arXiv subjects

C. P. Singh

Publications and source records attributed to C. P. Singh.

At least 19 recordsLinked to original sources

arXiv2024

Exploring interacting bulk viscous model with decaying vacuum density

In the present work, we study a cosmological model composed of a viscous dark matter interacting with decaying vacuum energy in a spatially flat Universe. In the first part, we find the analytical solution of different cosmological parameters by assuming the physically viable forms of bulk viscosity and decaying vacuum density with the interaction term. The second part is dedicated to constrain the free parameters of the interacting viscous model with decaying vacuum energy by employing latest observational data of $Pantheon+$, Cosmic Chronometer and $f(z)σ_{8}(z)$. We find that the interacting model just deviate very slightly from well-known concordance $Λ$CDM model and can alleviate effectively the current $H_0$ tension between local measurement by R21 and global measurement by Planck 2018, and the excess in the mass fluctuation amplitude $σ_{8}$ essentially vanish in this context. We report the Hubble constants as $H_0=72.150^{+0.989}_{-0.779}$, and $ 72.202^{+0.796}_{-0.937}$ \;$km s^{-1} Mpc^{-1}$, deceleration parameters as $q_0=-0.533 \pm 0.024$, and $-0.531 \pm 0.024$, and equation of state parameters as $w_0=-0.689 \pm 0.016$, and $ -0.687 \pm 0.016$ for $Λ$CDM and interacting models, respectively. It is found that the interacting model is in good agreement with $Λ$CDM. Further, we discuss the amplitude of matter power spectrum $σ_8$ and its associated parameter $S_8$ using $f(z)σ_8(z)$ data. Finally, the information selection criterion and Bayesian inference are discussed to distinguish the interacting model with $Λ$CDM model.

astro-ph.CO↗

arXiv2024

Viscous fluid dynamics with decaying vacuum energy density

In this work, we investigate the dynamics of bulk viscous models with decaying vacuum energy density (VED) in a spatially homogeneous and isotropic flat Friedmann-Lemaître- Robertson-walker (FLRW) spacetime. We particularly are interested to study the viscous model which considers first order deviation from equilibrium, i.e., the Eckart theory. In the first part, using the different forms of the bulk viscous coefficient, we find the main cosmological parameters, like Hubble parameter, scale factor, deceleration parameter and equation of state parameter analytically. We discuss some cosmological consequences of the evolutions and dynamics of the different viscous models with decaying VED. We examine the linear perturbation growth in the context of the bulk viscous model with decaying VED to see if it survives this further level of scrutiny. The second part of the work is devoted to constrain the viscous model of the form $ζ\propto H$, where $ζ$ is the bulk viscous coefficient and $H$ is the Hubble parameter, using three different combinations of data from type Ia supernovae (Pantheon), $H(z)$ (cosmic chronometers), Baryon Acoustic Oscillation and $f(z)σ_8(z)$ measurements with Markov Chain Monte Carlo (MCMC) method. We show that the considered model is compatible with the cosmological probes, and the $Λ$CDM recovered in late-time of the evolution of the Universe. Finally, we obtain selection information criteria (AIC and BIC) to study the stability of the models.

