kinetic term remains small over $0 \lesssim z \lesssim 2.5$, while $w(z)$ remains close to $-1$ within the uncertainties. Using only CC32 and DESI DR2, we find a mild ($1\sigma$) hint of a phantom-divide crossing near $z\sim0.5$. However, this feature depends on the supernova compilation and cannot be regarded as a firm conclusion with current data. The slope parameter $|\lambda(z)|$ shows mild evolution, with present-day central values around $0.8$--$1.0$, but large uncertainties due to its dependence on higher-order derivatives of the expansion history. The curvature parameter $\Gamma(z)$ requires even higher-order derivatives and is not constrained by current data; therefore, we do not report its reconstruction. Allowing small spatial curvature, $\Omega_k=0$--$0.02$, changes the results only slightly and remains within the existing uncertainty bands.

astro-ph.CO↗

arXiv2026

Dissipative Cosmology and the Nature of Dark Energy: Insights from Bulk Viscosity with DESI DR2 observations

We explore a cosmological model in which dark energy is described by a bulk viscous fluid, providing a dissipative mechanism for late-time cosmic acceleration. Considering both minimally and non-minimally coupled scenarios, we constrain the model using SNe Ia, DESI DR2 BAO, and Planck 2018 CMB data. We find that viscous effects can successfully mimic dynamical dark energy and yield improved fits over $\Lambda$CDM, particularly in the interacting non-minimal case. Our results demonstrate that dissipative processes offer a viable and physically motivated alternative to the cosmological constant in explaining the current accelerated expansion of the universe.

astro-ph.CO↗

arXiv2025

Observational constraints on early time non-phantom behaviour of dynamical dark energy

We investigate dynamical dark energy models that admit non-phantom behaviour at early times, including thawing, scaling--thawing, and effective fluid extensions. Using current cosmological observations, we find that late-time background parameters remain stable across all models. Time-dependent parametrizations such as CPL show a $\sim2\sigma$ preference for phantom evolution at low redshift. Imposing non-phantom scaling dynamics at early times leads to strong lower bounds on the potential steepness, $\lambda \gtrsim 20$--$30$, constraining the early dark energy density to below the percent level at matter--radiation equality. Consequently, early scaling behaviour does not alleviate the Hubble tension and is penalised by Bayesian model selection. Our results indicate that while late-time dynamics can mildly improve the fit, early non-phantom scaling is strongly disfavoured by current data.

astro-ph.CO↗

arXiv2025

Beyond CPL: Evidence for dynamical dark energy in three-parameter models

We introduce two three-parameter extensions of the minimal Akhtar-Hossain (mAH) dark energy parametrization, termed modified minimal AH (MmAH1 and MmAH2), which provide a smooth and bounded evolution of the dark energy equation of state while retaining $\Lambda$CDM as a limiting case. Using a joint analysis of the CMB compressed likelihood, DESI DR2 BAO, $H(z)$, redshift space distortions, and three SNeIa samples (PantheonPlus, Union3, and DESY5), we compare these models with $\Lambda$CDM, $w$CDM, mAH, CPL, and the three-parameter CPL-$w_{\rm b}$ extension. The standard cosmological parameters remain stable across all models, while CPL, MmAH1 and MmAH2 parametrizations yield modest but consistent improvements in fit ($\Delta\chi^2\simeq-6$ to $-12$ for PantheonPlus and Union3, and $\simeq-38$ for DESY5). Statistical consistency with $\Lambda$CDM, quantified via the Mahalanobis distance in one, two, and three dimensional parameter subspaces, reveals mild to moderate deviations, $\sim2$--$2.5\sigma$ for $+$PantheonPlus, $2$--$3\sigma$ for $+$Union3, and up to $4$--$5\sigma$ for $+$DESY5 combination, depending on model complexity. Among all extensions CPL, MmAH1 and MmAH2 provide the most stable and physically coherent representations of dynamical dark energy, maintaining moderate tensions with $\Lambda$CDM and well behaved parameter correlations. Overall, these results indicate consistent evidence for departures from $\Lambda$CDM.

