Effective Equation Of State Parameter Research Articles

Cosmology of Barrow holographic QCD ghost dark energy and a look into the thermodynamics

The present study endeavours to study the cosmology of QCD ghost dark energy based on Barrow holographic fluid, a particular example of Nojiri-Odintsov holographic dark energy (2006, General Relativity and Gravitation, 38, 1285–1304); (2017, The European Physical Journal C, 77, 1–8). The Hubble parameter is reconstructed and according the equation of state parameter is reconstructed for the Barrow holographic QCD ghost dark energy. It is observed that the effective equation of state parameter has a transition from quintessence to phantom and for the current universe the equation of state parameter is very close to −1. The deceleration parameter is computed based on the reconstructed Hubble parameter and it is observed that the model can have a transition from decelerated to accelerated universe. The statefinder trajectories are plotted and an interpolation between dust and ΛCDM phases is observed. Finally, the thermodynamics is studied considering apparent horizon as the enveloping horizon of the Universe.

Dark energy nature of logarithmic f(R,Lm)-gravity models with observational constraints

This work explores the dark energy nature of logarithmic [Formula: see text]-gravity models with observational constraints, where [Formula: see text] represents the matter Lagrangian for perfect fluid and R is the Ricci-scalar curvature. With a matter Lagrangian [Formula: see text], a flat FLRW space–time metric and an arbitrary function [Formula: see text], where [Formula: see text] is a positive model parameter, we have derived the field equations of this model. By using the Hubble function, we have been able to solve the energy conservation equation and create a relationship between [Formula: see text], [Formula: see text] and [Formula: see text]. Using the most recent two observational datasets as 32 [Formula: see text] and 1048 Pantheon SNe Ia datasets, we conducted MCMC analysis. We have obtained best fit values of model parameters with [Formula: see text] errors and using these values, we have explored the cosmological properties of the model. We have performed om diagnostic analysis for the model and estimated the present age of the universe. Thus our derived model presents a transit phase decelerating to accelerating model without dark energy term [Formula: see text]. We found that the effective equation of state parameter is in the range [Formula: see text] over [Formula: see text] with present value [Formula: see text].

Dynamical system analysis of scalar field cosmology in coincident f(Q) gravity

In this article, we investigate scalar field cosmology in the coincident f(Q) gravity formalism. We calculate the motion equations of f(Q) gravity under the flat Friedmann-Lemaître-Robertson-Walker background in the presence of a scalar field. We consider a non-linear f(Q) model, particularly f(Q) = − Q + α Q n , which is nothing but a polynomial correction to the Symmetric Teleparallel Equivalent to General Relativity (STEGR) case. Further, we assumed two well-known specific forms of the potential function, specifically the exponential from V(ϕ) = V 0 e −β ϕ and the power-law form V(ϕ) = V 0 ϕ −k . We employ some phase-space variables and transform the cosmological field equations into an autonomous system. We calculate the critical points of the corresponding autonomous systems and examine their stability behaviors. We discuss the physical significance corresponding to the exponential case for parameter values n = 2 and n = − 1 with β = 1, and n = − 1 with β=3 . Moreover, we discuss the same corresponding to the power-law case for the parameter value n = − 2 and k = 0.16. We also analyze the behavior of corresponding cosmological parameters such as scalar field and dark energy density, deceleration, and the effective equation of state parameter. Corresponding to the exponential case, we find that the results obtained for the parameter constraints in Case III is better among all three cases, and that represents the evolution of the Universe from a decelerated stiff era to an accelerated de-Sitter era via matter-dominated epoch. Further, in the power-law case, we find that all trajectories exhibit identical behavior, representing the evolution of the Universe from a decelerated stiff era to an accelerated de-Sitter era. Lastly, we conclude that the exponential case shows better evolution as compared to the power-law case.

