arXiv subjects

Narayan Banerjee

Publications and source records attributed to Narayan Banerjee.

At least 19 recordsLinked to original sources

arXiv2026

Towards a causal effective thermodynamics of scalar-tensor gravity

The thermal analogy between the effective fluid of scalar-tensor gravity and Eckart's irreversible thermodynamics is extended to the causal Israel-Stewart model, adopting the minimal ansatz of promoting the heat flux density to a timelike vector. This choice yields analytically manageable constitutive equations, allowing for the first consistent decoupling of the effective temperature $\mathcal{T}$ and the effective thermal conductivity $\mathcal{K}$ of scalar-tensor gravity. Crucially, this new framework preserves the interpretation of general relativity as the equilibrium state approached via a dynamical relaxation process in the vanishing-$\mathcal {KT}$ limit. This new causal formalism is applied to cosmology.

gr-qc↗

arXiv2024

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) $m(v,r)=\mu v$, (ii) $m(v,r)=\mu v +\nu r$, and (iii) $m(v,r)=\mu v -\frac{\mu^2 v^2}{2r}$, where $\mu$ and $\nu$ 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↗

arXiv2024

Interacting dark sector: a dynamical system perspective

We investigate the interaction between the dark sectors from the point of view of a dynamical system analysis. A general setup for interacting dark energy models that incorporates both quintessence and phantom fields through a switch parameter, allowing an interaction in the dark sectors, has been considered. In the first part of our analysis, we have not assumed any specific form of the interaction, and in the second part, we invoked examples in a general framework of the interaction. The potentials of the scalar field are classified into two broad classes of potentials: exponential and non-exponential. We identify the potential late-time attractors of the system, which have a complete dark energy domination. From our analysis, it is evident there could be an interaction between the dark sector. The interaction, if any, weakens over time. We find for the quintessence field the transfer of energy from dark matter to dark energy can flip the direction, and on the contrary, for the phantom field, it is only from dark matter to dark energy.

gr-qc↗

arXiv2024

A comprehensive data-driven odyssey to explore the equation of state of dark energy

For the first time, we reconstruct the dark energy equation of the state parameter $w$ from the combination of background and perturbation observations, specifically combining the Hubble parameter data from cosmic chronometer observations and the logarithmic growth rate data from the growth rate observations. We do this analysis using posterior Gaussian process regression without considering any specific cosmological model or parametrization. However there are three main assumptions: (I) a flat Friedmann-Lema\^itre-Robertson-Walker (FLRW) metric is considered for the cosmological background, (II) there is no interaction between dark energy and matter sectors, and (III) for the growth of inhomogeneity, sub-Hubble approximation and linear perturbations are considered. This study is unique in the sense that the reconstruction of $w$ is independent of any derived parameters such as the present values of the matter-energy density parameter and Hubble parameter. From the reconstruction, we look at how the dark energy equation of state evolves between redshifts 0 and 1.5, finding a slight hint of dynamical behavior in dark energy. However, the evidence is not significant. We also find a leaning towards non-phantom behavior over phantom behavior. We observe that the $\Lambda$CDM model ($w=-1$) nearly touches the lower boundary of the 1$\sigma$ confidence region in the redshift range $0.6 \lesssim z \lesssim 0.85$. However, it comfortably resides within the 2$\sigma$ confidence region in the whole redshift range under investigation, $0\leq z \leq 1.5$. Consequently, the non-parametric, model-independent reconstruction of dark energy provides no compelling evidence to deviate from the $\Lambda$CDM model when considering cosmic chronometer and growth rate observations.

astro-ph.CO↗

arXiv2023

Checking the second law at cosmic scales

Based on recent data about the history of the Hubble factor, it is argued that the second law of thermodynamics holds at the largest scales accessible to observation. This is consistent with previous studies of the same question.

astro-ph.CO↗

arXiv2023

Constraints on the speed of sound in the k-essence model of dark energy

We consider a particular k-essence scalar field model for the late-time cosmic acceleration in which the sound speed, parametrized as $c_s$ is constant. We compute the relevant background and perturbation quantities corresponding to the observables like cosmic microwave background, type Ia supernova, cosmic chronometers, baryon acoustic oscillations, and the $f\sigma_8$. We put constraints on the $c_s^2$ parameter from these observations along with other parameters. We find lower values of $c_s^2$ which are close to zero are tightly constrained. Particularly, we find mean value of $\log_{\rm 10} (c_s^2)$ to be $-0.61$ and $c_s^2 \leq 10^{-3}$ is more than 3$\sigma$ away from this mean value. This means these observations favor a homogeneous dark energy component compared to the clustering one.

astro-ph.CO↗

arXiv2023

Realisations of type III stress-energy tensors of the Hawking-Ellis classification in scalar-tensor gravity

The ``ugly duckling'' of the Segr\'e-Pleba\'nski-Hawking-Ellis classification of stress-energy tensors is believed to be either impossible or extremely difficult to realise in Einstein gravity. Effective stress-energy tensors in alternative gravity offer a wider range of possibilities. We report a class of type III realisations in ``first-generation'' scalar-tensor and in Horndeski gravity, and their physical interpretation. The ugly duckling may be a freak of nature of limited importance but it is not physically impossible.

gr-qc↗

arXiv2023

Signature flip in deceleration parameter: A thermodynamic phase transition?

