arXiv subjects

M. Shahalam

Publications and source records attributed to M. Shahalam.

At least 19 recordsLinked to original sources

arXiv2024

Dynamical system analysis in descending dark energy model

In this paper, we study the dynamical system analysis for a recently proposed decaying dark energy model, namely, Q-SC-CDM. First we investigate the stationary points to find the stable attractor solution under the conditions discussed recently in the literature. In this case, we do not find any stable attractor solution. Therefore, we avoid the parameter space of Q-SC-CDM model discussed in arXiv:2201.07704. Second, we make different choice for the model parameters and re-investigate the stationary points and their stability. Our analysis shows that a simple choice of model parameters allows to capture a stable attractor solution. Moreover, we obtain phase portrait where all trajectories move towards the stable attractor point.

gr-qc↗

arXiv2023

Initial conditions of pre-inflation with Hilltop potential in loop quantum cosmology

In this paper, we investigate the dynamics of pre-inflation with Hilltop potential in the framework of loop quantum cosmology. The initial conditions of inflaton field at the quantum bounce is categorized into two classes, first one is dominated by kinetic energy and second one by potential energy. In both cases, the physically viable initial values of inflaton field at the bounce are obtained numerically that generate the desired slow-roll inflation and also sufficient number of $e$-folds. To be consistent with observations at least 60 $e$-folds are required. In case of kinetic energy dominated initial conditions of inflaton field at the bounce, the numerical evolution of the background prior to preheating is divided into three different regions: {\it bouncing, transition and slow-roll inflation} whereas bouncing and transition phases disappear in the potential energy dominated case but still slow-roll inflation is achieved. This is true in case of $p=4$ and $v=1 M_{Pl}$ of Hilltop potential. However for other cases, slow-roll inflation can not be obtained. Moreover, we study the phase space analysis for Hilltop potential and discuss the phase space trajectories under the chosen parameters.

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arXiv2020

Dynamics of nonminimally coupled scalar field models with generic potentials in FLRW background

We study the phase space analysis of a nonminimally coupled scalar field model with different potentials such as KKLT, Higgs, inverse and inverse square. Our investigation brings new asymptotic regimes, and obtains stable de-Sitter solution. In case of KKLT, we do not find stable de-Sitter solution whereas Higgs model satisfies the de-Sitter condition but does not provide a stable de-Sitter solution in usual sense as one of the eigenvalue is zero. We obtain time derivative of Hubble constant $\dot{H}=0$, equation of state $w_ϕ\simeq -1$, scalar field $ϕ=$constant and the positive effective gravitational constant ($G_{eff}>0$), which are missed in our earlier work. Therefore, in case of $F(ϕ)R$ coupling with $F(ϕ)= 1-ξϕ^2 $ and the models of inverse and inverse square potentials$-$ a true stable de-Sitter solution is trivially satisfied.

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arXiv2020

Revisiting pre-inflationary universe of family of $α-$attractor in loop quantum cosmology

In this work, we revisit the dynamics of pre-inflationary universe with a family of $α-$attractor potentials, in the framework of loop quantum cosmology, in which the big bang singularity is generically resolved purely with quantum geometric effects, and replaced by a quantum bounce. At the bounce, the background evolution is divided into two distinct classes, the first is dominated by the kinetic energy of the inflaton field and the second by the potential energy. In both classes, we find the physically viable initial conditions numerically that provide not only the slow-roll inflation, but also sufficient e-folds to be compatible with observations. In the entire range of kinetic energy dominated initial conditions (except some subsets of Models 2 and 4), the background evolution prior to reheating is always split into three different phases: bouncing, transition and slow-roll inflation. In the bouncing phase, the numerical evolution of the scale factor is independent not only of the initial data, but also the inflationary potentials, as long as it is dominated by the kinetic energy, and can be well approximated by an analytical solution, whereas in the potential energy dominated case, such approximated results do not exist. Moreover, we study the phase space analysis for a class of $α-$attractor potentials, and discuss the phase space trajectories for physically viable initial conditions of the inflaton field.

