arXiv subjects

Debashis Gangopadhyay

Publications and source records attributed to Debashis Gangopadhyay.

At least 19 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↗

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↗

arXiv2026

Boltzmann Dynamics in K-essence Cosmology: Photon Propagation in an Emergent Spacetime

Recent cosmological tensions, notably the Hubble and $S_{8}$ tensions, necessitate extensions of the conventional $Λ$CDM framework, wherein additional dynamical fields alter the effective spacetime encountered by matter and radiation. In K-essence cosmology, the scalar field induces an emergent FLRW geometry that is disformally linked to the gravitational metric, resulting in a \emph{tilted causal structure} where the light cone propagation differs from that of gravity. This study develops a covariant Boltzmann formalism inside a homogeneous K-essence framework and derives the modified mass-shell condition, geodesic equations, and collision integrals for both massless and massive particles. We demonstrate that the photon distribution retains its thermal properties in the emergent frame, while it seems geometrically rescaled in the gravitational frame. The Thomson and Compton processes maintain their microscopic structure while obtaining effective masses and interaction rates governed by the scalar field. During the tightly coupled epoch, the photon-baryon fluid experiences acoustic oscillations characterized by a modified sound horizon. For the kinetic K-essence DBI-type Lagrangian, the interaction rate scales as $n_{e}σ_{T}^{\rm eff}a\propto a^{-8}$, indicating a strong coupling in the early universe. Additionally, the diffusion damping scale scales as $k_{D}^{-2}\propto a^{29/2}$, indicating that small-scale anisotropies become increasingly sensitive to the evolving geometry. The results provide a coherent kinetic description of particle transport in a tilted spacetime and demonstrate that CMB propagation effects may serve as an observational probe of K-essence and emergent gravity frameworks.

gr-qc↗

arXiv2026

Improved cryptographic security in teleportation with q-deformed non-maximal entangled states

In this work the machinery of q-deformed algebras are used to enhance cryptographic security during teleportation. We use q-deformed harmonic oscillator states to develop a novel method of teleportation. The deformed states can be expressed in terms of standard oscillator states and the expressions contain certain arbitrary functions of $q$. It is the presence of these arbitrary functions that allows an enhancement of cryptographic security. The specifics are : (a) q-deformed Bell-like states are constructed which reduce to the usual Bell states when the deformation parameter $q\rightarrow 1$. These deformed states form an orthonormal basis for q-deformed entangled bipartite states when certain arbitrary functions of $q$ satisfy a constraint. (b) We discuss the generalisation of the usual teleportation protocol with non-maximally entangled states. This generalisation is then employed to construct two new protocols using q-deformed non-maximally entangled states. These states have additional parameters and these have to be shared for decryption after teleportation. Consequently, the cryptographic security is improved.

quant-ph↗

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↗

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

Teleportation with non-maximally entangled states and underlying unitary algebras of certain bipartite systems

New convenient thumbrules are obtained to test entanglement of wavefunctions for bipartite qubit and qutrit systems. All results are analytic. The new results are: (a) For bipartite qubit systems there exists a matrix $A$ for which $\det A = 0$ implies unentanglement while $\det A \ne 0$ implies entanglement. There is an underlying SU(2) algebra. (2) Teleportation for a general qubit state is possible by using non-maximally entangled bipartite qubit states. This protocol has an additional parameter, viz., $\det A$, which enhances the cryptographic security of the teleportation. (c) For qutrits there is a matrix $P$ for which $\det P = 0$ simultaneously with ${\rm tr}P=\pm 1$ imply unentanglement. Any departure from these conditions implies entanglement. There exists an underlying SU(3) algebra. (d) Physical interpretation of the underlying algebras are given and plausible experimental scenarios are proposed for the SU(2) case in the context of two entangled electrons. (e) The entanglement entropy in both cases, viz., for qubits and qutrits respectively, are expressed in terms of the determinants and trace of the matrices mentioned above.

