Searcharxiv⌕ Search

arXiv subjects

Himanshu Chaudhary

Publications and source records attributed to Himanshu Chaudhary.

At least 55 records · Page 3Linked to original sources
arXiv2024

The estimation of parameters of generalized cosmic Chaplygin gas and viscous modified Chaplygin gas and Accretions around Black Hole in the background of Einstein-Aether gravity

In this paper, we have investigated the phenomenon of accelerated cosmic expansion in the late universe and the mass accretion process of a 4-dimensional Einstein-Aether black hole. Starting with the basics of Einstein-Aether gravity theory, we have first considered the field equations and two eminent models of Chaplygin gas, viz. generalized cosmic Chaplygin gas model and viscous modified Chaplygin gas model. Then, we obtained the energy density and Hubble parameters equations for these models in terms of some dimensionless density parameters and some unknown parameters. After finding the required parameters, we proceeded with the mass accretion process. For both models, we obtained the equation of mass in terms of the redshift function and represented the change of mass of the black hole graphically with redshift. At the same time, we have made a graphical comparison between the above-mentioned models and the $Λ$CDM model of the universe. Eventually, we have concluded that the mass of a 4-dimensional black hole will increase along the universe's evolution in the backdrop of Einstein-Aether gravity.

gr-qc↗
arXiv2024

Cosmological Test of Dark Energy Parametrizations in Horava-Lifshitz Gravity

In this work, we assume the FRLW Universe which is filled with dark matter along with dark energy in the framework of Horava-Lifshitz (HL) gravity. The dark energy is considered as the Linear (Model I) and CPL (Model II) parametrizations of the equation of state parameter. For both models, we express the Hubble parameter $H(z)$ in terms of the model parameters and redshift $z$. To rigorously constrain the model, we have employed a comprehensive set of recent observational datasets including cosmic chronometers (CC), Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), Gamma-ray Burst (GRB), Quasar (Q) and Cosmic Microwave Background Radiation (CMB). Through the joint analysis of this diverse collection of datasets, we have achieved tighter constraints on the model's parameters. This, in turn, allows us to delve into both the physical and geometrical aspects of the model with greater precision. Furthermore, our analysis has enabled us to determine the present values of crucial cosmological parameters, including $H_{0}$, $Ω_{m0}$, $Ω_{k0}$ and $Ω_{\Lambda0}$. It's noteworthy that our results are consistent with recent findings from Planck 2018, underscoring the reliability and relevance of our models in the current cosmological context. We also conduct analysis of cosmographic parameters and apply statefinder and diagnostic tests to explore the evolution of the Universe. In addition, the AIC and BIC suggest that the $Λ$CDM model is the preferred model among all our considered models. Our investigation into the models has unveiled intriguing features of the late universe

gr-qc↗
arXiv2024

Constraints on the parameters of modified Chaplygin-Jacobi and modified Chaplygin-Abel gases in $f(T)$ gravity

In this study, we investigate two dark energy models, MCJG and MCAG, in the context of $f(T)$ gravity within a non-flat FLRW Universe. Our analysis considers radiation, dark matter, and dark energy components. We compare the equation of state for MCJG and MCAG with $f(T)$ gravity. Using recent astronomical data (e.g., $H(z)$, type Ia supernovae, Gamma Ray Bursts, quasars, and BAO), we constrain the models' parameters and explore the Universe's behavior. The reduced Hubble parameter is expressed in terms of observable parameters like $Ω_{r0}$, $Ω_{m0}$, $Ω_{k0}$, $Ω_{CJ0}$, $Ω_{CA0}$, and $H_0$. We investigate cosmic evolution using deceleration, $\mathrm{Om}$, and statefinder diagnostics. Information criteria are employed to assess model viability, comparing against the standard $Λ$CDM model. Our objective is to deepen our understanding of dark energy, its relation to $f(T)$ gravity, and the mechanisms governing the accelerated expansion of the Universe.

gr-qc↗
arXiv2023

A study on matter accretion onto charged black hole solution in metric-affine gravity

This study deals with astrophysical accretion onto the charged black hole solution, which is sourced by the dilation, spin, and shear charge of matter in metric affine gravity. The metric affine gravity defines the link between torsion and nonmetricity in space-time geometry. In the current analysis, we study the accretion process of various perfect fluids that are accreting near the charged black hole in the framework of metric affine gravity. Within the domain of accretion, multiple fluids have been examined depending on the value of $f_1$. The ultra-stiff, ultra-relativistic, and sub-relativistic fluids are considered to discuss the accretion. In the framework of equations of state, we consider isothermal fluids for this investigation. Further, we explore the effect of polytropic test fluid in relation to accretion discs, and it is presented in phase diagrams. Some important aspects of the accretion process are investigated. Analyzing the accretion rate close to a charged black hole solution, typical behavior is created and discussed graphically.