gr-qc↗

arXiv2019

Late time evolution of a nonminimally coupled scalar field system

We revisit the dynamics of a nonminimally coupled scalar field model in case of $F(ϕ)R$ coupling with $F(ϕ)= 1-ξϕ^2 $, and the potentials $V(ϕ) = V_0 (1+ ϕ^p)^2$, $V(ϕ)= V_0 e^{λϕ^2}$. We use an autonomous system to bring out new asymptotic regimes, and find stable de-Sitter solution. Under the chosen functional form of $F(ϕ)$ and steep exponential potentials, a true de-Sitter solution is trivially satisfied for which the equation of state $w_ϕ\simeq -1$, the effective gravitational constant $G_{eff}$ and field $ϕ$ are constant that has been missed in the power law case and our previous study.

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arXiv2019

Constraints on quintessence scalar field models using cosmological observations

We consider a varieties of quintessence scalar field models in a homogeneous and isotropic geometry of the universe with zero spatial curvature aiming to provide stringent constraints using a series of cosmological data sets, namely, the cosmic microwave background observations (CMB), baryon acoustic oscillations (BAO), joint light curve analysis (JLA) from supernovae type Ia, redshift space distortions (RSD), and the cosmic chronometers (CC). From the qualitative evolution of the models, we find all of them are able to execute a fine transition from the past decelerating phase to the presently accelerating expansion where in addition, the equation of state of the scalar field (also the effective equation of state) might be close to that of the $Λ$CDM cosmology depending on its free parameters. From the observational analyses, we find that the scalar field parameters are unconstrained irrespective of all the observational datasets. In fact, we find that the quintessence scalar field models are pretty much determined by the CMB observations since any of the external datasets such as BAO, JLA, RSD, CC does not add any constraining power to CMB. Additionally, we observe a strong negative correlation between the parameters $H_0$ (present value of the Hubble parameter), $Ω_{m0}$ (density parameter for the matter sector, i.e., cold dark matter plus baryons) exists, while no correlation between $H_0$, and $σ_8$ (amplitude of the matter fluctuation) are not correlated. We also comment that the present models are unable to reconcile the tension on $H_0$. Finally, we conclude our work with the Bayesian analyses which report that the non-interacting $Λ$CDM model is preferred over all the quintessence scalar field models.

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arXiv2019

Cosmic acceleration from coupling of baryonic and dark matter components: Analysis and diagnostics

In this paper, we examine a scenario in which late-time cosmic acceleration might arise due to the coupling between baryonic matter and dark matter without the presence of extra degrees of freedom. In this case, one can obtain late-time acceleration in Jordan frame and not in Einstein frame. We consider two different forms of parametrization of the coupling function, and put constraints on the model parameters by using an integrated datasets of Hubble parameter, Type Ia supernova and baryon acoustic oscillations. The models under consideration are consistent with the observations. In addition, we perform the statefinder and $Om$ diagnostics, and show that the models exhibit a distinctive behavior due to the phantom characteristic in future which is a generic feature of the underlying scenario.

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arXiv2018

Preinflationary dynamics in loop quantum cosmology: Monodromy Potential

In this article we explore the pre-inflationary background dynamics of an FLRW universe sourced by a scalar field with monodromy potential in LQC framework. In particular we calculate the number of e-folds, $N_{inf}$, produced during the slowly rolling phase of the inflation and find out the critical value of the ratio of the kinetic to potential energy, $r_w^c$, at the quantum bounce that is required to produce $N_{inf}\simeq 60.$ Two different monodromy potentials, namely, linear and quadratic with a modulation term are investigated to this effect. The effects on the value of $N_{inf}$ due to parameters associated with the strength, decay constant and the phase factor of the modulation term are calculated. In addition to this we present the qualitative picture of the background dynamics by carrying out a dynamical system analysis. We produce the phase portraits and carry out a detailed linear stability analysis of the finite fixed points, if any, for each of the potentials.