gr-qc↗

arXiv2021

Friedmann cosmology with decaying vacuum density in Brans-Dicke theory

In this paper, we study Friedmann cosmology with time-varying vacuum energy density in the context of Brans-Dicke theory. We consider an isotropic and homogeneous flat space, filled with a matter-dominated perfect fluid and a dynamical cosmological term $Λ(t) $, obeying the equation of state of the vacuum. As the exact nature of a possible time-varying vacuum is yet to be found, we explore $Λ(t)$ given by the phenomenological law $Λ(t)=λ+σH$, where $λ$ and $σ$ are positive constants. We solve the model and then focus on two different cases $Λ_{H1}$ and $Λ_{H2}$ by assuming $Λ=λ$ and $Λ=σH$, respectively. Notice that $Λ_{H1}$ is the analog of the standard $Λ$CDM, but within the Brans-Dicke cosmology. We find the analytical solution of the main cosmological functions such as the Hubble parameter, the scale factor, deceleration and equation of state parameters for these models. In order to test the viability of the cosmological scenarios, we perform two sets of joint observational analyses of the recent Type Ia supernova data (Pantheon), observational measurements of Hubble parameter data, Baryon acoustic oscillation/Cosmic microwave background data and Local Hubble constant for each model. For the sake of comparison, the same data analysis is performed for the $Λ$CDM model. Each model shows a transition from decelerated phase to accelerated phase and can be viewed as an effective quintessence behavior. Using the model selection criteria AIC and BIC to distinguish from existing dark energy models, we find that the Brans-Dicke analog of the $Λ$-cosmology (i.e. our model $Λ_{H1}$) performs at a level comparable to the standard $Λ$CDM, whereas $Λ_{H2}$ is less favoured.

gr-qc↗

arXiv2017

Holographic Dark Energy Model in Brans-Dicke Theory with Future Event Horizon

In this paper, we study the dynamics of non-interacting and interacting holographic dark energy models in the framework of Brans-Dicke theory. As system's infra-red cut-off we consider the future event horizon. The motivation of this work is to use the logarithmic form of the Brans-Dicke scalar field, $ϕ\propto ln(α+βa)$, where $α$ and $β$ are constants and `a' is the scalar factor as proposed Kumar and Singh in a recent work to study the new agegraphic dark energy models. We find the time-dependent equation of state parameter and deceleration parameter which describe the phase transition of the universe. We observe that the model explains the early time inflation and late time acceleration including matter-dominated phase. It is also observed that the equation of state parameter may cross phantom divide line in late time evolution. The cosmic coincidence problem is also discussed for both the models. We observe that this logarithmic form of Brans-Dicke scalar field is more appropriate to achieve a less acute coincidence problem in non-interacting model whereas a soft coincidence can be achieved if coupling parameter in interacting model has small value.

gr-qc↗

arXiv2016

New agegraphic dark energy in Brans-Dicke theory with logarithmic form of scalar field

In a very recent paper, the current authors (arXiv:gr-qc/1609.01477) have proposed and analyzed in detail the logarithmic form of Brans-Dicke scalar field $ϕ$ as $ϕ\propto ln(α+βa)$, where $α$ and $β$ are positive constants, to alleviate the problems of interacting holographic dark energy models in Brans-Dicke theory. In this paper, the cosmological evolution of a new agegraphic dark energy (NADE) model within the framework of Friedmann-Robertson-Walker Universe is analyzed with the same form of scalar field in Brans-Dicke theory. We derive the equation of state parameter $w_D$ and deceleration parameter $q$ of NADE model. It is observed that $w_D\rightarrow -1$ when $a\rightarrow \infty$, i.e., the NADE mimics cosmological constant in the late time evolution. Indeed, due to the assumption of logarithmic form of Brans-Dicke scalar field the NADE in Brans-Dicke theory behaves like NADE in general relativity in the late time evolution. The NADE model shows a phase transition from matter dominated phase in early time to accelerated phase in late time. We further extend NADE model by including the interaction between dark matter and NADE. In this case, $w_D$ definitely crosses the phantom divide line ($w_D=-1$) in the late time evolution. The phase transition from matter dominated to NADE dominated phase may be achieved at early stage in interacting model. Further, we show that the interacting NADE model resolves the cosmic coincidence problem as the energy density ratio may evolve sufficiently slow at present.

gr-qc↗

arXiv2015

Holographic dark energy models with statefinder diagnostic in modified $f(R,T)$ gravity