gr-qc↗
arXiv2023

Joule-Thomson expansion and tidal force effects of AdS black holes surrounded by Chaplygin dark fluid

This study examines a recently hypothesized black hole. We study the Joule-Thomson coefficient, the inversion temperature and also the isenthalpic curves in the $T_i -P_i$ plane. A comparison is made between the Van der Waals fluid and the black hole to study their similarities and differences. The Joule-Thomson coefficient, the inversion curves and the isenthalpic curves are discussed inAdS black holes surrounded by Chaplygin dark fluid. In $T -P$ plane, the inversion temperature curves and isenthalpic curves are obtained with different parameters. Next, we explore the radial timelike geodesics that leads us to explore the tidal force effects for a radially in-falling particle in such black hole spacetime. We also numerically solve the geodesic deviation equation for two nearby radial geodesics for a freely falling particle. Our analysis shows that contrary to the Schwarzschild spacetime, the tidal forces don't become zero at spatial infinity due to the lack of asymptotic flatness because of the presence of a non-zero cosmological constant. The geodesic separation profile shows an oscillating trend and depends on the dynamic spacetime parameters $q, B$ and $Λ$.

gr-qc↗
arXiv2023

Cosmic evolution in $f(Q,T)$ gravity: Exploring a higher-order time-dependent function of deceleration parameter with observational constraints

In this research paper, we explore a well-motivated parametrization of the time-dependent deceleration parameter, characterized by a cubic form, within the context of late time cosmic acceleration. The current analysis is based on the $f(Q,T)$ gravity theory, by considering the background metric as the homogeneous and isotropic Friedmann Lemaître Robertson Walker (FLRW) metric. Investigating the model reveals intriguing features of the late universe. To constrain the model, we use the recent observational datasets, including cosmic chronometer (CC), Supernovae (SNIa), Baryon Acoustic Oscillation (BAO), Cosmic Microwave Background Radiation (CMB), Gamma Ray Burst (GRB), and Quasar (Q) datasets. The joint analysis of these datasets results in tighter constraints for the model parameters, enabling us to discuss both the physical and geometrical aspects of the model. Moreover, we determine the present values of the deceleration parameter ($q_0$), the Hubble parameter ($H_0$), and the transition redshift ($z_t$) from deceleration to acceleration ensuring consistency with some recent results of Planck 2018. Our statistical analysis yields highly improved results, surpassing those obtained in previous investigations. Overall, this study presents valuable insights into the higher order $q(t)$ model and its implications for late-time cosmic acceleration, shedding light on the nature of the late universe.

gr-qc↗
arXiv2023

Strong Gravitational Lensing by Sgr A* and M87* Black Holes embedded in Dark Matter Halo exhibiting string cloud and quintessential field

We investigate the strong gravitational lensing phenomena caused by a black hole with a dark matter halo. In this study, we examine strong gravitational lensing with two significant dark matter models: the universal rotation curve model and the cold dark matter model. To do this, we first numerically estimate the strong lensing coefficients and strong deflection angles for both the universal rotation curve and cold dark matter models. It is observed that the deflection angle, denoted as $α_D$, increases with the parameter $α$ while holding the value of $γ\cdot 2M$ constant. Additionally, it increases with the parameter $γ\cdot 2M$ while keeping the value of $α$ constant. The strong deflection angle, $α_D$, for the black hole with a dark matter halo, with parameters $α=0$ and $γ\cdot 2M$, greatly enhances the gravitational bending effect and surpasses the corresponding case of the standard Schwarzschild black hole ($A=0, B=0, α=0, γ\cdot 2M=0$). Furthermore, we investigate the astrophysical consequences through strong gravitational lensing observations, using examples of various supermassive black holes such as $M87^{*}$ and $SgrA^{*}$ located at the centers of several galaxies. It is observed that black holes with dark matter halos can be quantitatively distinguished and characterized from the standard Schwarzschild black hole ($A=0, B=0, α=0, γ\cdot 2M=0$). The findings in our analysis suggest that observational tests for black holes influenced by dark matter halos are indeed feasible and viable.

gr-qc↗
arXiv2023

Exploring Tidal Force Effects and Shadow Constraints for Schwarzschild-like Black Hole in Starobinsky-Bel-Robinson Gravity

