Interacting dark energy with inhomogeneous equation of state

We have investigated the model of dark energy interacting with dark matter by choosing inhomogeneous equations of state for the dark energy and a nonlinear interaction term for the underlying interaction. The equations of state have dependencies either on the energy densities, the redshift, the Hubble parameter or the bulk viscosity. We have considered these possibilities and have derived the effective equations of state for the dark energy in each case.     

Dark energy fluid with time-dependent,inhomogeneous equation of state

The four-dimensional flat Friedman universe filled with an ideal fluid with a linear (oscillating) inhomogeneous equation of state (EoS) depending on time is studied. The equations of motion are solved. It is shown that in some cases there appears a quasi-periodical universe that repeats the cycles of phantom-type space acceleration. The appearance of future singularities resulting from various choices for the input parameters is discussed.     

Interaction between scalar field and ideal fluid with inhomogeneous equation of state

In this Letter we study a model of interaction between the scalar field and an inhomogeneous ideal fluid. We have considered two forms of the ideal fluid and a power law expansion for the scale factor. We have solved the equations for the energy densities. Also we show that besides being a dark energy model to explain the cosmic acceleration, this model shows a decaying nature of the scalar field potential and the interaction parameter.     

Cosmological model with viscosity media (dark fluid) described by an effective equation of state

A generally parameterized equation of state (EOS) is investigated in the cosmological evolution with bulk viscosity media modelled as dark fluid, which can be regarded as a unification of dark energy and dark matter. Compared with the case of the perfect fluid, this EOS has possessed four additional parameters, which can be interpreted as the case of the non-perfect fluid with time-dependent viscosity or the model with variable cosmological constant. From this general EOS, a completely integrable dynamical equation to the scale factor is obtained with its solution explicitly given out. (i) In this parameterized model of cosmology, for a special choice of the parameters we can explain the late-time accelerating expansion universe in a new view. The early inflation, the median (relatively late time) deceleration, and the recently cosmic acceleration may be unified in a single equation. (ii) A generalized relation of the Hubble parameter scaling with the redshift is obtained for some cosmology interests. (iii) By using the SNe Ia data to fit the effective viscosity model we show that the case of matter described by p=0$p=0$ plus with effective viscosity contributions can fit the observational gold data in an acceptable level.     

Effective dark energy equation of state in interacting dark energy models

In models where dark matter and dark energy interact non-minimally, the total amount of matter in a fixed comoving volume may vary from the time of recombination to the present time due to energy transfer between the two components. This implies that, in interacting dark energy models, the fractional matter density estimated using the cosmic microwave background assuming no interaction between dark matter and dark energy will in general be shifted with respect to its true value. This may result in an incorrect determination of the equation of state of dark energy if the interaction between dark matter and dark energy is not properly accounted for, even if the evolution of the Hubble parameter as a function of redshift is known with arbitrary precision. In this Letter we find an exact expression, as well as a simple analytical approximation, for the evolution of the effective equation of state of dark energy, assuming that the energy transfer rate between dark matter and dark energy is described by a simple two-parameter model. We also provide analytical examples where non-phantom interacting dark energy models mimic the background evolution and primary cosmic microwave background anisotropies of phantom dark energy models.     

Observational constraints on FRW cosmologies with dark energy-dark matter interaction

In this paper, within the scope of FRW cosmology for $k=0, \pm 1$ , we investigate the dynamics of the universe in cosmological model where a scalar field nonminimally is coupled to matter field. By best-fitting the model parameters with the observational data, for the direct interaction between the dark sectors in the model, we obtain new constraints on cosmological parameters. The result with the best fitted model parameters supports the current universe acceleration in all models and shows that only in flat universe case the phantom crossing occurs twice in the past and once in the future. The best fitted reconstructed potential function and other physical functions are also obtained.     

A fundamental length as a candidate for dark energy: A DSR inspired FRW spacetime

We show that the existence of a fundamental length, introduced in Deformed Special Relativity (DSR) inspired minisuper-(phase-)space, causes the behavior of the scale factor of the universe to change from that of a universe filled with dust to an accelerating universe driven by a cosmological constant.     

Percus-Yevick equation for an inhomogeneous fluid: Critical region

The critical region of a locally nonuniform fluid with gaussian density inhomogeneity is investigated. Nonclassical behaviour is found; critical exponents agree very well with those obtained from the perturbation theory of liquids. The action of an external field shifts the critical region and weakens the critical behaviour. Discussed effects could be verified experimentally.     

A wave operator for a non-linear Klein-Gordon equation

We prove the existence of a set of initial data to which correspond solutions of the nonlinear Klein-Gordon equation with a polynomial nonlinear term, which converge asymptotically, when t→+∞, to solutions of the linear Klein-Gordon equation.     

Reconstruction of a kinetic k-essence Lagrangian from a modified of dark energy equation of state

In this paper the Lagrangian density of a purely kinetic k-essence model that describes the behavior of dark energy described by four parameterized equations of state proposed by Cooray and Huterer (Astrophys J 513:L95, 1999), Zhang and Wu (Mod Phys Lett A 27:1250030, 2012), Linder (Phys Rev Lett 90:091301, 2003), Efstathiou (Mon Not R Astron Soc 310:842, 2000), and Feng and Lu (J Cosmol Astropart Phys 1111:34, 2011) has been reconstructed. This reconstruction is performed using the method outlined by de Putter and Linder (Astropart Phys 28:263, 2007), which makes it possible to solve the equations that relate the Lagrangian density of the k-essence with the given equation of state (EoS) numerically. Finally, we discuss the observational constraints for the models based on 1049 SNIa data points from the Pantheon data set compiled by Scolnic et al. (Astrophys J 859(2):101, 2018)     

An exact FRW cosmology with radiation and imperfect fluid

An exact solution of the Einstein field equations for a combination of black-body radiation and an imperfect fluid, in which the geometrical background is a flat FRW metric, is presented. The solution exhibits an axial preferred direction along which the material content moves relative to the radiation field, the latter representing the cosmic background radiation. The solution is shown to be in excellent agreement with current observations.     