Using the Hayward-Kodama temperature for the apparent horizon, it is found that matter content in the Universe is not thermodynamically stable, and the entry to the late accelerated expansion is actually a second order phase transition. The cosmological model used for the purpose is one that imitates the $\Lambda$CDM model, the favoured model for the present Universe.

gr-qc↗

arXiv2023

Spatial Curvature and Thermodynamics

Reasonable parametrizations of the current Hubble data set of the expansion rate of our homogeneous and isotropic universe, after suitable smoothing of these data, strongly suggests that the area of the apparent horizon increases irrespective of whether the spatial curvature of the metric is open, flat or closed. Put in another way, any sign of the spatial curvature appears consistent with the second law of thermodynamics.

astro-ph.CO↗

arXiv2022

Model independent bounds on Type Ia supernova absolute peak magnitude

We put constraints on the peak absolute magnitude, $M_B$ of type Ia supernova using the Pantheon sample for type Ia supernova observations and the cosmic chronometers data for the Hubble parameter by a model independent and non-parametric approach. Our analysis is based on the Gaussian process regression. We find percent level bounds on the peak absolute magnitude given as $M_B=-19.384\pm0.052$. For completeness and to check the consistency of the results, we also include the Baryon acoustic oscillation data and the prior of the comoving sound horizon from Planck 2018 cosmic microwave background observations. The inclusion of these two data gives tighter constraints on $M_B$ at the sub-percent level. We obtain constraints on $M_B$ from the combination of pantheon compilation of type Ia supernova observations and baryon acoustic oscillation observations given as $M_B=-19.396\pm0.016$. When adding the cosmic chronometer observations with these observations, we find $M_B=-19.395\pm0.015$. The mean values of peak absolute magnitude from all these data are consistent with each other and the values are approximately equal to $-19.4$.

astro-ph.CO↗

arXiv2022

Constraining the curvature density parameter in cosmology

The cosmic curvature density parameter has been constrained in the present work independent of any background cosmological model. The reconstruction is performed adopting the non-parametric Gaussian Processes (GP). The constraints on $\Omega_{k0}$ are obtained via a Markov Chain Monte Carlo (MCMC) analysis. Late-time cosmological probes viz., the Supernova (SN) distance modulus data, the Cosmic Chronometer (CC) and the radial Baryon Acoustic Oscillations ($r$BAO) measurements of the Hubble data have been utilized for this purpose. The results are further combined with the data from redshift space distortions (RSD) which studies the growth of large scale structure in the universe. The only \textit{a priori} assumption is that the universe is homogeneous and isotropic, described by the FLRW metric. Results indicate that a spatially flat universe is well consistent in 2$\sigma$ within the domain of reconstruction $0<z<2$ for the background data. On combining the RSD data we find that the results obtained are consistent with spatial flatness mostly within 2$\sigma$ and always within 3$\sigma$ in the domain of reconstruction $0<z<2$.

astro-ph.CO↗

arXiv2021

Non-parametric reconstruction of interaction in the cosmic dark sector

The possibility of a non-gravitational interaction between the dark matter and the dark energy has been reconstructed using some recent datasets. The crucial aspect is that the interaction is not parametrized at the outset, but rather reconstructed directly from the data in a non-parametric way. The Cosmic Chronometer Hubble data, the Pantheon Supernova compilation of CANDELS and CLASH Multy-Cycle Treasury programs obtained by the HST, and the Baryon Acoustic Oscillation Hubble data have been considered in this work. The widely accepted Gaussian Process is used for the reconstruction. The results clearly indicate that a no interaction scenario is quite a possibility. Also, the interaction, if any, is not really significant at the present epoch. The direction of the flow of energy is clearly from the dark energy to the dark matter which is consistent with the thermodynamic requirement.

