State Parameter Of Dark Energy Research Articles

Phantom cosmologies from the simplest parameterization of the DE model using observational data in a BI type universe

To scrutinize the nature of dark energy, many equations of state have been proposed. In this context, we examine the simplest parameterization of the equation of state parameter of dark energy in an anisotropic Bianchi type I universe compared with the ΛCDM model. Using different combinations of data samples, including Pantheon and Pantheon + H(z), alongside applying the minimization of the function of the distance modulus of data samples, we obtain the constrained values of cosmographic parameters in the parameterization of the dark energy scenario. One condition of the phantom barrier crossing is acquired. Several physical properties of the universe are discussed by considering the anisotropy effect and different observational data points. One should note that the deductions of the cosmological parameter verify recent observational data.

Higher-Order mimetic gravity after GW170817

On 17 August 2017, the thriving discovery of gravitational wave event GW170817 and its optical counterpart GRB170817A, owing to coalescing of two neutron stars, divulged a very small amount of difference of around [Formula: see text] between traveling speed of light and the velocity of gravitational waves [Formula: see text]. This small deviation can be used as a strong constraint on modified gravity models. We concentrate on the Higher-Order expansion of Mimetic Gravity (HOMimG) model to specify the parametric space of three parameters of our model ([Formula: see text], [Formula: see text], and [Formula: see text]) utilizing the observational constraint from GW170817–GRB170817A on [Formula: see text], besides two theoretical constraints on [Formula: see text] and [Formula: see text] due to assurance of the stability of the model and subluminal promulgation of the scalar and tensor perturbations. Thereafter, we increase the accuracy of the parametric space with the aid of imposing further limitation of [Formula: see text] parameter (related to the age of the universe). In pursuance of determining the other parameter of the model [Formula: see text], the potential of the model is specified, and another observational bound related to the Equation of State parameter of dark energy is taken into account. In consequence, we attain a viable HOMimG model confined to numbers of observational and theoretical constraints. At the end, regarding the concluded numerical ranges for the model parameters, and cogitating two different potential (quadratic and quartic potentials) to specify [Formula: see text] parameter, we illustrate that the values of the model parameters are independent of the form of potential.

Forecast analysis on interacting dark energy models from future generation PICO and DESI missions

ABSTRACT The next-generation cosmic microwave background (CMB) satellite missions are expected to provide robust constraints on a wide range of cosmological parameters with unprecedented precision. But these constraints on the parameters could weaken if we do not attribute dark energy to a cosmological constant. The cosmological models involving interaction between dark energy and dark matter can give rise to comparable energy densities at the present epoch, thereby alleviating the so-called cosmic coincidence problem. In the present paper, we perform a forecast analysis to test the ability of the future generation high-sensitive CMB, and baryon acoustic oscillation (BAO) experiments to constrain phenomenological interacting dark energy models. We consider cosmic variance limited future CMB experiment Probe of Inflation and Cosmic Origins (PICO) along with BAO information from the Dark Energy Spectroscopic Instrument (DESI), to constrain the parameters of the interacting dark sector. Based on the stability of the cosmological perturbations, we consider two possibilities for the interaction scenario. We investigate the impact of both coupling constant and equation of state parameter of dark energy on CMB temperature power spectrum, matter power spectrum, and fσ8. We have used simulated temperature and polarization data from PICO within the multipole ranges (ℓ = 2–4000), and as expected, we do see PICO alone produces better constraints than Planck on the ΛCDM parameters. With the integration of PICO and DESI missions, we observe a significant improvement in the constraints on several cosmological parameters, especially the equation of state parameter of dark energy. However, we note that additional data are required to constrain a small positive coupling constant.

Crossing Phantom Divide in f(Q)$f(Q)$ Gravity

AbstractThe possibility of crossing a phantom divide line in the extension of symmetric teleparallel gravity or the gravity, where Q is the non‐metricity, is investigated. The cosmic evolution of the effective equation of state parameter for dark energy is studied considering exponential, logarithmic, and combined theories. Moreover, the exponential model behaves like the ΛCDM at high redshifts before deviating to or , respectively, depending on the value of model parameter. It also approaches a desitter phase asymptotically. However, the crossing of the phantom divide line, that is, , is realized in the combined theory. Furthermore, statefinder diagnostics are studied in order to differentiate between several dark energy models. To ensure the three model's stability, the stability analysis is employed using linear perturbations. It is demonstrated how to reassemble via a numerical inversion approach based on existing observational constraints on cosmographic parameters and the potential of bridging the phantom divide in the resulting model. It explicitly demonstrates that future crossings of the phantom dividing line are a generic feature of feasible gravity models.

