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Snehasish Bhattacharjee

Publications and source records attributed to Snehasish Bhattacharjee.

At least 19 recordsLinked to original sources
arXiv2025

The Ultraviolet Spectra of 2003fg-like Type Ia Supernovae

2003fg-like Type Ia supernovae (03fg-like SNe Ia) are rare subtype of SNe Ia, photometrically characterized by broader optical light curves and bluer ultraviolet (UV) colors compared to normal SNe Ia. In this work, we study four 03fg-like SNe Ia using Swift UltraViolet and Optical Telescope (UVOT) grism observations to understand their unique UV properties and progenitor scenario(s). We report 03fg-like SNe Ia to have similar UV features and elemental compositions as normal SNe Ia, but with higher UV flux relative to optical. Previous studies have suggested that the UV flux levels of normal SNe Ia could be influenced by their progenitor properties, such as metallicity, with metal-poor progenitors producing higher UV flux levels. While 03fg-like SNe were previously reported to occur in low-mass and metal-poor host environments, our analysis indicates that their UV excess cannot be explained by their host-galaxy parameters. Instead, we demonstrate that the addition of a hot blackbody component, likely arising from the interaction with the circumstellar material (CSM), to the normal SN Ia spectrum, can reproduce their distinctive UV excess. This supports the hypothesis that 03fg-like SNe Ia could explode in a CSM-rich environment.

astro-ph.HE↗
arXiv2023

White dwarf cooling in $f(R,T)$ gravity

In recent times, astounding observations of both over- and under-luminous type Ia supernovae have emerged. These peculiar observations hint not only at surpassing the Chandrasekhar limit but may also suggest potential modifications in the physical attributes of their progenitors, such as their cooling rate. This, in turn, can influence their temporal assessments and provide a compelling explanation for these intriguing observations. In this spirit, we investigate here the cooling process of white dwarfs in $f(R,T)$ gravity with the simplest model $f(R,T) = R + λT$, where $λ$ is the model parameter. Our modelling suggests that the cooling timescale of white dwarfs exhibits an inverse relationship with the model parameter $λ$, which implies that for identical initial conditions, white dwarfs in $f(R,T)$ gravity cool faster. This further unveils that in the realm of $f(R,T)$ gravity, the energy release rate for white dwarfs increases as $λ$ increases. Furthermore, we also report that the luminosity of the white dwarfs also depends on $λ$ and an upswing in $λ$ leads to an amplification in the luminosity, and consequently a larger white dwarf in general relativity can exhibit comparable luminosity to a smaller white dwarf in $f(R,T)$ gravity.

gr-qc↗
arXiv2023

Baryogenesis in $f(R,L_m)$ gravity

This paper aims to recreate the gravitational baryogenesis epoch in the framework of the $f(R,L_m)$ theory of gravity, where $R$ and $L_m$ are the curvature scalar and the matter Langragian, respectively. In particular, we examine the model, $f(R,L_m) = \frac{R}{2} + L_m ^α + ζ$, under the supposition that the universe is saturated with dark energy and perfect fluid, with a non-zero baryon to entropy ratio during a radiation dominance era. We confine the model with the gravitational baryogenesis scenario, emphasizing the appropriate values of model parameters compatible with the baryon-to-entropy ratio observation data. Our study found that $f(R,L_m)$ gravity can considerably and steadily make a contribution to the phenomenon of gravitational baryogenesis.

gr-qc↗
arXiv2021

Baryogenesis in $f(P)$ Gravity

In this work, we investigate gravitational baryogenesis in the framework of $f(P)$ gravity to understand the applicability of this class of modified gravity in addressing the baryon asymmetry of the Universe. For the analysis, we set $f(P) = αP$ where $α$ is the model parameter. We found that in $f(P)$ gravity, the CP-violating interaction acquires a modification through the addition of the nontopological cubic term $P$ in addition to the Ricci scalar $R$ and the mathematical expression of the baryon-to-entropy ratio depends not only on the time derivative of $R$ but also the time derivative of $P$. Additionally, we also investigate the consequences of a more complete and generalized CP-violating interaction proportional to $f(P)$ instead of $P$ in addressing the baryon asymmetry of the Universe. For this type of interaction, we report that the baryon-to-entropy ratio is proportional to $\dot{R}$, $\dot{P}$ and $f^{'}(P)$. We report that for both of these cases, rational values of $α$ and $χ$ generate acceptable baryon-to-entropy ratios compatible with observations.

