arXiv subjects

Sougata Bhunia

Publications and source records attributed to Sougata Bhunia.

2 recordsLinked to original sources

arXiv2026

Gravitational Wave Propagation in K-essence Cosmology: Theory and Observational Constraints

Gravitational waves (GWs) provide a powerful, theory-independent probe of the dynamical structure of spacetime and the cosmological background. We study linearized GW propagation in k-essence cosmology, where a non-canonical scalar field describes the dark sector. In the high-frequency (short-wavelength) approximation on a Friedmann--Lema\^ıtre--Robertson--Walker (FLRW) background, and restricting to the transverse-traceless tensor sector, we derive a modified evolution equation for tensor perturbations. The GW speed remains strictly luminal, consistent with multimessenger bounds such as GW170817, but the interaction with the background field $\barϕ$ induces a time-dependent effective mass-like term $m_{\rm eff}$. This background-induced mass modifies the dispersion relation without introducing additional propagating degrees of freedom, leading to a cumulative, frequency-dependent phase shift in the waveform over cosmological distances. We show that $m_{\rm eff}$ is uniquely determined by background cosmological parameters and can be written as a redshift-dependent function, $m_{\rm eff}(z)$, directly linking GW observables to scalar-field dynamics, while the GW luminosity distance remains identical to its electromagnetic counterpart, preserving standard-siren consistency. We test the scenario through a joint Bayesian analysis that combines cosmic chronometers (CC), BAO, Pantheon+SH0ES, and standard-siren data from GWTC-2.1/3/4. The reconstruction is consistent with current constraints and reproduces the late-time expansion history, while the evolution of $m_{\rm eff}(z)$ offers a new mechanism that may help alleviate the $H_0$ tension.

gr-qc↗

arXiv2025

From Geometry to Observation: Gravitational Waves and the Raychaudhuri Equation

Gravitational waves (GWs) are independent of any particular theory of gravity. The universality of this notion is highlighted by the Raychaudhuri equation (RE), which is independent of any theory of gravity and contains the Ricci tensor $R_{μν}$ as a key ingredient, thereby connecting spacetime geometry with matter-energy content. Under small metric perturbations, $R_{μν} \propto \Box h_{μν}$, where $h_{μν}$ is the perturbation, indicating that various gravity theories, via their corresponding $R_{μν}$, produce different gravitational wave equations. In the framework of Einstein's gravity, this leads to the standard wave equation. This study analyzes a modified form, {\it GW-inspired RE}, within the homogeneous and isotropic FLRW background to investigate late-time cosmic acceleration and structure formation. We employ {\it Pantheon+ SNe Ia, Hubble, and BAO} datasets to constrain model parameters through Bayesian inference utilizing NUTS in {\it NumPyro}. A nuisance parameter $μ_0$ is introduced to address residual systematics. This facilitates a robust estimation of $H_0$, $Ω_{DE,0}$, and $r_d$, which addresses the resolution of the Hubble tension. We analyze the redshift evolution of the deceleration parameter, $q(z)$, both with and without $μ_0$, emphasizing its influence on cosmic dynamics. The GW-inspired RE is reformulated as a harmonic oscillator, providing insight into expansion and geodesic focusing. A graphical comparison demonstrates the relationship $d^{GW}_L(z) = d^{EM}_L(z)$ utilizing GWOSC data. Thus, the RE in the context of small perturbation of the metric opens up whole new vistas of {\it observational astronomy.}

gr-qc↗