Interacting dynamical dark energy with stable high-scale cosmological perturbations at radiation-dominated epoch

In this paper, the cosmological perturbations of dynamical dark energy and dark matter, which interact non-gravitationally are studied. This dark energy–dark matter non-gravitational interaction is linearly dependent on the energy densities of dark components. However, in the interacting quintessence dark energy models with such type of interaction, the non-adiabatic instabilities of cosmological perturbations at radiation-dominated epoch arise. To avoid this problem, the model of dynamical dark energy was chosen as the basis. Here, the equation of state parameter of dark energy evolves in time but can be assumed constant at early epoch, so this model can be tuned in such a way that the non-adiabatic instabilities would not appear. The drawback of this cosmological model is that the energy densities of dark components can take the negative values for the certain range of interaction parameter, so the conditions for positivity of dark components densities were derived. Using obtained constraints on the model parameters, the impact of coupling parameter on modification of cosmological perturbations’ evolution is analyzed.

Anisotropic Universe with Manimally Interacting Matter and Holographic Dark Energy

The holographic dark energy (HDE), a form of dark energy, has been a useful tool in explaining the recent phase transition of the universe. In this paper, we study the anisotropic and homogeneous Bianchi type-III model of the universe filled with minimally interacting matter and holographic dark energy under the framework of the Brans-Dicke (BD) scalar tensor theory of gravitation. Considering two physically plausible conditions such as, (i) the special law of variation for Hubble parameter and (ii) the scalar expansion proportional to the shear scalar, we propose two new models namely, exponential expansion model and power law expansion model. We also show the dynamics of these models fit with existing observational data and literature thereof. The transit behavior of the equation of state parameter for dark energy has been analyzed graphically. The jerk parameter is also studied for both of the models describing cosmological evolution. The Chittagong Univ. J. Sci. 42(1): 39-57, 2020

Phase transition and shadow behaviors of quintessential black holes in M-theory/superstring inspired models

We study [Formula: see text]-dimensional black holes surrounded by dark energy (DE), embedded in [Formula: see text]-dimensional M-theory/superstring inspired models having [Formula: see text] space–time where [Formula: see text]. We focus first on the thermodynamical Hawking–Page phase transitions, whose microscopical origin is linked to [Formula: see text] coincident [Formula: see text]-branes supposed to live in such inspired models. Interpreting the cosmological constant as the number of colors, we compute various thermodynamical quantities in terms of the brane number, the entropy and the DE contribution. Calculating the ordinary chemical potential conjugated to the number of colors, we show that a generic black hole is more stable for a larger number of branes in lower dimensions. In the presence of DE, however, we find that the DE state parameter [Formula: see text] takes particular values, for [Formula: see text] models, providing nontrivial phase transitions. Then, we examine some optical properties. Concretely, we investigate shadow behaviors of quintessential black holes in terms of [Formula: see text]-brane physics. In terms of certain ratios, we find similar behaviors for critical quantities and shadow radius.

Interacting Chaplygin gas cosmology

In this paper, we study in FRW universe the interaction between the viscous Chaplygin gas and the dark energy in the modified gravity theory F(R,T), where T and R are the trace of the energy-momentum tensor and is the Ricci scalar respectively. In this model, the universe is dominated by both components Chaplygin gas and dark energy. We discuss the effect of the bulk viscosity on the dark energy state parameter, this study was carried for two interactions after obtaining the modified Friedmann equations. Finally, we found that both interaction models lead to quintessence phase of the dark energy.

Kerr-AdS black hole behaviors from dark energy

We investigate the critical behaviors of four-dimensional Kerr-AdS black holes from quintessential Dark Energy (DE) contributions. Using a moduli space, coordinated by the DE state parameter [Formula: see text] and the quintessence field intensity [Formula: see text], we deal with three different [Formula: see text]-models. By elaborating analytical formulas of relevant thermodynamical quantities denoted by [Formula: see text], we find significant similarities and distinctions. Precisely, for the [Formula: see text]-model, we show that DE contributions stabilize such black holes. For the [Formula: see text]-model, however, we get a reversed DE effect. In the [Formula: see text]-model, Kerr-AdS black holes reveal a resistance regarding the usual DE effects. Exploiting the explicit formulas of such thermodynamical quantities, we give certain physical interpretations for thermal behaviors. Although such relevant distinctions, we show that the [Formula: see text]-models involve similar universal ratios associated with certain critical thermodynamical quantities. Then, we analyze the photon orbits in the presence of DE.

