arXiv subjects

Samit Ganguly

Publications and source records attributed to Samit Ganguly.

6 recordsLinked to original sources

arXiv2026

Multi-Probe Cosmological Constraints on Clustering DBI Dark Energy Scenario

We present a multi-probe observational analysis of a clustering tachyonic Dirac-Born-Infeld~(DBI) dark energy model. Its noncanonical kinetic term gives a subluminal sound speed, so dark energy perturbations can grow on subhorizon scales. Working in a multifluid framework that tracks both the background expansion and linear perturbations, we constrain the model with nested sampling, using Planck 2018 compressed CMB distance priors, DESI DR2 BAO, cosmic chronometers, $fσ_8$ growth data, and three Type Ia supernova compilations: Pantheon$+$, DES-SN5YR, and Union3. The standard cosmological parameters agree with $Λ$CDM within $2σ$, with DBI preferring a slightly lower $H_0$ and higher $Ω_{m0}$, with the largest shift being $\simeq1.8σ$ in $H_0$ for Pantheon$+$. The Bayesian evidence favours DBI in every compilation, moderately for Pantheon$+$ ($Δ\ln\mathcal{Z}\simeq3.4$) and inconclusively for DES-SN5YR and Union3 ($Δ\ln\mathcal{Z}\simeq0.6$-$0.7$). The field thaws naturally. Dark energy perturbations stay several orders of magnitude below matter perturbations, yet the noncanonical sound speed still alters the Hubble friction term, raising $S_8$ by only $\approx0.5σ$ relative to $Λ$CDM. Four independent diagnostics, namely the CPL parametrization, $Om(z)$, the statefinder hierarchy, and the growth index, all point to genuinely evolving dark energy rather than a cosmological constant. The thermal history is left intact, with negligible early dark energy. However, one limitation remains. Because $w_ϕ=\dotϕ^2-1\geq-1$, the phantom regime is out of reach, which is in line with the no-go theorem for a single minimally coupled scalar field, so the model cannot follow the DESI DR2 preference for an effective equation of state crossing $w=-1$.

astro-ph.CO↗

arXiv2026

Distinguishing Coupled Dark Matter Dark Energy from Kinematic Phantom Crossing

Recent measurements of cosmic expansion have renewed interest in dark-energy scenarios in which the effective equation of state exhibits phantom-like evolution and may cross $w=-1$. Such behavior, however, does not require a fundamental phantom field: in coupled dark-energy-dark-matter~(CDEDM) models, energy exchange between a canonical scalar field and cold dark matter, happens Lagrangian labels, can reproduce the same effective equation of state evolution as that of phenomenological $w_0w_a$CDM model. We show that this microscopic interaction leaves distinctive, testable signatures in structure growth by forming a two-fluid system in which only cold dark matter feels the scalar-mediated fifth force (and an associated drag term), while baryons remain uncoupled. When combined with neutrino-free streaming, this setup generates a characteristic coupling, neutrino-mass degeneracy, whose leading scaling $\sum m_ν\propto f_c^2 α^2$ follows analytically from the two-fluid growth equations. Using DESI DR2 BAO, CMB distance information, ACT DR6 lensing, redshift-space distortions, and three supernova compilations, we find that this scaling is recovered for both exponential and inverse-power-law potentials. We further show that the dominant contribution to the growth response arises from the coupling-induced modification of the background evolution, with the fifth force enhancing growth and the drag partially counteracting it. Finally, the interaction induces a running dark-matter mass, changing it at recombination by $4.3\%$--$5.5\%$, so CMB distance calculations must account for the evolving dark-matter density. These correlated growth and recombination effects provide a route to distinguish interaction-driven phantom crossing from purely kinematic dark-energy parametrizations.

astro-ph.CO↗

arXiv2026

Observational Insights on DBI K-essence Models Using Machine Learning and Bayesian Analysis