Energy conditions in f(R,A) gravity

In this paper, we consider the framework of the newly proposed modified theory of gravity named [Formula: see text] gravity, wherein [Formula: see text] and [Formula: see text] signify the Ricci and anticurvature scalars, respectively. For our cosmic discussions, we evaluate a renowned [Formula: see text] model, incorporating various forms of the scale factor within the context of Friedmann–Robertson–Walker (FRW) spacetime. Our exploration delves into energy conditions, the effective equation of state parameter and the stability of models within the [Formula: see text] theory. These models are further elucidated through graphical representations plotted against time and their respective model constants. Notably, it is observed that scale factors with exponential terms yield more consistent results in comparison to those of power-law forms, given specific values of model constants.

Cosmic dynamics of isotropic models with inhomogeneous EoS: A dynamical system perspective

We examine the homogeneous and isotropic models sourced by the fluid having an inhomogenous equation of state (EoS) using qualitative methods. The flat Friedmann–Lemaitre–Robertson–Walker (FLRW) model explains the accelerating universe expansion of the present era having past of decelerating era dominated by radiation and matter. The open FLRW model incorporates the radiation, matter and dark energy dominated era in order with the spatial curvature playing role just before the accelerated universe evolution. The dominating behaviors of radiation, matter and spatial curvature affect the evolution trajectories of the deceleration and effective EoS parameter, which are obtained by the numerical solutions of autonomous system. The model parameters affect the nature of dark energy and therefore the EoS parameter may also lie in the quintessence or phantom regions. The closed FLRW model having constant positive curvature may yield the bouncing and turnaround universe evolution with dark energy-dominated phase at late-times. The application of statefinder diagnostic in the cosmological dynamical systems highlights that the universe may approach the [Formula: see text] cold dark matter ([Formula: see text]CDM) universe as a particular case in the future.

Constrained evolution of effective equation of state parameter in non-linear $$\qquad\qquad f\left( {R,L_{m} } \right)$$ dark energy model: insights from Bayesian analysis of cosmic chronometers and Pantheon samples

Constrained evolution of effective equation of state parameter in non-linear $$\qquad\qquad f\left( {R,L_{m} } \right)$$ dark energy model: insights from Bayesian analysis of cosmic chronometers and Pantheon samples

Impact of dark energy on the equation of state in light of the latest cosmological data

Abstract We reconstruct the effective equation of state (EoS) within the framework of the general theory of relativity in a homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker universe, which is assumed to be composed of matter and dark energy (DE). Our analysis employs a dataset consisting of 31 cosmic chronometer data points, six data points of baryon acoustic oscillations, and 1048 type Ia supernovae from the Pantheon sample, and we determine the best-fitting values of the model parameters through Markov chain Monte Carlo simulation. We then use these parameter values to calculate various cosmological parameters, such as the DE EoS parameter, the energy density, the deceleration parameter, the state-finder parameters, and the Om(z) diagnostic. All the analyzed cosmological parameters show behavior consistent with the accelerated universe scenario.

Constraining the cosmological model using recent observational data* *Supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) ( IMSIU-RG23008)

In this study, we conduct a comprehensive investigation of the cosmological model described by (where λ represents a free parameter) in light of the most recent observational data. By constraining the model using the and datasets, we determine its compatibility with the observed behavior of the Universe. For this purpose, we adopt a parametric form for the effective equation of state (EoS) parameter. This parametric form allows us to describe the evolution of the EoS parameter with respect to redshift and investigate its behavior during different cosmic epochs. The analysis of the deceleration parameter reveals an accelerating Universe with a present value of , indicating the current phase of accelerated expansion. The transition redshift is found to be , marking the epoch of transition from deceleration to acceleration. We also analyze the evolution of important cosmological parameters, including the density parameter, pressure, effective EoS, and stability. These findings collectively demonstrate the viability of the cosmological model as a robust candidate capable of engendering the requisite negative pressure, thereby efficiently propelling cosmic expansion. Moreover, the undertaken stability analysis underscores the model's stability within the broader cosmic landscape. By providing the best-fit values for the coupling parameter λ, this approach motivates and encourages further exploration into the extensive landscape of this model and its potential applications across diverse realms of cosmology and astronomy.