Understanding the phenomenology of interacting dark energy scenarios and their theoretical bounds

Non-gravitational interaction between dark matter and dark energy has been considered in a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) universe. The interaction rate is assumed to be linear in the energy densities of dark matter and dark energy and it is also proportional to the Hubble rate of the FLRW universe. This kind of interaction model leads to an autonomous linear dynamical system, and depending on the coupling parameters, could be solved analytically by calculating the exponential of the matrix, defining the system. We show here that such interaction rate has a very deep connection with some well known cosmological theories. We then investigate the theoretical bounds on the coupling parameters of the interaction rate in order that the energy densities of the dark sector remain positive throughout the evolution of the universe and asymptotically converge to zero at very late times. Our analyses also point out that such linear interacting model may encounter with finite time future singularities depending on the coupling parameters as well as the dark energy state parameter.

Cosmological implications and thermodynamics of some reconstructed modified gravity models

Abstract In the context of extended teleparallel gravity theory with Gauss–Bonnet term, we reconstruct some gravitational Lagrangian functions f ( T , T G ) = T G g ( T ) and f ( T , T G ) = − T + T G g ( T ) , using some holographic dark energy models. These models include Tsallis, Renyi and Sharma–Mittal holographic dark energy models. We consider the flat FRW universe model with perfect fluid. We check the behavior of these reconstructed models through equation of state parameter for dark energy dominated era along with its plane of evolutionary parameter to check the thawing and freezing regions. In order to check the stability of the models, we explore the squared speed of sound for these models. Also, we discuss the thermodynamical analysis by checking the validity of generalized second law of thermodynamics of these reconstructed models.

Dark energy model in higher-dimensional FRW universe with respect to generalized entropy of Sharma and Mittal of flat FRW space–time

In this study, we report the state parameter of dark energy in higher dimensional Friedmann–Robertson–Walker (FRW) space–time according to generalized entropy of Sharma and Mittal. In this case we analyze the state parameter of dark energy according to today’s observational evidence.

Improving cosmological parameter estimation with the future 21 cm observation from SKA

Future observations of 21 cm emission from neutral hydrogen survey will become a promising approach to probe the large scale structure of the Universe. In this paper, we investigate the impacts of Square Kilometre Array (SKA) 21 cm observation on the estimation of cosmological parameters. We use the simulated data of the baryonic acoustic oscillation (BAO) measurements based on the future SKA experiment with the intensity mapping (IM) technique to do the analysis. For the current observations, we use the latest cosmic microwave background (CMB) observation from Planck, the optical BAO measurements, and the Type Ia supernovae (SN) observation (Pantheon compilation). We find that the SKA mock data could break the degeneracy between the matter density and the Hubble constant, further improving the cosmological constraints to a great extent. We also find that the constraint on the equation of state parameters of dark energy could be significantly improved by including the SKA mock data into the cosmological global fit.

Axially magnetized dark energy cosmological model

We investigate the behavior of the skewness parameters for an anisotropic universe in the framework of General Relativity. Non-interacting dark energy is considered in presence of electromagnetic field. A time-varying deceleration parameter simulated by a hybrid scale factor is considered. The dynamics of the universe is investigated in presence and absence of magnetic field. The equation of state parameter of dark energy evolves within the range predicted by the observations. Magnetic field is observed to have a substantial effect on the cosmic dynamics and the skewness parameters. The models discussed here end in a big rip and become isotropic at finite time.

Revisiting the Instability Problem of the Interacting Dark Energy Model in the Parameterized Post-Friedmann Framework

Abstract Dark energy might interact with dark matter in a direct, non-gravitational way, which can help remedy several theoretical defects. In order to find out the properties of interacting dark energy (IDE) models, it is necessary to investigate the cosmological perturbations in detail. However, due to the improper use of pressure perturbation of dark energy, a large-scale instability at the early stage occurs occasionally. In recent years, parameterized post-Friedmann approach has been used to calculate the perturbation equations in the IDE scenario. Under this framework, the dark energy pressure perturbation was replaced by the relationship between the momentum density of dark energy and other components on a large scale. However, this paper shows that if the interaction terms are related to the velocity perturbation of dark energy, the density perturbation of dark matter, and the matter power spectrum will diverge when the equation of state parameter of dark energy w is close to −1. A simple parameterization steering clear of this problem is proposed in this paper, which is a more general form and can be applied to explore the interaction between dark matter and dark energy by using various cosmological data.