quant-ph↗

arXiv2024

NEC violation in $f(\bar{R},\bar{T})$ gravity in the context of a non-canonical theory via modified Raychaudhuri equation

In this work, we develop the Raychaudhuri equation in $f(\bar{R},\bar{T})$ gravity in the setting of a non-canonical theory, namely K-essence theory. We solve the modified Raychaudhuri equation for the additive form of $f(\bar{R},\bar{T})$, which is $f_{1}(\bar{R})+f_{2}(\bar{T})$. For this solution, we employ two different scale factors to give two types of $f(\bar{R},\bar{T})$ solutions. The ongoing debate between Fisher et. al. and Harko et. al. in 2020 regarding the additive form of $f(\bar{R},\bar{T})$ may provide a resolution within the modified $f(\bar{R},\bar{T})$ gravity theory. By conducting a viability test and analyzing energy conditions, we have determined that in the first scenario, the null energy condition (NEC) is violated between two regions where the NEC is satisfied. Additionally, we have observed that this violation of the NEC exhibits a symmetric property during the phase transition. These observations indicate that bouncing events may occur as a result of the symmetrical violation of the NEC during the expansion of the universe. Moreover, this model indicates that resonant-type quantum tunneling may take place during the period when the NEC is violated. The findings of NEC violation through the power law of scale factor may have empirical relevance in contemporary observations. In the second scenario, our model indicates that the strong energy condition is violated, but the NEC and weak energy conditions are satisfied. The effective energy density decreases and is positive, while the effective pressure and equation of state parameters are negative. This suggests that the universe is expanding with acceleration and is dominated by dark energy.

gr-qc↗

arXiv2024

Form Invariance of Raychaudhuri equation in the presence of Inflaton-type fields

We show that the Raychaudhuri equation remains form invariant for certain solutions of scalar fields $ϕ$ whose Lagrangian is non-canonical and of the form $\mathcal{L}(X,ϕ)=-V(ϕ)F(X)$, with $X=\frac{1}{2} g_{μν} \nabla^μϕ\nabla^ν ϕ$ and $V(ϕ)$ the potential. Solutions exist for both homogeneous and inhomogeneous fields that are like inflatons. Certain recent observations indicate that the cosmos is inhomogeneous and thus our results are in sync with the latest observations. So the Raychaudhuri equation can accommodate primordial inhomogeneities as well as cosmologically relevant scenarios.

gr-qc↗

arXiv2021

An accelerated universe with negative equation of state parameter in Inhomogeneous Cosmology with $k$-essence scalar field

We obtain a scaling relation for spherically symmetric k-essence scalar fields $ϕ(r,t)$ for an inhomogeneous cosmology with the Lemaitre-Tolman- Bondi (LTB) metric. We show that this scaling relation reduces to the known relation for a homogeneous cosmology when the LTB metric reduces to the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric under certain identifications of the metric functions. A k-essence lagrangian is set up and the Euler-Lagrangian equations solved assuming $ϕ(r,t)=ϕ_{1}(r) + ϕ_{2}(t)$. The solutions enable the LBT metric functions to be related to the fields. The LTB inhomogeneous universe exhibits accelerated expansion i.e.cosmic acceleration driven by negative pressure.

gr-qc↗

arXiv2017

Three Flavoured neutrino oscillations and the Leggett Garg Inequality

Three flavoured neutrino oscillations are investigated in the light of the Leggett-Garg inequality. The outline of an experimental proposal is suggested whereby the findings of this investigation may be verified. The results obtained are: (a) The maximum violation of the Leggett Garg Inequality (LGI) is $2.17036$ for neutrino path length $L_{1}=140.15 $ Km and $ΔL=1255.7 $ Km.(b) Presence of the mixing angle $θ_{13}$ enhances the maximum violation of LGI by $4.6\%$.(c) The currently known mass hierarchy parameter $α= 0.0305$ increases the the maximum violation of LGI by $3.7\%$. (d)Presence of CP violating phase parameter enhances the maximum violation of LGI by $0.24\%$, thus providing an \textit{alternative indicator of CP violation} in 3-flavoured neutrino oscillations.

