Kalyan Bhuyan

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• Doctor of Philosophy
• Professor (Full) at Dibrugarh University

This work explores the influence of viscous fluids on cosmological dynamics within the framework of General Relativity. We introduce a novel time-dependent parametrization for the bulk viscosity coefficient, given by \(\zeta = \zeta_0 (t - t_0)^{-2n} \rho^{1/2}\), where \(\zeta_0\), \(t_0\), and \(n\) are model parameters. This formulation is desig...

In this study, we investigated the effects of incorporating barotropic fluids on cosmological solutions within the general relativity (GR) framework. We proposed a modified version of the barotropic fluid with the EoS, $p=\zeta _0 \rho +\zeta _1 \rho \left(t-t_0\right){}^{-2 n}$, where $\zeta_0$, $\zeta_1$, $t_0$ and $n$ are some constants. Our goa...

We investigate photon geodesics in a static black hole spacetime sourced by nonlinear electrodynamics (NLED)--a model inspired by the confinement of heavy quark-antiquark pairs that recovers Maxwell's theory in the strong-field limit. This solution generalizes the Schwarzschild metric by introducing both an electric charge $Q$ and a nonlinear param...

Climate variability has significant impacts on agricultural productivity in the Northeastern region (NER), especially during the monsoon season. This study investigates the trends and implications of key climate parameters — maximum temperature (Tmax), minimum temperature (Tmin), rainfall anomaly index (RAI), net shortwave radiation flux (NSWRF), s...

We investigate the properties of the shadow cast by a black hole in a non-commutative spacetime surrounded by a plasma medium. Black holes in a plasma environment can exhibit complex interactions that profoundly affect their shadow properties. In specific terms, we consider a special type of plasma medium motivated from the interactions among axion...

Tropical Cyclones (TCs) are significant atmospheric phenomena in the earth’s lower atmosphere leading to interactions between various atmospheric layers as well as destroy lives and properties owing to associated extreme weather conditions. The densely populated coastal areas of India are worst affected by TCs attributed to the distinctive geo-clim...

The study of black hole shadows after the discovery of the M87 black hole by the Event Horizon Telescope (EHT) has gained major interest recently. In this work, we have studied the shadows of a renormalization group improved (RGI) black hole system surrounded by two types of plasma media, viz homogeneous and inhomogeneous plasma. We have found that...

The long‐term (2009–2019) TEC variation over a southern low latitude station Cocos Islands (12.2°S, 96.8°E) lying in edge of the Equatorial Ionization Anomaly (EIA) region and around the WN4 peak longitude are investigated and compared with its magnetic conjugate station Dibrugarh (27.5°N, 95°E). The causative mechanisms for the observed intra‐ and...

This book, titled Physics Frontier (Volume-I), serves as a comprehensive compilation of research articles presented at the national conferenece "Physics Frontiers-2024: Bridging Theories and Experiment",held on 2-4 July 2024 hosted by Bhawanipur Anchalik College.

In this paper, we have put forwarded a detailed investigation on the cosmic evolution of Bianchi type III model within the realm of Born-Infeld f(R) gravity executing the Palatini approach. Using a very eminent tool known as Dynamical System Approach (DSA), we have curtailed the complexity of the non linear field equations and study the dynamics fo...

In this paper, we explore the possibility of formation of a traversable wormhole in General Relativity supported by particle creation mechanism. The repulsive back-reaction pressure generated through this mechanism can be thought of as a source of sustaining a traversable wormhole. In the first part of this paper, we model a wormhole geometry by as...

The study of compact stellar systems in extended theories of gravity is crucial for understanding the behaviour and nature of the matter within these systems. We systematically study an anisotropic compact stellar model utilizing a Tolman–Kuchowicz spacetime configuration in the simplest form of f(R, T)-gravity where R is the Ricci scalar and T is...

In this paper, we investigate the quasinormal modes and chaotic phenomena associated with the Frolov black hole in the presence of a quintessence field. We investigated the effects of a quintessence field on quasinormal modes (QNMs) and the chaos bound of circular photon orbits. The obtained results on the QNM shows that increasing the magnitude of...