Primordial gravitational waves by chaotic potential with a sharp step

We calculate the primordial gravitational wave spectrum observed today, Ω0gw(f) produced by a chaotic inflationary model featuring a step in its potential. Considering an inflaton field, slowly rolling down the potential, we analyze the scalar and tensor power spectra for the step potential model and found oscillatory density fluctuations within a specific range of wavenumbers around the feature. Localized small anomalies are observed in the tensor power spectrum also. We estimate the observational parameters, including the scalar spectral index (ns), tensor tilt (nt) and tensor-to-scalar ratio (r), which are found to be consistent with Planck satellite results. Additionally, by employing the relation between Ω0gw(f) and r, we determine the effective equation of state parameter wˆ(f) at the end of inflation. Evolution of wˆ(f) from 0.2 to 0.33, followed by constancy is obtained in the ntˆ(f)−wˆ(f) plot. This evolution of wˆ(f) is attributed to the reheating process, which is crucial for transitioning from the inflationary phase to the subsequent radiation-dominated era.

Gravitational wave production after inflation for a hybrid inflationary model

We discuss a cosmological scenario with a stochastic background of gravitational waves sourced by the tensor perturbation due to a hybrid inflationary model with cubic potential. The tensor-to-scalar ratio for the present hybrid inflationary model is obtained as [Formula: see text]. Gravitational wave spectrum of this stochastic background, for large-scale CMB modes, [Formula: see text] to [Formula: see text] is studied. The present-day energy spectrum of gravitational waves [Formula: see text] is sensitively related to the tensor power spectrum and r which is, in turn, dependent on the unknown physics of the early cosmos. This uncertainty is characterized by two parameters: [Formula: see text] logarithmic average over the primordial tensor spectral index and [Formula: see text] logarithmic average over the effective equation-of-state parameter. Thus, exact constraints in the [Formula: see text]-[Formula: see text] plane can be obtained by comparing theoretical constraints of our model on r and [Formula: see text]. We obtain a limit on [Formula: see text] around the modes probed by CMB scales.

Noether symmetry approach in scalar-torsion f(T,phi ) gravity

The Noether Symmetry approach is applied to study an extended teleparallel f(T,phi ) gravity that contains the torsion scalar T and the scalar field phi in the context of an Friedmann–Lemaître–Robertson–Walker space-time. We investigate the Noether symmetry approach in f(T,phi ) gravity formalism with the specific form of f(T,phi ) and analyze how to demonstrate a nontrivial Noether vector. The Noether symmetry method is a helpful resource for generating models and finding out the exact solution of the Lagrangian. In this article, we go through how the Noether symmetry approach enables us to define the form of the function f(T,phi ) and obtain exact cosmological solutions. We also find the analytical cosmological solutions to the field equations, that is consistent with the Noether symmetry. Our results demonstrate that the obtained solutions enable an accelerated expansion of the Universe. We have also obtained the present value of the Hubble parameter, deceleration parameter, and effective equation of state parameter, which is fit in the range of current cosmological observations.

Cosmological constraints on dark energy in [formula omitted] gravity: A parametrized perspective

In this paper, we focus on the parametrization of the effective equation of state (EoS) parameter within the framework of f(Q) symmetric teleparallel gravity. Here, the gravitational action is represented by an arbitrary function of the non-metricity scalar Q. By utilizing a specific parametrization of the effective EoS parameter and a power-law model of f(Q) theory, namely f(Q)=βQm+1 (where β and m are arbitrary constants), we derive the cosmological solution of the Hubble parameter H(z). To constrain model parameters, we employ recent observational data, including the Observational Hubble parameter Data (OHD), Baryon Acoustic Oscillations data (BAO), and Type Ia supernovae data (SNe Ia). The current constrained value of the deceleration parameter is found to be q0=−0.50−0.01+0.01, indicating that the current Universe is accelerating. Furthermore, we examine the evolution of the density, EoS, and Om(z) diagnostic parameters to deduce the accelerating nature of the Universe. Finally, we perform a stability analysis with linear perturbations to confirm the model’s stability.