astro-ph.CO↗

arXiv2021

The Raychaudhuri equation for a quantized timelike geodesic congruence

A recent attempt to arrive at a quantum version of Raychaudhuri's equation is looked at critically. It is shown that the method, and even the idea, has some inherent problems. The issues are pointed out here. We have also shown that it is possible to salvage the method in some limited domain of applicability. Although no generality can be claimed, a quantum version of the equation should be useful in the context of ascertaining the existence of a singularity in the quantum regime. The equation presented in the present work holds for arbitrary $n + 1$ dimensions. An important feature of the Hamiltonian in the operator form is that it admits a self-adjoint extension quite generally. Thus, the conservation of probability is ensured.

gr-qc↗

arXiv2021

Raychaudhuri equation in scalar-tensor theory

Implications of the Raychaudhuri equation in focusing of geodesic congruences are studied in the framework of scalar--tensor theory of gravity. Specifically, we investigate the Brans--Dicke theory and Bekenstein's scalar field theory. In both of these theories, we deal with a static spherically symmetric distribution and a spatially homogeneous and isotropic cosmological model as specific examples. We find that it is possible to violate the convergence condition under reasonable physical assumptions. This leads to the possibility of avoiding a singularity.

gr-qc↗

arXiv2020

Perturbations in a scalar field model with virtues of $\Lambda$CDM

In the era of precision cosmology, the cosmological constant $\Lambda$ gives quite an accurate description of the evolution of the Universe, but it is still plagued with the fine-tuning problem and the cosmic coincidence problem. In this work, we investigate the perturbations in a scalar field model that drives the recent acceleration in a similar fashion that the cosmological constant does and has the dark energy (DE) density comparable to the dark matter (DM) energy density at the recent epoch starting from arbitrary initial conditions. The perturbations show that this model, though it keeps the virtues of a $\Lambda$CDM model, has a distinctive qualitative feature, particularly it reduces the amplitude of the matter power spectrum on a scale of $8 h^{-1}\, \mbox{Mpc}$, $\sigma_{8}$ at the present epoch.

astro-ph.CO↗

arXiv2020

Revisiting a non-parametric reconstruction of the deceleration parameter from combined background and the growth rate data

The cosmic deceleration parameter $q$ has been reconstructed in a non-parametric way using various combinations of recent observational datasets. The Pantheon compilation of the Supernova (SN) distance modulus data, the Cosmic Chronometer (CC) measurements of the Hubble parameter including the full systematics and the Baryon Acoustic Oscillation (BAO) data have been considered in this work. The redshift $z_t$, where the transition from a past decelerated to a late-time accelerated phase of evolution occurs, is estimated from the reconstructed $q$. The possible effect of a non-zero spatial curvature from the Planck 2020 estimate is checked. The outcome of including different $H_0$ measurements from recent Planck 2020 and Riess 2021 probes having a maximum discrepancy at the $4.2\sigma$ level, is investigated. Results indicate that the transition from a past decelerated phase to the late-time accelerated phase occurs within the redshift range $0.5 1$, the reconstructed $q$ is observed to have a non-monotonic evolution in case of the combined CC and SN data. On introducing the BAO data, the reconstructed $q$ shows an oscillating behaviour for $z\gtrsim1$. To investigate the effect of matter perturbations, the growth rate data from the Redshift-Space Distortions (RSD) are utilized in reconstructing $q$. Using the $\mathcal{O}m(z)$ diagnostic, we draw inferences on the validity of $\Lambda$CDM as a consistency check. The $\Lambda$CDM model is well consistent and included at the 2$\sigma$ level in the domain of all the reconstructions.

astro-ph.CO↗

arXiv2020

Non-parametric reconstruction of the cosmological \textit{jerk} parameter

The cosmological jerk parameter $j$ is reconstructed in a non-parametric way from observational data independent of a fiducial cosmological model. From this kinematical quantity, the equation of state parameter for composite matter distribution is also found out. The result shows that there is a deviation from the $\Lambda$CDM model close to $z=1.5$, at the $3\sigma$ confidence level.

astro-ph.CO↗

arXiv2020

Echoes from a singularity

Though the cosmic censorship conjecture states that spacetime singularities must be hidden from an asymptotic observer by an event horizon, naked singularities can form as the end product of a gravitational collapse under suitable initial conditions, so the question of how to observationally distinguish such naked singularities from standard black hole spacetimes becomes important. In the present paper, we try to address this question by studying the ringdown profile of the Janis-Newman-Winicour (JNW) naked singularity under axial gravitational perturbation. The JNW spacetime has a surfacelike naked singularity that is sourced by a massless scalar field and reduces to the Schwarzschild solution in absence of the scalar field. We show that for low strength of the scalar field, the ringdown profile is dominated by echoes which mellows down as the strength of the field increases to yield characteristic quasinormal mode frequency of the JNW spacetime.

gr-qc↗