The current manuscript deals with the tidal force effects, geodesic deviation, and shadow constraints of the Schwarzschild-like black hole theorised in Starobinsky-Bel-Robinson gravity exhibiting M-theory compactification. In the current analysis, we explore the radial and angular tidal force effects on a radially in-falling particle by the central black hole, which is located in this spacetime. We also numerically solve the geodesic deviation equation and study the variation of the geodesic separation vector with the radial coordinate for two nearby geodesics using suitable initial conditions. All the obtained results are tested for Sag A* and M87* by constraining the value of the stringy gravity parameter $β$ using the shadow data from the event horizon telescope observations. All the results are compared with Schwarzschild black hole spacetime. In our study, we found that both the radial and angular tidal forces experienced by a particle switch their initial behaviour and turn compressive and stretching, respectively, before reaching the event horizon. The geodesic deviation shows an oscillating trend as well for the chosen initial condition. For the constrained value of $β$, we see that the spacetime geometry generated by Sag A* and M87* is effectively same for both Schwarzschild and Starobinsky-Bel-Robinson black hole. Furthermore, we also calculated the angular diameter of the shadow in Starobinsky-Bel-Robinson black hole and compared with the Schwarzschild black hole. It is observed that the angular diameter of shadow for M87* and Sgr A* in Starobinsky-Bel-Robinson black hole is smaller than the Schwarzschild black hole. The calculated results satisfy the event horizon telescope observational constraints. Finally, we have concluding remarks.

gr-qc↗
arXiv2023

Exploring Late-Time Cosmic Acceleration with EoS Parameterizations in Horava-Lifshitz Gravity via Baryon Acoustic Oscillations

In our study, we have adopted the framework of Horava-Lifshitz gravity to model the Universe's dark matter and dark energy components. Specifically, we have considered two recent parametrizations for dark energy models: the CBDRM-type and CADMM-type parameterizations. In our analysis, we have explicitly expressed the Hubble parameter, denoted as $H(z)$, for these two distinct dark energy models. By doing so, we have aimed to investigate and quantify the accelerated cosmic expansion rate characterizing the late-time Universe. Our study uses a wide range of datasets. This dataset consists of recent measurements of baryon acoustic oscillations (BAO) collected over a period of twenty years with the Cosmic Chronometers (CC) dataset, Type Ia supernovae (SNIa) dataset, the Hubble diagram of gamma-ray bursts (GRBs), quasars (Q), and the latest measurement of the Hubble constant (R22). Consequently, we present a crucial aspect of our study by plotting the $r_{d}$ vs. $H_{0}$ plane. In the context of the $Λ$CDM model, after incorporating all the datasets, including the R22 prior, we obtain the following results: $H_{0}$ = $71.674089 \pm 0.734089$ $km s^{-1} Mpc^{-1}$ and $r_d = 143.050380 Mpc \pm 3.702038$. For the CBDRM model, we find $H_{0}$ = $72.355058 \pm 1.004604$ $km s^{-1} Mpc^{-1}$ and $r_d = 144.835069 Mpc \pm 2.378848$. In the case of the CADMM model, our analysis yields $H_{0}$ = $72.347804 \pm 0.923328$ $km s^{-1} Mpc^{-1}$ and $r_d = 144.466836 Mpc \pm 4.288758$. We have conducted cosmographic analyses for both of the proposed parameterizations in comparison to the $Λ$CDM paradigm. Additionally, we have applied Diagnostic tests to investigate the evolution of both models. Finally, the Information Criteria test demonstrates that the $Λ$CDM model emerges as the preferred choice among the models we have considered.

astro-ph.CO↗
arXiv2023

Constraints on the Parameterized Deceleration Parameter in FLRW Universe

Confirmation of accelerated expansion of the universe probed the concept of dark energy theory, and since then, numerous models have been introduced to explain its origin and nature. The present work is based on reconstructing dark energy by parametrization of the deceleration parameter in the FRW universe filled with radiation, dark matter, and dark energy. We have chosen some well-motivated parametrized models 1-3 in an attempt to investigate the energy density in terms of deceleration parameters by estimating the cosmological parameters with the help of different observational datasets. Also, we have introduced a new model 4 for the parametrization of the deceleration parameter. Then we analyzed the cosmography parameters using the best-fit values of the parameters. Using the information criteria, we have examined the viability of the models.

gr-qc↗
arXiv2023

Observational Constraints and Cosmological Implications of Scalar-Tensor $f(R, T)$ Gravity