astro-ph.CO↗

arXiv2025

Bouncing Cosmologies in modified gravity with space time torsion

We explore the possibility of realizing a non-singular bounce in the early universe within the framework of modified gravity with spacetime torsion. In Einstein Cartan theory, torsion is embedded in the spacetime by adding an antisymmetric part in affine connection . We consider generalized version of the framework as $f(\bar{R})$, $\bar{R}$ being the scalar of the modified curvature tensor. $f(\bar{R})$ gravity is recast in Einstein frame as non-minimally coupled scalar tensor theory where the scalar field gets coupled with a rank 2 antisymmetric torsion field through derivative couplings. We investigate whether the introduction of three additional torsion-dependent terms in Einstein frame help to realize a bounce. We first explore this cosmological system in the background of a homogeneous and isotropic FRW spacetime but inclusion of the torsion terms are insufficient to produce a bounce in this symmetric setting. Motivated by this limitation, we relax the symmetry and generalize the background to include inhomogeneity and anisotropy. In this setup, the dynamics is modified in such a way that a bouncing solution is possible without invoking phantom fields or energy condition violations. We have found the exact solutions of all the fields and reconstructed the modified gravity form. We have addressed the behaviour of the fields under perturbation and investigated the stability of the solutions. Constraints on the model parameters have also been derived based on cosmological observations.

gr-qc↗

arXiv2024

Quintessential early dark energy

We introduce a unified model of early and late dark energy. We call it {\it quintessential early dark energy} model where early and late dark energy are explained by a single scalar field {\it i.e.}, two different energy scales are related by a single scalar field potential. To achieve this we introduce the modified steep exponential potential, which is chosen phenomenologically. This potential has a hilltop nature during the early time which consists of a flat region followed by a steep region. This nature of the potential plays a crucial role in achieving early dark energy solution. During recent time, the potential can almost mimic the cosmological constant which can result into late time acceleration. But, at the perturbation level the potential shows significant difference with the $\Lambda$CDM model. We also constrain and compare the models for steep exponential, modified steep exponential, axionlike and power law potentials by using the available background cosmological data from CMB, BAO (including DESI DR1 2024), supernovae (Pantheon$+$, DESY5 and Union3) and Hubble parameter measurements. Even after the presence of required EDE solution in all four potentials we don't get any significant improvement in the value of $H_0$. The maximum improvement we get in the present value of Hubble parameter compared to the standard $\Lambda$CDM model is for the axionlike potential. For other potentials the constraints are similar to the $\Lambda$CDM model. We also see that the data prefers $\Lambda$CDM model over the considered scalar field models at least for the data combinations with Pantheon$+$ and Union3.

astro-ph.CO↗

arXiv2024

Absence of antisymmetric tensor fields : Clue from f(R) model of gravity

One of the surprising aspects of the present Universe, is the absence of any noticeable observable effects of higher-rank antisymmetric tensor fields in any natural phenomena. Here, we address the possible explanation of the absence of the higher rank antisymmetric tensor fields within the framework of a general class of $f(R)$ gravity represented by $f (R) = R +\alpha_n R^n$. We explore the setup in Einstein frame, where the higher curvature is manifested in terms of a scalar field with a potential through a conformal transformation. The evolution of different cosmological parameters is studied in the background of FRW universe. We show that while different cosmological parameters mimic their standard behaviour at different epochs for different forms of higher curvature gravity (i.e. different values of n ), the positive values of the scalar field in the models provide an additional suppression for the massless modes of higher rank antisymmetric field, The Starobinsky model identified with $n=2$, provides heavier suppression compared to the others $n\neq2$. The result does not change even with the inclusion of the Cosmological Constant. Thus, our result reveals that a general class of modified gravity models can successfully explain the suppression of the massless modes of higher rank antisymmetric tensor fields leading to their invisibility in the present universe.

astro-ph.CO↗