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arXiv2018

Preinflationary dynamics of $α-$attractor in loop quantum cosmology

We systematically study the preinflationary dynamics of the spatially flat Friedmann-Lemaitre-Robertson-Walker universe filled with a single scalar field that has the generalized $α-$attractor potentials, in the framework of loop quantum cosmology, in which the big bang singularity is replaced generically by a non-singular quantum bounce due to purely quantum geometric effects. The evolution can be divided into two different classes, one is dominated initially (at the quantum bounce) by the kinetic energy of the scalar field, and one is not. In both cases, we identify numerically the physically viable initial conditions that lead to not only a slow-roll inflationary phase, but also enough $e$-folds to be consistent with observations, and find that the output of such a viable slow-roll inflationary phase is generic. In addition, we also show that in the case when the evolution of the universe is dominated initially by the kinetic energy of the scalar field (except for a very small set in the phase space), the evolution before reheating is aways divided into three different phases: {\em bouncing, transition and slow-roll inflation}. This universal feature does not depend on the initial conditions of the system nor on the specific potentials of the scalar field, as long as it is dominated initially by the kinetic energy of the scalar field at the bounce. Moreover, we carry out phase space analyses for the models under consideration and compare our results with the power-law and Starobinsky potentials.

astro-ph.CO↗

arXiv2018

Preinflationary dynamics of power-law potential in loop quantum cosmology

In this article, I mainly discuss the dynamics of the pre-inflationary Universe for the potential $V(ϕ) \propto ϕ^n$ with $n=5/3$ in the context of loop quantum cosmology, in which the big bang singularity is resolved by a non-singular quantum bounce. In the case of the kinetic energy-dominated initial conditions of the scalar field at the bounce, the numerical evolution of the Universe can be split up into three regimes: {\em bouncing, transition,} and {\em slow-roll inflation}. In the bouncing regime, the~numerical evolution of the scale factor does not depend on a wide range of initial values, {or on}~the inflationary potentials. I calculate the number of $e$-folds in the slow-roll regime, by which observationally identified initial conditions are obtained. Additionally, I display the phase portrait for the model under consideration.

gr-qc↗

arXiv2018

Late-time acceleration with steep exponential potentials

In this letter, we study the cosmological dynamics of steeper potential than exponential. Our analysis shows that a simple extension of an exponential potential allows to capture late-time cosmic acceleration and retain the tracker behavior. We also perform statefinder and $Om$ diagnostics to distinguish dark energy models among themselves and with $Λ$CDM. In addition, to put the observational constraints on the model parameters, we modify the publicly available CosmoMC code and use an integrated data base of baryon acoustic oscillation, latest Type Ia supernova from Joint Light Curves sample and the local Hubble constant value measured by the Hubble Space Telescope.

gr-qc↗

arXiv2017

Pre-inflationary dynamics in loop quantum cosmology: Power-law potentials

In this paper, we study the pre-inflationary dynamics for the power-law potential $(V(ϕ) \propto ϕ^n)$ with $n<2$ in the framework of loop quantum cosmology. In the case where the kinetic energy of the inflaton dominates at the initial, the evolution of the universe can always be divided into three different phases prior to preheating: {\em bouncing, transition and slow-roll inflation}. During the bouncing phase, the evolution of the expansion factor is independent not only on the initial conditions but also the inflationary potentials, and is given explicitly by an analytical solution. In contrast, for the potential energy dominated initial conditions, this universality is lost. We also obtain total number of e-folds during the slow-roll inflation, whereby physically viable models are identified. In addition, we present phase space analysis for the inflationary potentials under consideration and compare our results with the ones obtained previously for different potentials.