Recently, the scalar-tensor representation of $f (R,T)$ gravity was used to explore gravitationally induced particle production/annihilation. Using the framework of irreversible thermodynamics of open systems in the presence of matter creation/annihilation, the physical and cosmological consequences of this setup were investigated in detail. In this paper, we test observationally the scalar-tensor representation of $f(R,T)$ gravity in the context of the aforementioned framework, using the Hubble and Pantheon+ measurements. The best fit parameters are obtained by solving numerically the modified Friedmann equations of two distinct cosmological models in scalar tensor $f(R, T)$ gravity, corresponding to two different choices of the potential, and by performing a Markov Chain Monte Carlo analysis. The best parameters are used to compute the cosmographic parameters, i.e., the deceleration, the jerk and the snap parameters. Using the output resulting from the Markov Chain Monte Carlo analysis, the cosmological evolution of the creation pressure and of the matter creation rates are presented for both models. To figure out the statistical significance of the studied scalar-tensor $f(R,T)$ gravity, the Bayesian and the corrected Akaike information criteria are used. The latter indicates that the first considered model in scalar tensor $f(R,T)$ gravity is statistically better than $Λ$CDM, i.e., it is more favored by observations. Besides, a continuous particle creation process is present in Model 1. On the other hand, for large redshifts, in Model 2 the particle creation rate may become negative, thus indicating the presence of particle annihilation processes. However, both models lead to an accelerating expansion of the Universe at late times, with a deceleration parameter equivalent to that of the $Λ$CDM model.

gr-qc↗
arXiv2023

Data Analysis of three parameter models of deceleration parameter in FRW Universe

Constraining the dark energy deceleration parameter is one of the fascinating topics in the recent cosmological paradigm. This work aims to reconstruct the dark energy using parametrization of the deceleration parameter in a flat FRW universe filled with radiation, dark energy, and pressure-less dark matter. Thus, we have considered four well-motivated parameterizations of q(z), which can provide the evolution scenario from the deceleration to acceleration phase of the Universe. We have evaluated the expression of the corresponding Hubble parameter of each parametrization by imposing it into the Friedmann equation. We have constrained the model parameter through H(z), Pantheon, and baryons acoustic oscillation (BOA) data. Next, we have estimated the best-fit values of the model parameters by using Monte Carlo Markov Chain (MCMC) technique and implementing H(z)+ BAO+SNe-Ia dataset. Then we analyzed the cosmographic parameter, such as deceleration, jerk, and snap parameters, graphically by employing the best-fit values of the model parameter. Moreover, we have analyzed statefinder and Om diagnostics parameters for each scenario to discriminate various dark energy models. Using the information criteria, the viability of the models have examined. In the end, we have analogized our outcomes with the standard ΛCDM model to examine the viability of our models

gr-qc↗
arXiv2023

Model-independent study for a quintessence model of dark energy: Analysis and Observational constraints

In this paper, a well-motivated parametrization of the Hubble parameter ($H$% ) is revisited that renders two models of dark energy showing some intriguing features of the late-time accelerating Universe. A general quintessence field is considered as a source of dark energy. We have obtained tighter constraints using recently updated cosmic observational datasets for the considered models. The two models described here show a nice fit to the considered uncorrelated Hubble datasets, Standard candles, Gamma Ray Bursts, Quasars, and uncorrelated Baryonic Acoustic Oscillations datasets. Using the constrained values of the model parameters, we have discussed some features of the late-time accelerating models and obtained the present value of the deceleration parameter ($q_{0}$), the present value of the Hubble parameter ($H_{0}$) and the transition redshift ($z_{t}$) from deceleration to acceleration. The current value of the deceleration parameter for both models is consistent with the Planck 2018 results. The evolution of the geometrical and physical parameters is discussed through graphical representations for both models with some diagnostic analysis. The statistical analysis performed here shows greater results and overall, the outcomes of this investigation are superior to those previously found.