The Frolov black hole (BH) is a charged extension of the Hayward BH, having regularity at the central point $r = 0$ and an asymptotically Schwarzschild form for large values of $r$. Such a BH is parameterized by a length scale parameter, $\alpha_0$. In this paper, we analyze the thermodynamic properties, null and timelike geodesics, and shadows of...

In this work, we have studied how incorporating viscous fluids leads to exact bounce cosmological solutions in general relativity (GR) framework. Specifically, we propose a novel parameterization of bulk viscosity coefficient of the form [Formula: see text], where [Formula: see text], with [Formula: see text] being some positive constants and [Form...

Non-singular cosmological models, particularly the idea of emergent cosmology have been explored to describe the non-singular origin of the Universe. Recently, zero-point length cosmology has shown some positive insights into some non-singular aspects of the early Universe. In addition, topological defects are well-known for their major impact on t...

In this work, we have studied how incorporating viscous fluids leads to exact bounce cosmological solutions in general relativity (GR) framework. Specifically, we propose a novel parameterization of bulk viscosity coefficient of the form $\zeta = \zeta_0 (t-t_0)^{-2n} H$, where $\zeta_0$, $n$ being some positive constants and $t_0$ is the bounce ep...

In this work, we derive a novel black hole solution surrounded by a Dehnen-(1,4,0) type dark matter halo by embedding a Schwarzschild black hole within the halo, constituting a composite dark matter-black hole system. The thermodynamics of the resulting effective black hole spacetime is then studied with particular attention to the influence of the...

In this paper, in an FLRW background and a perfect fluid equation of state, we explore the possibility of the realization of an emergent scenario in a 4D regularized extension of Einstein-Gauss-Bonnet gravity, with the field equations particularly expressed in terms of scalar-tensor degrees of freedom. By assuming non-zero spatial curvature (k = ±...

In this paper, we have investigated the consequences of emergent cosmology in a magnetised Bianchi-VI spacetime inf(R,T)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$...

Tropospheric ozone is an air pollutant at the ground level and a greenhouse gas which significantly contributes to the global warming. Strong anthropogenic emissions in and around urban environments enhance surface ozone pollution impacting the human health and vegetation adversely. However, observations are often scarce and the factors driving ozo...

In this paper, we have investigated the possibility of deriving exact non-singular cosmological solutions in a combined scenario of bulk viscous fluid and symmetric teleparallel gravity (f (Q) gravity). Interestingly, we have found that the general solutions obtained from our models are flexible enough for demonstrating the emergent Universe scenar...

In this paper, we attempt to compare the Antarctic SSW of 2019 with the Arctic SSW of 2013. Despite being a minor SSW, the rare Antarctic event of September 2019 caused a significant enhancement of stratospheric temperature at high latitudes with more temperature deviation than the Arctic major SSW of 2013. On the basis of the duration and extent o...

In Saez–Ballester gravitational theory, we studied the role of particle creation and bulk viscosity in the evolution of a spatially homogeneous and anisotropic Bianchi type I universe. We treated particle creation and bulk viscosity as two distinct irreversible processes, and we modified the energy momentum tensor to account for viscous and particl...

Recently, scalar field approaches have been widely considered in cosmological models due to their potential for useful investigation of the cosmological evolution of the Universe. The use of a scalar field as a matter source in a cosmological model within symmetric teleparallel gravity is as interesting as many other modified gravity models sourced...

Hořava–Lifshitz (HL) gravity belongs to a class of modified gravity models capable of providing interesting possibilities from a cosmological perspective. In this paper, we obtain three exact solutions to the Friedmann equations corresponding to the HL model. Out of these three solutions, two of the solutions are physical, using which we studied th...

Northeast India (NEIN) is one of India's wettest areas. Due to its geographical location, this region is vulnerable to water-related calamities. Because of the heavy precipitation, NEIN is subject to severe natural disasters such as sporadic flooding, erosion, and so on. These natural disasters are common among the people of NEIN and act as a barri...

We have examined the performance of the International Reference Ionosphere (IRI 2016), International Reference Ionosphere extended to Plasmasphere (IRI-Plas 2017), and Sami2 is Another Model of the Ionosphere (SAMI2) models for long and deep solar minimum quiet time period around 100ºE by comparing the models with the total electron content (TEC) o...