We study non-viscous and viscous holographic dark energy models for a homogeneous and isotropic flat Friedmann-Robertson-Walker Universe in $f(R,T)$ gravity. We find that the Hubble horizon as an IR cut-off is suitable for both the models to explain the recent accelerated expansion of the Universe. The cosmological parameters like deceleration parameter and statefinder parameters are discussed in each model. In non-viscous model a constant deceleration parameter is found which shows that there is no phase transition. The constraints on the parameters are obtained to analyse the fixed point values of statefinder parameters of SCDM and $Λ$CDM models. We know that the phase transition is required to explain the accelerated expansion of the Universe and this is possible if both the parameters would be time-dependent. Therefore, we extend our analysis to viscous holographic dark energy model to investigate whether this viscous model with the same IR cut-off could be helpful to find the phase transition. We find that this model gives a time-dependent deceleration parameter which achieves a smooth phase transition of the Universe. We also find the time-varying statefinder pair which matches with $Λ$CDM model. We plot the trajectories in $r-s$ and $r-q$ plans to discriminate our model with the existing dark energy models and obtain the quintessence like behaviour for the suitable values of parameters.

gr-qc↗

arXiv2014

Friedmann model with viscous cosmology in modified $f(R,T)$ gravity theory

In this paper, we introduce bulk viscosity in the formalism of modified gravity theory in which the gravitational action contains a general function $f(R,T)$, where $R$ and $T$ denote the curvature scalar and the trace of the energy-momentum tensor, respectively within the framework of a flat Friedmann-Robertson-Walker model. As an equation of state for prefect fluid, we take $p=(γ-1)ρ$, where $0 \leq γ\leq 2$ and viscous term as a bulk viscosity due to isotropic model, of the form $ζ=ζ_{0}+ζ_{1}H$, where $ζ_{0}$ and $ζ_{1}$ are constants, and $H$ is the Hubble parameter. The exact non-singular solutions to the corresponding field equations are obtained with non- viscous and viscous fluids, respectively by assuming a simplest particular model of the form of $f(R,T) = R+2f(T)$, where $f(T)=αT$ ( $α$ is a constant). A big-rip singularity is also observed for $γ<0$ at a finite value of cosmic time under certain constraints. We study all possible scenarios with the possible positive and negative ranges of $α$ to analyze the expansion history of the universe. It is observed that the universe accelerates or exhibits transition from decelerated phase to accelerated phase under certain constraints of $ζ_0$ and $ζ_1$. We compare the viscous models with the non-viscous one through the graph plotted between scale factor and cosmic time and find that bulk viscosity plays the major role in the expansion of the universe. A similar graph is plotted for deceleration parameter with non-viscous and viscous fluids and find a transition from decelerated to accelerated phase with some form of bulk viscosity.

gr-qc↗

arXiv2014

Friedmann cosmology with particle creation in modified $f(R,T)$ gravity

The theoretical and observational consequences of thermodynamics of open systems, which allow particle creation are investigated in modified $f(R,T)$ ($R$ is the Ricci scalar and $T$ is the trace of energy-momentum tensor) theory of gravity within the framework of a flat Friedmann-Robertson-Walker line element. A simplest particular model $f(R,T)=R+2f(T)$ and "gamma-law" equation of state $p=(γ-1)ρ$ are assumed to explore the role of particle creation in the early and late time evolution of the universe. A power-law expansion model is proposed for $0\leq γ\leq 2$ by considering the natural phenomenological particle creation rate $ψ=3βnH$, where $β$ is a pure number of the order of unity. A Big Rip singularity is observed for $γ<0$, which describes the phantom cosmology. We observe that the accelerated expansion of the universe is driven by the particle creation without any exotic "quintessence" component or a cosmological constant. It is also found by density parameter that the model becomes of negative curvature due to particle creation and the entropy increases with the evolution of the universe. Some kinematics tests such as lookback time, luminosity distance, proper distance, angular diameter versus redshift are discussed in detail to observe the role of particle creation in early and late time evolution of the universe.