Dark energy and dark matter interaction in light of the second law of thermodynamics

In the context of thermodynamics we discuss the way inevitable emerge an interaction between dark components, and in this way, provide a mechanism to understand the limits of the LCDM model and the class of interaction models between dark components. Using observational data we have tested two particular models of explicit interaction between dark components and reconstructed the evolution of temperatures for both components. We found that observations suggest the interaction exist with energy flowing from dark energy to dark matter. The best fit also suggest a phantom equation of state parameter for dark energy. We discuss the results having in mind the constraints imposed by thermodynamics.

Generalization of the Randall-Sundrum Model Using Gravitational Model F(T,Θ)

In this letter, we explore a generalized model based on two scenarios including the Randall-Sundrum model and gravity model F(T,Θ). We first study the standard Randall-Sundrum gravitational model and then add a function containing two parameters as torsion and trace energy-momentum tensor to the main action of the model. Next, we derive the equations of the generalized model and obtain a new critical value for the energy density of the brane. The results showed that inflation and the dark energy-dominated stage can be realized in this model. We pointed out one significant category of dark energy models that had greatly developed the knowledge about dark energy. To be specific, dark energy could either be quintessence-like, phantom-like, or the so-called “quintom”-like. The models of quintom type suggest that the equation of state parameter of dark energy can cross the cosmological constant boundary ω=-1. Interestingly, this quintom scenario exactly appeared in this paper.

Scenario of two-fluid dark energy models in Bianchi type-III Universe

In this paper, we have analyzed the equation of state parameter for dark energy in the spatially homogeneous and anisotropic Bianchi type-III spacetime filled with a barotropic fluid and dark energy by considering a variable deceleration parameter. To solve the Einstein field equations, we assume that the expansion scalar is proportional to the shear scalar. We find that during the evolution of the Universe, the equation of state and the cosmological parameters are positive decreasing functions of cosmic time [Formula: see text]. Also, we discuss the physical parameters, as well as the jerk and cosmic span parameters, which predict that the Universe in this model approaches the [Formula: see text]CDM model at late times.

Exploring the distance-redshift relation with gravitational wave standard sirens and tomographic weak lensing

Gravitational waves from inspiraling compact objects provide us with information of the distance scale since we can infer the absolute luminosity of the source from analysis of the wave form, which is known as standard sirens. The first detection of the gravitational wave signal of the binary black hole merger event by Advanced LIGO has opened up the possibility of utilizing standard sirens as cosmological probe. In order to extract information of the distance-redshift relation, we cross-correlate weak lensing, which is an unbiased tracer of matter distribution in the Universe, with the projected number density of gravitational wave sources. For weak lensing, we employ tomography technique to efficiently obtain information of large-scale structures at wide ranges of redshifts. Making use of the cross-correlations along with the auto-correlations, we present forecast of constraints on four cosmological parameters, i.e., Hubble parameter, matter density, the equation of state parameter of dark energy, and the amplitude of matter fluctuation. To fully explore the ability of cross-correlations, which require large overlapping sky coverage, we consider the specific case with the upcoming surveys by \textit{Euclid} for weak lensing and Einstein Telescope for standard sirens. We show that cosmological parameters can be tightly constrained solely by these auto- and cross-correlations of standard sirens and weak lensing. For example, the $1\text{-}\sigma$ error of Hubble parameter is expected to be $\sigma (H_0) = 0.33 \, \mathrm{km} \, \mathrm{s}^{-1} \, \mathrm{Mpc}^{-1}$. Thus, the proposed statistics will be a promising probe into the distance scale.

Dark energy cosmological models with cosmic string

In this paper we have studied the anisotropic Kantowski-Sachs, locally rotationally symmetric (LRS) Bianchi type-I and LRS Bianchi type-III geometries filled with dark energy and one dimensional cosmic string in the Saez-Ballester theory of gravitation. To get physically valid solution we take hybrid expansion law of the average scale factor which is a product of power and exponential type of functions that results in time dependent deceleration parameter ( $q$ ). The equation of state parameter of dark energy ( $\omega _{\mathit{de}}$ ) has been discussed and we have observed that for the three models it crosses the phantom divide line ( $\omega _{\mathit{de}} = -1$ ) and shows quintom like behavior. The density of dark energy ( $\rho _{\mathit{de}}$ ) is an increasing function of redshift and remains positive throughout the evolution of the universe for the three models. Moreover in Kantowski-Sachs and LRS Bianchi type-I geometries the dark energy density dominates the string tension density ( $\lambda $ ) and proper density ( $\rho $ ) throughout the evolution of the universe. The physical and geometrical aspects of the statefinder parameters ( $r,s$ ), squared speed of sound ( $v_{s}^{2} $ ) and $\omega _{\mathit{de}}$ – $\omega ^{\prime }_{\mathit{de}}$ plane are also discussed.