quant-ph↗

arXiv2016

A Scaling Relation in Inhomogeneous Cosmology with k-essence scalar fields

We obtain a scaling relation for spherically symmetric k-essence scalar fields $ϕ(r,t)$ for an inhomogeneous cosmology with the Lemaitre-Tolman- Bondi (LTB) metric. We show that this scaling relation reduces to the known relation for a homogeneous cosmology when the LTB metric reduces to the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric under certain identifications of the metric functions. A k-essence lagrangian is set up and the Euler-Lagrangian equations solved assuming $ϕ(r,t)=ϕ_{1}(r) + ϕ_{2}(t)$. The solutions enable the LBT metric functions to be related to the fields. The LTB inhomogeneous universe exhibits late time accelerated expansion i.e.cosmic acceleration driven by negative pressure.

gr-qc↗

arXiv2015

f(R) Gravity with k-essence scaling relation and Cosmic acceleration

A modified gravity theory with $f(R)=R^2$ coupled to a dark energy lagrangian $L=-V(ϕ)F(X)$ , $X=\nabla_μϕ\nabla^μϕ$, gives plausible cosmological scenarios when the modified Friedman equations are solved subject to the scaling relation $X (\frac{dF}{dX})^{2}=Ca(t)^{-6}$. This relation is already known to be valid, for constant potential $V(ϕ)$, when $L$ is coupled to Einstein gravity. $ϕ$ is the k-essence scalar field and $a(t)$ is the scale factor. The various scenarios are: (1) Radiation dominated Ricci flat universe with deceleration parameter $Q=1$. The solution for $ϕ$ is an inflaton field for small times. (2) $Q$ is always negative and we have accelerated expansion of the universe right from the beginning of time and $ϕ$ is an inflaton for small times. (3)The deceleration parameter $Q= -5$, i.e. we have an accelerated expansion of the universe. $ϕ$ is an inflaton for small times.(4)A generalisation to $f(R)= R^n$ shows that whenever $n > 1.780$ or $n < - 0.280$ , $Q$ will be negative and we will have accelerated expansion of the universe. At small times $ϕ$ is again an inflaton.

astro-ph.CO↗

arXiv2015

Cosmology in presence of dark energy in an emergent gravity scenario

We obtain the analogues of the Friedman equations in an emergent gravity scenario in the presence of dark energy. The background metric is taken to be Friedman-Lemaitre-Robertson-Walker (FLRW). We show that if $\dotϕ^{2}$ is the dark energy density (in units of the critical density) then (a) for total energy density greater than the pressure (non-relativistic scenario, matter domination) the deceleration parameter $q(t)\approx\frac {1}{2} [1 + 27 \dotϕ^{2}+...] > \frac{1}{2}$ (b) for total energy density equal to 3 times the pressure (relativistic case, radiation domination), the deceleration parameter $q(t)\approx 1 + 18\dotϕ^{2} +... > 1$ and (c) for total energy density equal to the negative of the pressure (dark energy scenario), the deceleration parameter $q(t)< -1$. Our results indicate that many aspects of standard cosmology can be accommodated with the presence of dark energy right from the beginning of the universe where the time parameter $t\equiv \frac{t}{t_{0}}$, $t_{0}$ being the present epoch.

gr-qc↗

arXiv2014

Semi-classical treatment of $k$-essence effect on cosmic temperature

A phenomenological model is described for Cosmic Microwave Background Radiation (CMBR) evolution with dark energy an essential ingredient in the form of a $k-$essence scalar field. The following features of this evolution can be successfully obtained from this model: (a) the {\it observed} variation of the rate of change of scale factor $a(t)$, i.e. $\dot a$, with time and (b) the {\it observed} value of the epoch when the universe went from a decelerating phase to an accelerated phase. These two features have been matched with graphical transcriptions of SNe Ia data. The model also indicates that the evolution is sensitive to the presence of inhomogeneity and this sensitivity increases as one goes further into the past. Further, the value of the inhomogeneity parameter determines the epoch of switch over to an accelerated phase. A positive value of inhomogeneity parameter leads to switch over at earlier epochs, while a negative value leads to switch over at later epochs. If the value of the inhomogeneity parameter is a bit negative then the crossover point from deceleration to acceleration gives better agreement with the observed value.