gr-qc↗

arXiv2013

Charged Hadron Multiplicity Distribution at Relativistic Heavy Ion Colliders

The present article reviews facts and problems concerning charge hadron production in high energy collisions. Main emphasis is laid on the qualitative and quantitative description of general characteristics and properties observed for charged hadrons produced in such high energy collisions. Various features of available experimental data e.g., the variations of charged hadron multiplicity and pseudo-rapidity density with the mass number of colliding nuclei, center-of-mass energies and the collision centrality obtained from heavy-ion collider experiments are interpreted in the context of various theoretical concepts and their implications. Finally, several important scaling features observed in the measurements mainly at RHIC and LHC experiments are highlighted in the view of these models to draw some insight regarding the particle production mechanism in heavy-ion collisions.

hep-ph↗

arXiv2013

Upsilon suppression at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider in a modified color screening scenario

The suppression of heavy quarkonia e.g. $J/ψ$, $Υ$ etc. is considered as a suitable probe to identify the nature of the matter created in heavy ion collisions. Recently we have presented a modified colour screening model for $J/ψ$ suppression in the quark gluon plasma (QGP) using quasiparticle model as the equation of state. In this paper, we extend our model to calculate the anomalous suppression of various states of $Υ$ arising due to QGP medium alone. We obtain the suppression patterns of different bottomonia states with respect to centrality at various available collision energies and compare them with the available experimental data.

hep-ph↗

arXiv2013

Production of Strange, Non-strange particles and Hypernuclei in an Excluded-Volume Model

We present a systematic study of production of strange and non-strange hadron yields and their ratios obtained in various experiments using our thermodynamically consistent excluded-volume model. We also analyze the production of light nuclei, hypernuclei and their antinuclei in terms of our excluded-volume model over a broad energy range starting from Alternating Gradient Synchrotron (AGS) to Large Hadron Collider (LHC) energies. Further, we extend our model for studying rapidity spectra of hadrons produced in heavy-ion collisions.

hep-ph↗

arXiv2013

Particle production in Ultra-relativistic Heavy-Ion Collisions : A Statistical-Thermal Model Review

The current status of various thermal and statistical descriptions of particle production in the ultra-relativistic heavy-ion collisions experiments is presented in detail. We discuss the formulation of various types of thermal models of a hot and dense hadron gas (HG) and the methods incorporated in implementing the interactions between hadrons. We first obtain the parameterization of center-of-mass energy ($\sqrt{s_{NN}}$) in terms of temperature ($T$) and baryon chemical potential ($μ_B$) obtained by analyzing the particle ratios at the freeze-out over a broad energy range from the lowest Alternating Gradient Synchrotron (AGS) energy to the highest Relativistic Heavy-Ion Collider (RHIC) energies. The results of various thermal models together with the experimental results for the various ratios of yields of produced hadrons are then compared. We have derived some new universal conditions emerging at the chemical freeze-out of HG fireball which demonstrate the independence with respect to the energy as well as the structure of the nuclei used in the collision. Further, we perform the calculation of various transport properties of HG such as shear viscosity-to-entropy density ratio ($η/s$) etc. using thermal model and compare with the results of other models. We also present the calculation of the rapidity as well as transverse mass spectra of various hadrons in the thermal HG model. The purpose of this review article is to organize and summarize the experimental data obtained in various experiments with heavy-ion collisions and then to examine and analyze them using thermal models so that a firm conclusion regarding the formation of quark-gluon plasma (QGP) can be obtained.

hep-ph↗

arXiv2013

Strongly Interacting Matter at Finite Chemical Potential : Hybrid Model Approach

Search for a proper and realistic equation of state (EOS) for strongly interacting matter used in the study of the QCD phase diagram still appears as a challenging problem. Recently, we constructed a hybrid model description for the quark gluon plasma (QGP) as well as hadron gas (HG) phases where we used an excluded volume model for HG and a thermodynamically consistent quasiparticle model for the QGP phase. The hybrid model suitably describes the recent lattice results of various thermodynamical as well as transport properties of the QCD matter at zero baryon chemical potential ($μ_{B}$). In this paper, we extend our investigations further in obtaining the properties of QCD matter at finite value of $μ_{B}$ and compare our results with the most recent results of lattice QCD calculation.