gr-qc↗

arXiv2017

Observational constraints on the generalized $α$ attractor model

We study the generalized $α$ attractor model in the context of the late time cosmic acceleration. The model interpolates between the scaling freezing and thawing dark energy models. In the slow roll region, the original potential is modified whereas the modification ceases in the asymptotic region and the effective potential behaves as the quadratic one. In our setting, the field rolls slowly around the present epoch and mimics the scaling behavior in the future. We obtain observational constraints on the model parameters by using an integrated data base (SN+Hubble+BAO+CMB).

astro-ph.CO↗

arXiv2017

Dynamics of coupled phantom and tachyon fields

In this paper, we apply the dynamical analysis to a coupled phantom field with scaling potential taking particular forms of the coupling (linear and combination of linear), and present phase space analysis. We investigate if there exist late time accelerated scaling attractor that has the ratio of dark energy and dark matter densities of the order one. We observe that the scrutinized couplings cannot alleviate the coincidence problem, however acquire stable late time accelerated solutions. We also discuss coupled tachyon field with inverse square potential assuming linear coupling.

gr-qc↗

arXiv2016

Galileons, phantom fields, and the fate of the Universe

In this paper we study cosmological dynamics of phantom as well as non-phantom fields with linear potential in presence of Galileon correction $(\partial_μϕ\partial^μϕ) \Box ϕ$. We show that the Big Crunch singularity is delayed compared to the standard case; the delay crucially depends upon the strength of Galileon correction. As for the phantom Galileon, $ρ_ϕ$ is shown to grow more slowly compared to the standard phantom delaying the approach to singularity. In case, $V\sim ϕ^n, n>4$, Big Rip is also delayed, similar phenomenon is shown to take place for potentials steeper than the exponential.

gr-qc↗

arXiv2015

Dynamics of interacting quintessence

In this paper, we investigate coupled quintessence with scaling potential assuming specific forms of the coupling as $A$ namely, $α\dot{ρ_m}$, $β\dot{ρ_ϕ}$ and $σ(\dot{ρ_m}+\dot{ρ_ϕ})$, and present phase space analysis for three different interacting models. We focus on the attractor solutions that can give rise to late time acceleration with $Ω_{DE}/Ω_{DM}$ of order unity in order to alleviate the coincidence problem.

gr-qc↗

arXiv2015

Light mass galileon and late time acceleration of the Universe

We study Galileon scalar field model by considering the lowest order Galileon term in the lagrangian, $(\partial_μ ϕ)^2 \Boxϕ$ by invoking a field potential. We use Statefinder hierarchy to distinguish the light mass galileon models with different potentials amongst themselves and from the $Λ$CDM behaviour. The $Om$ diagnostic is applied to cosmological dynamics and observational constraints on the model parameters are studied using SN+Hubble+BAO data.

gr-qc↗

arXiv2015

Constraints on cosmological parameters in power-law cosmology

In this paper, we examine observational constraints on the power law cosmology; essentially dependent on two parameters $H_0$ (Hubble constant) and $q$ (deceleration parameter). We investigate the constraints on these parameters using the latest 28 points of H(z) data and 580 points of Union2.1 compilation data and, compare the results with the results of $Λ$CDM. We also forecast constraints using a simulated data set for the future JDEM, supernovae survey. Our studies give better insight into power law cosmology than the earlier done analysis by Kumar [arXiv:1109.6924] indicating it tuning well with Union2.1 compilation data but not with H(z) data. However, the constraints obtained on $ $ and $ $ i.e. $H_0$ average and $q$ average using the simulated data set for the future JDEM, supernovae survey are found to be inconsistent with the values obtained from the H(z) and Union2.1 compilation data. We also perform the statefinder analysis and find that the power-law cosmological models approach the standard $Λ$CDM model as $q\rightarrow -1$. Finally, we observe that although the power law cosmology explains several prominent features of evolution of the Universe, it fails in details.

gr-qc↗