In this paper, we have derived exact solutions to the Raychaudhuri equation in the presence of a bulk viscous fluid in f(R, T) modified gravity. We obtained the solutions for two distinct cases: the case of general relativity (GR) and non-GR. We have identified that these solutions can describe a non-singular inflationary Universe, i.e. the Emergen...

A quiet day empirical model of electron density (Ne) for the Indian equatorial zone at the altitude of 500 km is presented. The model is applicable to all levels of solar activity and is based on the observation that electron density in the F region of the Indian equatorial and low latitude ionosphere are best correlated with F10.7 cm solar flux at...

The Atmospheric Boundary Layer (ABL) has always played a critical role in determining global aerosol dispersion and distribution in the atmosphere. Absorbing aerosols constituting of black carbon (BC) influences our atmosphere in several ways which includes absorption of the incoming solar radiation. There have been several studies on the spatio-te...

Introduction Alcohol use disorder is a global problem and is associated with various physical, mental and social complications. Symptom network analysis can provide an unique insight into the complex interaction of the various factors as regards to their contribution to all the social and mental health-related issues and vice versa. Objective This...

In the case of a homogeneous and anisotropic Bianchi type I cosmological model, we analyse the consequences of a past geodesically complete emergent Universe scenario. We explore the development of the Universe as well as its anisotropies by analysing the cosmological parameters under an emergent scenario in a flat Bianchi type I background. We als...

Due to its geographical location, Northeast India (NEI) is prone to water-related disasters. The high precipitation in Assam, an NEI state, results in annual severe floods and delays in the region’s overall development. The purpose of this work is to create a flood susceptibility map for the flood-prone district of Dibrugarh in Assam using the anal...

We present a study of the major and minor SSWs that have occurred in the Northern Hemisphere (NH) over the past four decades (1981-2020) from a statistical perspective. This study investigates the frequency and timing of occurrence of major and minor SSWs during the specified period. A correlation is observed between the timing of the earliest mino...

Sudden stratospheric warmings (SSWs) are large and rapid temperature increases in the winter polar stratosphere associated with reversal of the climatological wintertime westerly winds. A comparative study of Major and Minor SSW events which occurred in the Northern Hemisphere has been carried out in this work. We have considered one major SSW and...

A systematic study on the Born–Infeld theory within the Palatini approach of f(R) gravity has been performed and the field equations are investigated considering the Hu–Sawicki model. We study the dynamics of the system in the background of isotropic Friedmann–Leimaître–Robertson–Walker (FLRW) model as well as anisotropic Bianchi I and V models. A...

Northeast India (NEIN) is one of India's wettest areas. Due to its geographical location, this region is vulnerable to water-related calamities. Because of the heavy precipitation, NEIN is subject to severe natural disasters such as sporadic flooding, erosion, and so on. These natural disasters are common among the people of NEIN and act as a barri...

Copernicus Atmosphere Monitoring Service (CAMS) data are used to evaluate the impact of the lockdown (24 March-3 May 2020) on the concentrations of surface O 3 , NO x , CO, SO 2 , PM and BC compared to those measured during the same period in 2015-2019 over northeast India and adjoining areas. Measurements made at Dibrugarh complements the CAMS obs...

Gravity waves are primarily generated in the lower atmosphere and can propagate up to thermospheric heights. In recent years, atmospheric gravity waves have been a subject of research because of their dynamical and thermodynamical importance. This chapter includes various observational and modeling techniques, and recent progress in understanding t...

The geomagnetic storm is an important weather issue in the earth’s ionosphere-magnetosphere system. Many linear and nonlinear systems are involved in this earth-space environment. In order to understand the nonlinearly evolving dynamical system of magnetosphere and ionosphere, Time series analysis of foF2 data, Disturbance Storm Index Dst, Geomagne...

Alcohol use disorders in adolescents has been a major health and social concern across the globe demanding timely intervention in order to prevent the great deal of morbidity and mortality that are often associated with maladaptive pattern of substance use. Understanding the causation, maintenance and progression of alcohol use disorder is of utmos...

The present study is aimed at discerning the ionospheric response to great geomagnetic storms (GGS) (Dst≤−200 nT) during solar cycles 19-23 along with their dependence on solar activity. The statistical analysis of the effect of geomagnetic storms on the F2 layer has been performed here by calculating the normalized deviation ΔfoF2 of the critical...