hep-ph↗

arXiv2013

String Cosmology with Magnetic field in Anisotropic Space-time

In this paper we study the effect of the magnetic field in string cosmology for a spatially homogenous and anisotropic Bianchi type -V space-time model. In order to study the effect of magnetic field, the standard form of the energy momentum tensor for cosmic strings is modified by including an additional term for magnetic field. The magnetic field is due to an electric current produced along the x-axis with infinite electrical conductivity. The field equations are solved for different string models such as geometric string (Nambu string), Takabayashi string (p-string) and Reddy string using string equation of state. We also find the solution for string models with uniform energy density. The physical and geometrical properties of each string model with and without magnetic field are discussed in detail.

gr-qc↗

arXiv2013

Wounded Quarks and Multiplicity at Relativistic Ion Colliders

In this paper, we propose a parameterization which is based on a phenomenological model involving the wounded quarks interactions for explaining the average charged particle multiplicity $\ < n_{ch}\ >$, the central pseudo-rapidity density $\ < (dn/dη)_{η=0}\ >$ and complete rapidity dependence of $dn/dη$ in relativistic heavy-ion collider experiments. The model also interrelates nucleus-nucleus (A-A) collisions with p-A and p-p interactions. Our parameterization rests on simple assumptions regarding mean number of participating quarks and their average number of collisions. The results for $\ < n_{ch}\ >$ and their variations with the mass number of colliding nuclei, center-of-mass energy ($\sqrt{s_{NN}}$) and collision centrality are well supported by the available experimental data. Finally we give the predictions from our model for A-A collisions at the Large Hadron Collider (LHC) and Compressed Baryonic Matter (CBM) experiments. Our results indicate the existence of a possible universal production mechanism for p-p, p-A and A-A collisions.

hep-ph↗

arXiv2013

Colour screening scenario for quarkonia suppression in a quasiparticle model compared with data obtained from experiments at the CERN SPS, BNL RHIC, and CERN LHC

We present a modified colour screening model for $J/ψ$ suppression in the Quark-Gluon Plasma (QGP) using quasi-particle model (QPM) as equation of state (EOS). Other theoretical ingredients incorporated in the model are feed-down from higher resonances namely, $χ_c$, and $ψ^{'}$, dilated formation time for quarkonia and viscous effects of the QGP medium. Assuming further that the QGP is expanding with Bjorken's hydrodynamical expansion, the present model is used to analyze the centrality dependence of the $J/ψ$ suppression in mid-rapidity region and compare it with the data obtained from SPS, RHIC and LHC experiments. We find that the centrality dependence of the data for the survival probability at all energies is well reproduced by our model. We further compare our model predictions with the results obtained from the bag model EOS for QGP which has usually been used earlier in all such calculations.

hep-ph↗

arXiv2013

Hot and Dense Hadron Gas (HG): A New Excluded-volume approach

We formulate a thermodynamically consistent equation of state (EOS), based on excluded-volume approach, for a hot, dense hadron gas (HG). We calculate various thermodynamical quantities of HG and various hadron ratios and compare our model results with the results of other excluded-volume models and experimental data. We also calculate various transport coefficients such as $η/s$ etc. and compare them with other HG model results. Furthermore, we test the validity of our model in getting the rapidity spectra of various hadrons and the effect of flow on them is investigated by matching our predictions with the experimental data.

hep-ph↗

arXiv2013

QCD Phase Transition in a new Hybrid Model Formulation

Search of a proper and realistic equations of state (EOS) for strongly interacting matter used in the study of QCD phase diagram still appears as a challenging task. Recently, we have constructed a hybrid model description for the quark gluon plasma (QGP) as well as hadron gas (HG) phases where we use a new excluded-volume model for HG and a thermodynamically-consistent quasiparticle model for the QGP phase. We attempt to use them to get a QCD phase boundary and a critical point. We test our hybrid model by reproducing the entire lattice QCD data for strongly interacting matter at zero baryon chemical potential ($μ_{B}$)and predict the results at finite $μ_{B}$ and $T$.

hep-ph↗