gr-qc↗
arXiv2021

Configurational Entropy in Chaplygin Gas Models

The present work employs the Linder parametrization of a constant growth index \cite{linder/index} to investigate the evolution of growth rate of clustering and the dissipation of configurational entropy in some of the most widely studied Chaplygin gas models, such as the generalized Chaplygin gas and the modified Chaplygin gas. The model parameters of the Chaplygin gas models are found to play a vital role in the evolution of growth rate, dark energy density parameter, EoS parameter, and configurational entropy. Furthermore, the work communicates the rate of change of configurational entropy to attain a minimum which depend solely on the choice of model parameters and that there exist suitable parameter combinations giving rise to a viable dissipation of configurational entropy, and therefore certifying its time derivative to hit a minimum at a scale factor which complies with the current observational constraints on the redshift of transition from a dust to an accelerated Universe and thereby making Chaplygin gas models a viable candidate for dark energy.

gr-qc↗
arXiv2021

Inflation in mimetic $f(R,T)$ gravity

In this paper, we employ mimetic $f(R,T)$ gravity coupled with Lagrange multiplier and mimetic potential to yield viable inflationary cosmological solutions consistent with latest Planck and BICEP2/Keck Array data. We present here three viable inflationary solutions of the Hubble parameter ($H$) represented by $H(N)=\left(A \exp βN+B α^N\right)^{γ}$, $H(N)=\left(A α^N+B \log N\right)^{γ}$, and $H(N)=\left(A e^{βN}+B \log N\right)^{γ}$, where $A$, $β$, $B$, $α$, $γ$ are free parameters, and $N$ represents the number of e-foldings. We carry out the analysis with the simplest minimal $f(R,T)$ function of the form $f(R,T)= R + χT$, where $χ$ is the model parameter. We report that for the chosen $f(R,T)$ gravity model, viable cosmologies are obtained compatible with observations by conveniently setting the Lagrange multiplier and the mimetic potential.

gr-qc↗
arXiv2021

Energy Conditions in $f(P)$ Gravity

$f(P)$ gravity is a novel extension of ECG in which the Ricci scalar in the action is replaced by a function of the curvature invariant $P$ which represents the contractions of the Riemann tensor at the cubic order \cite{p}. The present work is concentrated on bounding some $f(P)$ gravity models using the concept of energy conditions where the functional forms of $f(P)$ are represented as \textbf{a)} $f(P) = α\sqrt{P}$, and \textbf{b)} $f(P) = α\exp (P)$, where $α$ is the sole model parameter. Energy conditions are interesting linear relationships between pressure and density and have been extensively employed to derive interesting results in Einstein's gravity, and are also an excellent tool to impose constraints on any cosmological model. To place the bounds, we ensured that the energy density must remain positive, the pressure must remain negative, and the EoS parameter must attain a value close to $-1$ to make sure that the bounds respect the accelerated expansion of the Universe and are also in harmony with the latest observational data. We report that for both the models, suitable parameter spaces exist which satisfy the aforementioned conditions and therefore posit the $f(P)$ theory of gravity to be a promising modified theory of gravitation.

gr-qc↗
arXiv2021

Growth Rate and Configurational Entropy in Tsallis Holographic Dark Energy

In this work, we analyzed the effect of different prescriptions of the IR cutoffs, namely the Hubble horizon cutoff, particle horizon cutoff, Granda and Oliveros horizon cut off, and the Ricci horizon cutoff on the growth rate of clustering for the Tsallis holographic dark energy (THDE) model in an FRW universe devoid of any interactions between the dark Universe. Furthermore, we used the concept of configurational entropy to derive constraints (qualitatively) on the model parameters for the THDE model in each IR cutoff prescription from the fact that the rate of change of configurational entropy hits a minimum at a particular scale factor $a_{DE}$ which indicate precisely the epoch of dark energy domination predicted by the relevant cosmological model as a function of the model parameter(s). By using the current observational constraints on the redshift of transition from a decelerated to an accelerated Universe, we derived constraints on the model parameters appearing in each IR cutoff definition and on the non-additivity parameter $δ$ characterizing the THDE model and report the existence of simple linear dependency between $δ$ and $a_{DE}$ in each IR cutoff setup.