We perform a late-time cosmological study; we compare the performance of two Dirac-Born-Infeld (DBI)-type k-essence scalar field extensions of the $Λ$CDM model to the standard framework and a wCDM scenario using the Chevallier-Polarski-Linder (CPL) equation of state parametrization. We solve background dynamics numerically as functions of redshift and incorporate them into a Bayesian inference pipeline accelerated by machine learning. We use a Flax-based surrogate emulator to replace repeated direct integrations of the ODE system, reducing computational cost. A hybrid scheme that combines Stochastic Variational Inference (SVI) with No-U-Turn Hamiltonian Monte Carlo constrains cosmological parameters using the Pantheon$+$SH0ES Type Ia supernova sample, DESI BAO (DR2) data, and cosmic chronometer $H(z)$ measurements without CMB-based priors. In both DBI k-essence formulations, present-day dark energy equations of state are consistent with cosmic acceleration, indicating a $Λ$CDM-like regime with a modest redshift dependence. The $w$CDM model is marginally favored by conventional model selection measures such as $χ^2$, AIC, BIC, and DIC, which are based on goodness of fit and penalized. However, Bayesian predictive measures like WAIC and PSIS-LOO show no significant differences between $Λ$CDM, $w$CDM, and DBI k-essence scenarios. All have similar model weights and out-of-sample predictive performance for the datasets. Thus, DBI k-essence models mimic the success of the classic $Λ$CDM paradigm while allowing controlled, redshift-dependent deviations from a strict cosmological constant that are consistent with present late-time observations.

astro-ph.CO↗

arXiv2025

Non-Affine Extensions of the Raychaudhuri Equation in the K-essence Framework

We present a new avenue of the Raychaudhuri Equation (RE) by introducing a non-affine parametrization within the k-essence framework. This modification accounts for non-geodesic flow curves, leading to emergent repulsive effects in cosmic evolution. Using a DBI-type k-essence Lagrangian, we derive a modified RE and demonstrate its ability to address the Hubble tension while predicting a natural emergence of a dynamical dark energy equation of state. Our Bayesian analysis, constrained by cosmological data, supports the theoretical scaling relation of the k-essence field ($\dotϕ$) and the cosmic scale factor ($a$). Furthermore, we reinterpret the modified RE as an anti-damped harmonic oscillator, we found a caustic avoidance signature, it may reveal classical or quantum-like effects in cosmic expansion. These results suggest a deep connection between scalar field dynamics and modified gravity, offering new perspectives on the nature of the expansion history of the universe.

gr-qc↗

arXiv2025

Connecting Gravity and Quantum Physics: Primordial Black Holes and Accelerated Evolution of the Universe

This study presents a new framework to explore the fundamental relationship between gravity and quantum mechanics, with particular emphasis on the fundamental role of primordial black holes (PBHs) in cosmology. Through the concept of self-gravitating condensed light in the form of the experimentally discovered quantum photon Bose-Einstein condensate, this work examines the quantized gravitational, informational, thermodynamical, traditional, and other attributes of PBHs and their implications for early universe dynamics, baryogenesis, the very early formation of galaxies, supermassive black holes (SMBHs) and large-scale structures. Precise calculations have shown that primordial protogalaxies with supermassive black holes at their centers of gravity were formed before the recombination epoch. By solving issues like the cosmological constant problem and the information loss paradox, dark matter and dark energy, this work provides insights into Planck-scale physics and it's impact to cosmology. In such a way PBHs serve as a bridge between quantum theory and general relativity. This study ultimately posits that presented PBH physics is essential to resolving major cosmological and astrophysical issues, paradoxes and "mysteries", such as the accelerated evolution of the Universe established by JWST and other observations.

gr-qc↗

arXiv2025

Particle production rate for a dynamical system using the path integral approach

In this work, we investigate the particle creation rate in a dynamical (Vaidya) spacetime using Feynman's path integral formalism within the framework of the effective action approach. We examine three distinct cases involving the following mass functions, each representing dynamical geometries: (i) $m(v,r)=μv$, (ii) $m(v,r)=μv +νr$, and (iii) $m(v,r)=μv -\frac{μ^2 v^2}{2r}$, where $μ$ and $ν$ are positive constants that satisfy all known energy conditions. We analyze particle production rates in the region of dynamical horizons, revealing an initial high rate followed by a rapid decline in all cases. Additionally, we explore the thermodynamic properties by calculating the surface gravity and corresponding Hayward-Kodama temperatures for each scenario. Graphical representations show the variation of surface gravity over time for the three cases, offering insights into the system's thermodynamic evolution. Our research investigates the connection between background geometry and the particle creation process, placing it within the broader context of quantum field theory in curved spacetime. The non-stationary nature of Vaidya geometry is highlighted as a valuable framework for examining the dynamic aspects of particle creation. This in-depth analysis enhances our understanding of quantum processes in curved spacetime and may offer insights relevant to thermodynamics and studies of gravitational collapse.

gr-qc↗