The main objective of this work is the pursuit of a detailed analysis of the cosmological solutions corresponding to each fixed point belonging to an exclusive form of f(R) initially introduced by Hu and Sawicki in the background of Palatini f(R) gravity. Fixed points and their associated solutions for the anisotropic Bianchi type I and V models ar...

We investigate the dynamics of a number of f(R) theories in the background of the spatially homogeneous as well as isotropic Friedmann-Leimaître-Robertson-Walker (FLRW) model and anisotropic Bianchi type I and V models in Palatini formalism using the dynamical systems approach. Considering theories of the type f(R)=R+γRm−β/Rn, it is examined whethe...

The cosmological dynamics of spatially homogeneous but anisotropic Bianchi type-III space-time is investigated in presence of a perfect fluid within the framework of Hu–Sawicki model. We use the dynamical system approach to perform a detailed analysis of the cosmological behaviour of this model for the model parameters \(n=1, c_1=1\), determining a...

We apply the dynamical systems approach to investigate the spatially homogeneous and anisotropic Bianchi type V models for the Palatini version of f(R) gravity. In particular, we examine the existence of equilibrium points along with their exact solutions and stability properties for two different forms of f(R). Moreover, the evolution of shear and...

The statistical analysis of geomagnetic storms on the F2 layer has been emphasized here by calculating the normalized deviation DfoF2 of the critical frequency of F2 layer foF2 in the two ionosonde station of Indian region i.e Ahmedabad (LAT→23.0˚, LONG→72.6˚) and Bombay (LAT→19.0˚, LONG→72.8˚) during the rise of Solar Cycle 19 to perceive certain...

We derive exact solutions for anisotropic Bianchi type-III cosmological model in the Palatini formalism of \(f (R)\) gravity using Dynamical System Approach. For the \(f (R)\) of the form \(f(R) =R-\beta /R^{n} \) and \(f(R) =R+\alpha R^{m} \), we have found the fixed points describing the radiation-dominated, matter dominated and de Sitter evoluti...

We study the cosmological dynamics for Rp exp(λR) gravity theory in the metric formalism, using dynamical systems approach. Considering higher-dimensional FRW geometries in case of an imperfect fluid which has two different scale factors in the normal and extra dimensions, we find the exact solutions, and study its behaviour and stability for both...

A detailed analysis of the dynamics of homogeneous and anisotropic Bianchi I geometries has been performed in f(R) gravity theory in the Palatini formalism, using dynamical systems approach. The exact solutions have been found and the behavior and stability of these solutions have been studied for three different models based on f(R) gravity. These...

We investigate the cosmological dynamics of homogeneous and anisotropic Bianchi I geometries in exp(λR) gravity. We find the exact solutions and study the behavior and stability of these solutions for both vacuum and matter cases. It is found that stable solutions exist corresponding to accelerated expansion at late times, which can describe the in...

The effect of geomagnetic storms on the F2 region was studied by calculating the deviation, ΔfoF2, of foF2 during 40 magnetic storms, ranging from moderate (Dst < −50 nT) to very intense (Dst < −200 nT) of the 21st solar cycle. In order to study the variation of storm-time foF2 with latitude, season and storm strength, ionosonde data were obtained...

All the geomagnetic storms (dst<-50nt) for a period of 55 yrs (1957-2012) were identified by examining the monthly dst index and their seasonal variation studied. The seasonal variation clearly showed a equinoxial maxima indicating enhanced geomagnetic activity during the equinoxes. In addition to the equinoxial maxima, geomagnetic activity was fou...

Measurement of black carbon aerosol mass concentration have been made in Dibrugarh (94.6°E, 27.3°N, 111 m amsl), Assam using a seven channel Aethalometer for the period May, 2007-November, 2010. The diurnal variation of black carbon mass concentration (MBC) shows a primary peak around 2000-2200 hrs (local time) and a less prominent secondary maximu...

Latitudinal and seasonal variability of total columnar ozone from September 2007 to August 2008 across the Indian longitude sector within 10.5° N to 34.5° N and 70.5° E to 94.5° E using satellite data obtained from Aura Ozone Monitoring Instrument (OMI) of National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) is presente...