gr-qc↗
arXiv2021

BBN Constraints on $f(Q,T)$ Gravity

$f(Q,T)$ gravity is a novel extension of the symmetric teleparallel gravity where the Lagrangian $L$ is represented through an arbitrary function of the nonmetricity $Q$ and the trace of the energy-momentum tensor $T$ \cite{fqt}. In this work, we have constrained a widely used $f(Q,T)$ gravity model of the form $f(Q,T) = Q^{n+1} + m T$ from the primordial abundances of the light elements to understand its viability in Cosmology. We report that the $f(Q,T)$ gravity model can elegantly explain the observed abundances of Helium and Deuterium while the Lithium problem persists. From the constraint on the expansion factor in the range $0.9425 \lesssim Z \lesssim1.1525$, we report strict constraints on the parameters $m$ and $n$ in the range $-1.13 \lesssim n \lesssim -1.08$ and $-5.86 \lesssim m \lesssim12.52$ respectively.

gr-qc↗
arXiv2020

Do galactic bars depend on environment?: An information theoretic analysis of Galaxy Zoo 2

We use an information theoretic framework to analyze data from the Galaxy Zoo 2 project and study if there are any statistically significant correlations between the presence of bars in spiral galaxies and their environment. We measure the mutual information between the barredness of galaxies and their environments in a volume limited sample ($M_r \leq -21$) and compare it with the same in datasets where (i) the bar/unbar classifications are randomized and (ii) the spatial distribution of galaxies are shuffled on different length scales. We assess the statistical significance of the differences in the mutual information using a t-test and find that both randomization of morphological classifications and shuffling of spatial distribution do not alter the mutual information in a statistically significant way. The non-zero mutual information between barredness and environment arises due to the finite and discrete nature of the dataset which can be entirely explained by mock Poisson distributions. We also separately compare the cumulative distribution functions of the barred and unbarred galaxies as a function of their local density. Using a Kolmogorov-Smirnov test, we find that the null hypothesis can not be rejected even at $75\%$ confidence level. Our analysis indicates that environments do not play a significant role in the formation of a bar, which is largely determined by the internal processes of the host galaxy.

astro-ph.GA↗
arXiv2020

Growth of matter fluctuations in $f(R,T)$ Gravity

In this work, I present for the first time the analysis concerning the growth of matter fluctuations in the framework of $f(R,T)$ modified gravity where I presume $f(R,T) = R + λT$, where $R$ denote the Ricci scalar, $T$ the trace of the energy-momentum tensor and $λ$ a constant. I first solve the Friedman equations assuming a dust universe ($ω=0$) for the Hubble parameter $H(z)$ and then employ it in the equation of matter density fluctuations $δ(z)$ to solve for $δ(z)$ and the growth rate $f(z)$. Next, I proceed to show the behavior of $f(z)$ and $δ(z)$ with redshift for some values of $λ$ with observational constraints. Finally, following the prescription of \cite{growft41}, I present an analytical expression for the growth index $γ$ which is redshift dependent and the expression reduces to $3/5$ for $λ=0$, which is the growth index for a dust universe.

gr-qc↗
arXiv2020

Configurational entropy in $f(T)$ gravity

The evolution of the configurational entropy of the universe relies on the growth rate of density fluctuations and on the Hubble parameter. In this work, I present the evolution of configurational entropy for the power-law $f(T)$ gravity model of the form $f(T) = ζ(-T)^ b$, where, $ζ= (6 H_{0}^{2})^{(1-s)}\frac{Ω_{P_{0}}}{2 s -1}$ and $b$ a free parameter. From the analysis, I report that the configurational entropy in $f(T)$ gravity is negative and decreases with increasing scale factor and therefore consistent with an accelerating universe. The decrease in configurational entropy is the highest when $b$ vanishes since the effect of dark energy is maximum when $b=0$. Additionally, I find that as the parameter $b$ increases, the growth rate, growing mode, and the matter density parameter evolve slowly whereas the Hubble parameter evolves rapidly. The rapid evolution of the Hubble parameter in conjunction with the growth rate for the $b=0$ may provide an explanation for the large dissipation of configurational entropy.