gr-qc↗
arXiv2023

Cosmological tests of the osculating Barthel-Kropina dark energy model

We further investigate the dark energy model based on the Finsler geometry inspired osculating Barthel-Kropina cosmology. The Barthel-Kropina cosmological approach is based on the introduction of a Barthel connection in an osculating Finsler geometry, with the connection having the property that it is the Levi-Civita connection of a Riemannian metric. From the generalized Friedmann equations of the Barthel-Kropina model, obtained by assuming that the background Riemannian metric is of the Friedmann-Lemaitre-Robertson-Walker type, an effective geometric dark energy component can be generated, with the effective, geometric type pressure, satisfying a linear barotropic type equation of state. The cosmological tests, and comparisons with observational data of this dark energy model are considered in detail. To constrain the Barthel-Kropina model parameters, and the parameter of the equation of state, we use 57 Hubble data points, and the Pantheon Supernovae Type Ia data sample. The st statistical analysis is performed by using Markov Chain Monte Carlo (MCMC) simulations. A detailed comparison with the standard $Λ$CDM model is also performed, with the Akaike information criterion (AIC), and the Bayesian information criterion (BIC) used as the two model selection tools. The statefinder diagnostics consisting of jerk and snap parameters, and the $Om(z)$ diagnostics are also considered for the comparative study of the Barthel-Kropina and $Λ$CDM cosmologies. Our results indicate that the Barthel-Kropina dark energy model gives a good description of the observational data, and thus it can be considered a viable alternative of the $Λ$CDM model.

gr-qc↗
arXiv2022

FairGen: Fair Synthetic Data Generation

With the rising adoption of Machine Learning across the domains like banking, pharmaceutical, ed-tech, etc, it has become utmost important to adopt responsible AI methods to ensure models are not unfairly discriminating against any group. Given the lack of clean training data, generative adversarial techniques are preferred to generate synthetic data with several state-of-the-art architectures readily available across various domains from unstructured data such as text, images to structured datasets modelling fraud detection and many more. These techniques overcome several challenges such as class imbalance, limited training data, restricted access to data due to privacy issues. Existing work focusing on generating fair data either works for a certain GAN architecture or is very difficult to tune across the GANs. In this paper, we propose a pipeline to generate fairer synthetic data independent of the GAN architecture. The proposed paper utilizes a pre-processing algorithm to identify and remove bias inducing samples. In particular, we claim that while generating synthetic data most GANs amplify bias present in the training data but by removing these bias inducing samples, GANs essentially focuses more on real informative samples. Our experimental evaluation on two open-source datasets demonstrates how the proposed pipeline is generating fair data along with improved performance in some cases.

cs.LG↗
arXiv2022

Simulating magnetized neutron stars with discontinuous Galerkin methods

Discontinuous Galerkin methods are popular because they can achieve high order where the solution is smooth, because they can capture shocks while needing only nearest-neighbor communication, and because they are relatively easy to formulate on complex meshes. We perform a detailed comparison of various limiting strategies presented in the literature applied to the equations of general relativistic magnetohydrodynamics. We compare the standard minmod/$ΛΠ^N$ limiter, the hierarchical limiter of Krivodonova, the simple WENO limiter, the HWENO limiter, and a discontinuous Galerkin-finite-difference hybrid method. The ultimate goal is to understand what limiting strategies are able to robustly simulate magnetized TOV stars without any fine-tuning of parameters. Among the limiters explored here, the only limiting strategy we can endorse is a discontinuous Galerkin-finite-difference hybrid method.

gr-qc↗
arXiv2021

A Software Simulator for Noisy Quantum Circuits

We have developed a software library that simulates noisy quantum logic circuits. We represent quantum states by their density matrices in the Pauli basis, and incorporate possible errors in initialisation, logic gates, memory and measurement using simple models. Our quantum simulator is implemented as a new backend on IBM's open-source Qiskit platform. In this document, we provide its description, and illustrate it with some simple examples.

quant-ph↗
arXiv2020

Cosmic Censorship of Trans-Planckian Field Ranges in Gravitational Collapse

A classical solution where the (scalar) field value moves by an ${\cal O}(1)$ range in Planck units is believed to signal the breakdown of Effective Field Theory (EFT). One heuristic argument for this is that such a field will have enough energy to be inside its own Schwarzschild radius, and will result in collapse. In this paper, we consider an inverse problem: what kind of field ranges arise during the gravitational collapse of a classical field? Despite the fact that collapse has been studied for almost a hundred years, most of the discussion is phrased in terms of fluid stress tensors, and not fields. An exception is the scalar collapse made famous by Choptuik. We re-consider Choptuik-like systems, but with the emphasis now on the evolution of the scalar. We give strong evidence that generic spherically symmetric collapse of a massless scalar field leads to super-Planckian field movement. But we also note that in every such supercritical collapse scenario, the large field range is hidden behind an apparent horizon. We also discuss how the familiar perfect fluid models for collapse like Oppenheimer-Snyder and Vaidya should be viewed in light of our results.

hep-th↗