Columnar aerosol properties retrieved from Multi-Wavelength solar Radiometer (MWR) measurements during the period 2001e2010 over Dibrugarh (27.3�N, 94.6�E, 111 m amsl), North-Eastern India are analyzed to identify the types of aerosols in the atmospheric column. Highest Aerosol optical depth (AOD) characterizes the pre-monsoon (MarcheMay), while lo...

For the first time, micro-meteorological measurements were made at Dibrugarh (27.45N;94.92E) in the north-eastern Indian region by installing a Mini Boundary Layer Mast (MBLM, 15 m high) instrumented with slow sensors for studying the near surface response to the Total Solar Eclipse (TSE) of 22 July, 2009. The duration of the eclipse was for 02 h 0...

The total and size segregated near surface aerosol mass concentrations obtained using a Quartz Crystal Microbalance (QCM) impactor over Dibrugarh (27.3°N, 94.6°E, 111 m amsl), in the extreme northeastern part of India, during the period 2007 to 2009 are analyzed with concurrent measurements of columnar spectral aerosol optical depths (AOD) to infer...

Measurements of aerosol optical depth (AOD) and mass concentration of composite and black carbon (BC) aerosols made with collocated instruments over Dibrugarh in Northeast India are used to estimate the aerosol radiative forcing for the period June 2008 to May 2009. AOD shows seasonal variation with maximum in premonsoon (0.69 ± 0.13 at 500 nm in M...

Mass concentrations within the well-mixed atmospheric boundary layer (ABL) showed systematic temporal variations over Dibrugarh, a tropical rural location. Month to month variation of aerosol mass concentration shows prominent contribution of nucleation and accumulation mode aerosols to composite aerosol mass concentration. Maximum and minimum valu...

Long term diurnal and seasonal variations of aerosol optical depth are studied using data from a Multiwavelength Solar Radiometer for 2001 to 2010 over Dibrugarh (27.3°N, 94.6°E) and results are presented in this study. The seasonal mean AOD for full day observation is highest during the pre monsoon season and lowest in the retreating monsoon for a...

In this study, we investigate the characteristics of the total columnar ozone (TCO) over Dibrugarh (27.3° N and 94.6° E), India for the period September 2007 to February 2009 using Microtops II Ozonometer. These measurements are compared with TCO data available from Total Ozone Mapping Spectrometer (TOMS) on board Nimbus-7 satellite for the same pe...

Optical and physical properties of columnar and near surface aerosols studied using measurements made with a Multi Wavelength Radiometer and a QCM Impactor respectively over Dibrugarh (27.3°N, 94.6°E) for the period October 2007 to November 2009 are presented in this study. Results indicate that the seasonal mean AOD is highest during the pre monso...

Characteristics of columnar aerosols measured using a Multiwavelength Solar Radiometer from period 2001 to 2010 and that of near surface aerosols measured using a Quartz Crystal Microbalance Impactor for the period 2007- 2010 and an Aethalometer for 2008- 2010 over Dibrugarh (27.27°N, 94.54°E) have been investigated and results are presented in thi...

Electron density measured by the Indian satellite SROSS C2 at the altitude of ∼500 km in the 75°E longitude sector for the ascending half of the solar cycle 22 from 1995 to 1999 are used to study the position and density of the equatorial ionization anomaly (EIA). Results show that the latitudinal position and peak electron density of the EIA crest...

The electron density and temperature distribution of the equatorial and low latitude ionosphere in the Indian sector has been investigated by simultaneously solving the continuity, momentum and energy balance equations of ion and electron flux along geomagnetic field lines from the Northern to the Southern hemisphere. Model algorithm is presented a...

The Global Positioning System Satellite (GPS) provides an opportunity for studying lonospheric total electron content (TEC). We used GPS TEC data collected at 16 stations across India since 2003 to obtain correlation distance of ionospheric total electron content in the Indian low latitude ionosphere. The correlation coefficients at each local time...

In recent years, radio ray tomography of the ionosphere has developed into a very useful technique for the study of the ionosphere. Compact generalized singular value decomposition (CGSVD) is used to study the feasibility of using the International Reference Ionosphere (IRI) model as a regularization profile for ionospheric tomography experiments....