gr-qc↗
arXiv2020

Constraining $f(T,B)$ teleparallel gravity from energy conditions

$f(T,B)$ teleparallel gravity is a recently proposed straightforward generalization of the popular $f(T)$ teleparallel gravity by the incorporation of a boundary term $B=\frac{2}{e}\partial_{i}(e T ^{i}) = \bigtriangledown_{i}T^{i}$ where $T$ denote the torsion scalar \cite{ftb13}. In this work, I investigate the viability of some well motivated $f(T,B)$ teleparallel gravity models of the forms $f=αB^n+βT^m$, $f=αB^n T^m$ and $f=α\log (B)+βT$ where $α, β, n$ and $m$ are free parameters from the inequalities imposed the the weak energy condition. I use the recent estimates of Hubble, deceleration, jerk and snap parameters in finding corners in parameter spaces for the cosmological models for which the energy density remain positive and the weak energy condition ( i.e, $ρ+p \geq 0$, where $p$ and $ρ$ represent respectively the cosmological pressure and energy density) attains a minute positive value, as this implies the EoS parameter $ω= p/ρ\simeq-1$ and therefore consistent with an accelerating universe.

gr-qc↗
arXiv2020

Can a conditioning on stellar mass explain the mutual information between morphology and environment?

Recent studies with SDSS have shown that a statistically significant non-zero mutual information between morphology and environment persists up to several tens of Mpc, which awaits an explanation. Galaxies in different environments acquire their stellar mass through accretion and merger and the stellar mass function of galaxies is known to depend on both environment and morphology. Naturally, stellar mass can be an important link between morphology and environment which may explain the non-zero mutual information between the two. Measuring the mutual information between morphology and environment by conditioning the stellar mass would allow us to test this possibility. We employ here a volume and stellar mass limited sample from the $16^{th}$ data release (DR16) of the SDSS and find a non-zero conditional mutual information throughout the entire length scales probed. We compare the results with three different semi-analytic models implemented on the Millennium simulation and find their predictions to be in fairly good agreement with SDSS on smaller length scales ( $\lesssim 30 h^{-1}$ Mpc ), with a clear discrepancy observed at larger length scales ( $\gtrsim 30 h^{-1}$ Mpc ) where the models predict significantly lower conditional mutual information than the SDSS. Our analysis therefore suggests that only environmental and morphology dependence of stellar mass are inadequate in explaining the observed mutual information between morphology and environment and that physical processes which alters morphology may not necessarily have an impact on the stellar mass of galaxies and vice versa.

astro-ph.GA↗
arXiv2020

Late-Time Viscous Cosmology in $f(R,T)$ Gravity

The article communicates an alternative route to suffice the late-time acceleration considering a bulk viscous fluid with viscosity coefficient $ζ=ζ_{0}+ ζ_{1} H + ζ_{2} H^{2}$, where $ζ_{0}, ζ_{1}, ζ_{2}$ are constants in the framework of $f(R,T)$ modified gravity. We presume the $f(R,T)$ functional form to be $f=R+2αT$ where $α$ is a constant. We then solve the field equations for the Hubble Parameter and study the cosmological dynamics of kinematic variables such as deceleration, jerk, snap and lerk parameters as a function of cosmic time. We observe the deceleration parameter to be highly sensitive to $α$ and undergoes a signature flipping at around $t\sim 10$ Gyrs for $α=-0.179$ which is favored by observations. The EoS parameter for our model assumes values close to $-1$ at $t_{0}=13.7$Gyrs which is in remarkable agreement with the latest Planck measurements. Next, we study the evolution of energy conditions and find that our model violate the Strong Energy Condition in order to explain the late-time cosmic acceleration. To understand the nature of dark energy mimicked by the bulk viscous baryonic fluid, we perform some geometrical diagnostics like the $\{r,s\}$ and $\{r,q\}$ plane. We found the model to mimic the nature of a Chaplygin gas type dark energy model at early times while a Quintessence type in distant future. Finally, we study the violation of continuity equation for our model and show that in order to explain the cosmic acceleration at the present epoch, energy-momentum must violate.