Viscous Fluid Cosmology in Symmetric Teleparallel Gravity

AbstractIn this article, the viscous fluid cosmological model is analyzed in the framework of recently proposed gravity by assuming three different forms of bulk viscosity coefficients, specifically, , , and and a linear model, particularly, where is free model parameter. The bulk viscosity coefficients and the model parameter values using the combined H(z)+Pantheon+BAO data set are estimated. The asymptotic behavior of our cosmological bulk viscous model is studied by utilizing the phase space method. It is found that corresponding to all three cases, our model depicts the evolution of the universe from matter dominated decelerated epoch (a past attractor) to a stable de‐sitter accelerated epoch (a future attractor). Furthermore, the physical behavior of effective pressure, effective equation of state (EoS), and the statefinder parameters are studied. It is also found that the pressure component in the presence of bulk viscosity shows negative behavior and the effective EoS parameter predicts the accelerated expansion phase of the universe for all three cases. Moreover, the trajectories of the model lie in the quintessence region and it converges to the ΛCDM fixed point in the far future. It is found that the accelerated de‐Sitter like phase comes purely from the case without any geometrical modification to GR. Moreover, the late‐time behavior of all three cases of viscosity coefficients are identical. Furthermore, a non‐linear model is considered, specifically, , and then analyzed the behavior of model using dynamical approach. It is also found that the late‐time behavior of the considered non‐linear model with is similar to the linear case, whereas for the case results are quite different.

The constrained cosmological model in Lyra geometry

In this paper, we study a flat homogeneous FLRW model in Lyra geometry which is described by a time-dependent displacement vector. We consider an appropriate parametrization of the energy density of scalar field [Formula: see text] in terms of the cosmic scale factor. The result shows two transitions from deceleration to acceleration. Furthermore, we constrain the model parameter [Formula: see text] and the displacement field vector [Formula: see text] using the recent supernovae data, Hubble dataset of 77 points and their joint data which predict the accelerated expanding phase of the universe in late times. The effective equation of state parameter [Formula: see text] speculates [Formula: see text]CDM in late times. Finally, we use the statefinder diagnostic to differentiate our model from the various dark energy models.

Model independent bounds for the number of e-folds during the evolution of the universe

We present a simple procedure to obtain universal bounds for quantities of cosmological interest, such as the number of e-folds during inflation, reheating, and radiation, as well as the reheating temperature. The main assumption is to represent each of the various epochs of evolution of the universe as being due to a single substance changing instantaneously into the next, describing a new era of evolution of the universe. This assumption, commonly used to obtain solutions of the Friedmann equations for simple cosmological models, is implemented here to find model-independent bounds on cosmological quantities of interest. In particular, we find that the bound Nk ≈ 56 for -1/3 < ω re < 1/3 is very robust as an upper bound on the number of e-folds during inflation and also as a lower bound when ω re > 1/3, where ω re is the effective equation of state parameter during reheating. These are model-independent results that any single-field model of inflation should satisfy. As an example we illustrate with the basic α attractor model the usual model dependent approach, and the one presented here, and show how they complement each other.

Study of cosmic acceleration in modified theories of gravity through Kaniadakis holographic dark energy

In this paper, we investigate the cosmic analysis of Kaniadakis holographic dark energy model in the frameworks of [Formula: see text] and [Formula: see text] modified theories of gravity. We reconstruct functional forms of [Formula: see text] and [Formula: see text] models by using a well-known power-law form of scale factor and flat FRW metric. The behavior of these functional forms with respect to their arguments is discussed. Using these models, various cosmic parameters like energy conditions, effective equation of state parameter, deceleration parameter and squared speed of sound parameters are explored to discuss the accelerated expansion of the universe. We obtain some consistent results for accelerated expansion of the universe related to specific values of model parameters.