gr-qc↗

arXiv2014

The Hawking Temperature in the context of Dark Energy for Reissner-Nordstrom and Kerr background

For emergent gravity metrics, presence of dark energy modifies the Hawking temperature. We show that for the spherically symmetric Reissner-Nordstrom (RN) background metric, the emergent metric can be mapped into a Robinson-Trautman blackhole. Allowed values of the dark energy density follow from rather general conditions. For some allowed value of the dark energy density this blackhole can have zero Hawking temperature i.e. the blackhole does not radiate. For a Kerr background along $θ=0$, the emergent blackhole metric satisfies Einstein's equations for large $r$ and always radiates. Our analysis is done in the context of emergent gravity metrics having $k-$essence scalar fields $ϕ$ with a Born-Infeld type lagrangian. In both cases the scalar field $ϕ(r,t)=ϕ_{1}(r)+ϕ_{2}(t)$ also satisfies the emergent gravity equations of motion for $r\rightarrow\infty$ and $θ=0$. \keywords{dark energy, k-essence, Reissner-Nordstrom and Kerr blackholes} \pacs{98.80.-k ;95.36.+x}

physics.gen-ph↗

arXiv2012

The Hawking temperature in the context of dark energy

An emergent gravity metric incorporating $k-$essence scalar fields $ϕ$ having a Born-Infeld type lagrangian is mapped into a metric whose structure is similar to that of a blackhole of large mass $M$ that has swallowed a global monopole. However, here the field is not that of a monopole but rather that of a $k-$essence scalar field. If $ϕ_{emergent}$ be solutions of the emergent gravity equations of motion under cosmological boundary conditions at $\infty$, then for $r\rightarrow\infty$ the rescaled field $\frac {ϕ_{emergent}}{2GM-1}$ has exact correspondence with $ϕ$ with $ϕ(r,t)=ϕ_{1}(r)+ϕ_{2}(t)$. The Hawking temperature of this metric is $T_{\mathrm emergent}= \frac{\hbar c^{3}}{8πGM k_{\mathrm B}}(1-K)^{2}\equiv \frac{\hbar}{8πGM k_{\mathrm B}}(1-K)^{2}$, taking the speed of light $c=1$. Here $K=\dotϕ_{2}^{2}$ is the kinetic energy of the $k-$essence field $ϕ$ and $K$ is always less than unity, $k_{\mathrm B}$ is the Boltzmann constant. This is phenomenologically interesting in the context of Belgiorno {\it et al's} gravitational analogue experiment.

physics.gen-ph↗

arXiv2012

The $k$-essence scalar field in the context of Supernova Ia Observations

A $k$-essence scalar field model having (non canonical) Lagrangian of the form $L=-V(ϕ)F(X)$ where $X=1/2g^{μν}\nabla_μϕ\nabla_νϕ$ with constant $V(ϕ)$ is shown to be consistent with luminosity distance-redshift data observed for type Ia Supernova. For constant $V(ϕ)$, $F(X)$ satisfies a scaling relation which is used to set up a differential equation involving the Hubble parameter $H$, the scale factor $a$ and the $k$-essence field $ϕ$. $H$ and $a$ are extracted from SNe Ia data and using the differential equation the time dependence of the field $ϕ$ is found to be: $ϕ(t) \sim λ_0 + λ_1 t + λ_2 t^2$. The constants $λ_i$ have been determined. The time dependence is similar to that of the quintessence scalar field (having canonical kinetic energy) responsible for homogeneous inflation. Furthermore, the scaling relation and the obtained time dependence of the field $ϕ$ is used to determine the $X$-dependence of the function $F(X)$.

astro-ph.CO↗