gr-qc↗
arXiv2020

$f(Q,T)$ gravity models with observational constraints

The paper presents late time cosmology in $f(Q,T)$ gravity where the dark energy is purely geometric in nature. We start by employing a well motivated $f(Q,T)$ gravity model, $f(Q,T)=mQ^{n}+bT$ where $m,n$ and $b$ are model parameters. Additionally we also assume the universe to be dominated by pressure-less matter which yields a power law type scale factor of the form $% a(t)=c_{2}(At+c_{1})^{\frac{1}{A}}$, where $A=\dfrac{3(8π+b)}{n(16π+3b)% }$ and $c_{1}$ \& $c_{2}$ are just integration constants. To investigate the cosmological viability of the model, constraints on the model parameters were imposed from the updated 57 points of Hubble data sets and 580 points of union 2.1 compilation supernovae data sets. We have thoroughly investigated the nature of geometrical dark energy mimicked by the parametrization of $f(Q,T)=mQ^{n}+bT$ with the assistance of statefinder diagnostic in $\{s,r\}$ and $\{q,r\}$ planes and also performed the $Om$ -diagnostic analysis. The present analysis makes it clear-cut that $f(Q,T)$ gravity can be promising in addressing the current cosmic acceleration and therefore a suitable alternative to the dark energy problem. Further studies in other cosmological areas are therefore encouraging to further investigate the viability of $f(Q,T)$ gravity.

gr-qc↗
arXiv2020

Inflation in $f(R,T)$ Gravity

The article presents modeling of inflationary scenarios for the first time in the $f(R,T)$ theory of gravity. We assume the $f(R,T)$ functional from to be $R + ηT$, where $R$ denotes the Ricci scalar, $T$ the trace of the energy-momentum tensor and $η$ the model parameter (constant). We first investigated an inflationary scenario where the inflation is driven purely due to geometric effects outside of GR. We found the inflation observables to be independent of the number of e-foldings in this setup. The computed value of the spectral index is consistent with latest Planck 2018 dataset while the scalar to tensor ratio is a bit higher. We then proceeded to analyze the behavior of an inflation driven by $f(R,T)$ gravity coupled with a real scalar field. By taking the slow-roll approximation, we generated interesting scenarios where a Klein Gordon potential leads to observationally consistent inflation observables. Our results makes it clear-cut that in addition to the Ricci scalar and scalar fields, the trace of energy momentum tensor also play a major role in driving inflationary scenarios.

gr-qc↗
arXiv2020

Interacting Tsallis and Rényi Holographic Dark Energy with Hybrid Expansion Law

The manuscript presents the dynamics of Tsallis holographic dark energy (THDE) and Rényi holographic dark energy (RHDE) prescribed by a non-linear interaction in the FRW spacetime and for a scale factor evolving with a composite power law-exponential (hybrid) form. To construct the energy densities of these holographic dark energy models, I assume the Hubble cutoff to be the IR limit. I find that the deceleration parameter undergoes a signature flipping at a redshift $z$ consistent with observations. The EoS parameter $ω_{de}$ for both the HDE models exhibit quite contrasting dynamical behavior despite assuming values close to $-1$ at $z=0$ and therefore consistent with current observations. Next, I find the squared sound speed $c_{s}^{2}$ to be positive for the THDE model ensuring stability against perturbations, whereas for the RHDE model, $c_{s}^{2}<0$ implying instability against perturbations. Furthermore, I analyzed the evolutionary behavior of the EoS parameter of the HDE models by constructing the $ω_{de}-ω_{de}^{'}$ plane and find that the plane lies in the freezing region for the THDE model and in the thawing region for the RHDE model.

gr-qc↗