Cosmic aspects of matter creation models in modified gravities

This paper is devoted to study some cosmological models in the scenario of two gravitational theories like fractal universe and dynamical Chern–Simons modified theory in particle creation mechanism. In both theories, we consider three particle creation rate models including constant and variable mode. We use Gibbs equation to find out pressure of particle creation rate using adiabatic matter phenomenon. The Hubble parameter along with its normalized form are constructed for each case. We discuss the deceleration parameter and effective equation of state parameter by taking into account the normalized Hubble parameter and model parameters. The results of these parameters represent consistent behavior with recent observational data.

Dark energy constraint on equation of state parameter in the Weyl type [formula omitted] gravity

The equation of state parameter is a significant method for characterizing dark energy models. We investigate the evolution of the equation of state parameter with redshift using a Bayesian analysis of recent observational datasets (the Cosmic Chronometer data (CC) and Pantheon samples). The Chevallier–Polarski–Linder parametrization of the effective equation of state parameter, ωeff=ω0+ωaz1+z, where ω0 and ωa are free constants, is confined to the Weyl type f(Q,T) gravity, where Q represents the non-metricity and T is the trace of the energy–momentum tensor. We observe the evolution of the deceleration parameter q, the density parameter ρ, the pressure p, and the effective equation of state parameter ω. The cosmic data limit for ω does not exclude the possibility of ω<−1. It is seen that the parameter ω shows a transition from deceleration to acceleration, as well as a shift from ω>−1 to ω<−1.

Bulk Viscous Fluid in Symmetric Teleparallel Cosmology: Theory versus Experiment

The standard formulation of General Relativity Theory, in the absence of a cosmological constant, is unable to explain the responsible mechanism for the observed late-time cosmic acceleration. On the other hand, by inserting the cosmological constant in Einstein’s field equations, it is possible to describe the cosmic acceleration, but the cosmological constant suffers from an unprecedented fine-tuning problem. This motivates one to modify Einstein’s spacetime geometry of General Relativity. The f(Q) modified theory of gravity is an alternative theory to General Relativity, where the non-metricity scalar Q is the responsible candidate for gravitational interactions. In the present work, we consider a Friedmann–Lemâitre–Robertson–Walker cosmological model dominated by bulk viscous cosmic fluid in f(Q) gravity with the functional form f(Q)=αQn, where α and n are free parameters of the model. We constrain our model with the Pantheon supernovae dataset of 1048 data points, the Hubble dataset of 31 data points, and the baryon acoustic oscillations dataset consisting of 6 data points. We find that our f(Q) cosmological model efficiently describes the observational data. We present the evolution of our deceleration parameter with redshift, and it properly predicts a transition from decelerated to accelerated phases of the universe’s expansion. Furthermore, we present the evolution of density, bulk viscous pressure, and the effective equation of state parameter with redshift. Those show that bulk viscosity in a cosmic fluid is a valid candidate to acquire the negative pressure to drive the cosmic expansion efficiently. We also examine the behavior of different energy conditions to test the viability of our cosmological f(Q) model. Furthermore, the statefinder diagnostics are also investigated in order to distinguish among different dark energy models.

Cosmological aspects of anisotropic chameleonic Brans–Dicke gravity

We study the cosmological implications of non-minimal coupling of a scalar field with matter sector in the Brans–Dicke gravity. In particular, in the context of locally rotationally symmetric Bianchi-I (LRS BI) metric, we derived the field equations and used these equations to find a relation between scale factor and scalar field. We use this relation to find two different solutions, namely an eternally accelerating universe and a transit universe. By the power-law solutions for scalar field and coupling function, we find the allowed range of exponents for an accelerating universe. We further explore the direction and magnitude of energy transfer between scalar field and matter sector. We show by using the second law of thermodynamics that the direction and magnitude of energy transfer are closely related to the behavior of coupling function. In the transit universe, we investigate the dependence of the effective equation of state parameter and the effective energy density on the anisotropy parameter. The model explains the late-time accelerating universe evolution.