Investigating the possibility of a turning point in the dark energy equation of state

We investigate a second order parabolic parametrization,w(a)=wt+wa(at-a)2,which is a direct characterization of a possible turning in w.The cosmological consequence of this parametrization is explored by using the observational data of the SNLS3 type Ia supernovae sample,the CMB measurements from WMAP9 and Planck,the Hubble parameter measurement from HST,and the baryon acoustic oscillation(BAO)measurements from 6dFGS,BOSS DR11 and improved WiggleZ.We found the existence of a turning point in w at a～0.7 is favored at 1σCL.In the epoch 0.55a0.9,w-1 is favored at 1σCL,and this significance increases near a=0.8,reaching a 2σCL.The parabolic parametrization achieve equivalent performance to theΛCDM and Chevallier-Polarski-Linder(CPL)models when the Akaike information criterion was used to assess them.Our analysis shows the value of considering high order parametrizations when studying the cosmological constraints on w.     

Equation of state of a supercritical fluid based on the Ornstein-Zernike equation

A numerical modeling of the thermodynamic properties of a fluid is performed using the method of integral equations. The predictions are compared with the results of MC and MD simulations. The problem of stability of the numerical solution is examined. The methods for correcting the correlation functions and for estimating their uncertainties are proposed.     

An accurate equation of state of a hard convex body fluid mixture

A method enabling to calculate the contact-point values of the pair correlation function of convex body fluids from a semi-empirical equation of state is presented and the accurate Nezbeda equation of the pure hard convex body fluid is extended to mixtures. Comparison of results for one- and two-component systems with Monte Carlo simulation data shows excellent agreement.     

Probing dark energy using its density instead of its equation of state

The variation of dark energy density with redshift, ρX(z)${\rho }_X(z)$, provides a critical clue to the nature of dark energy. Since ρX(z)${\rho }_X(z)$ depends on the dark energy equation of state wX(z)$w_X(z)$ through an integral, ρX(z)${\rho }_X(z)$ can be constrained more tightly than wX(z)$w_X(z)$ given the same observational data. We demonstrate this explicitly using current type Ia supernova (SN Ia) data [the Tonry/Barris sample], together with the Cosmic Microwave Background (CMB) shift parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure (LSS) growth factor from 2dF galaxy survey data. We assume a flat universe, and use Markov Chain Monte Carlo (MCMC) technique in our analysis. We find that, while wX(z)$w_X(z)$ extracted from current data is consistent with a cosmological constant at 68% C.L., ρX(z)${\rho }_X(z)$ (which has far smaller uncertainties) is not. Our results clearly show the advantage of using ρX(z)${\rho }_X(z)$, instead of wX(z)$w_X(z)$, to probe the variation of dark energy.     

Constraints on the phase plane of the dark energy equation of state

Classification of dark energy models in the plane of w   and w^′$w^{\prime }$, where w   is the dark energy equation of state and w^′$w^{\prime }$ its time-derivative in units of the Hubble time, has been studied in the literature. We take the current SN Ia, CMB and BAO data, invoke a widely used parametrization of the dark energy equation of state, and obtain the constraints on the w–w^′$w\text{–}{w}^{\prime }$ plane. We find that several dark energy models including the cosmological constant, phantom, non-phantom barotropic fluids, and monotonic up-rolling quintessence are ruled out at the 68.3% confidence level based on the current observational data. On the other hand, down-rolling quintessence, including the thawing and the freezing models, is consistent with the current observations. All the above-mentioned models are still consistent with the data at the 95.4% confidence level.     

The equation of state of the hard-disc fluid revisited

Some points about the search for analytical expressions for the equation of state of the hard-disc fluid are discussed in the light of the most recent advances in the field. New and accurate equations of state for this fluid are proposed.     

Oscillations in the dark energy equation of state: New MCMC lessons

We study the possibility of detecting oscillating patterns in the equation of state (EoS) of the dark energy using different cosmological datasets. We follow a phenomenological approach and study three different oscillating models for the EoS, one of them periodic and the other two damped (proposed here for the first time). All the models are characterized by the amplitude, the center and the frequency of oscillations. In contrast to previous works in the literature, we do not fix the frequency to a fiducial value related to the time extension of chosen datasets, but consider a discrete set of values, so to avoid arbitrariness and try to detect any possible time period in the EoS. We test the models using a recent collection of SNeIa, direct Hubble data and Gamma Ray Bursts data. Main results are: I. even if constraints on the amplitude are not too strong, we detect a trend of it versus the frequency, i.e. decreasing (and even negatives) amplitudes for higher frequencies; II. the center of oscillation (which corresponds to the present value of the EoS parameter) is very well constrained, and phantom behavior seems statistically disfavored; III. the frequency is hard to constrain, showing similar statistical validity for all the values of the discrete set chosen, but the best fit of all the considered scenarios is associated with a period which is in the redshift range depicted by our cosmological data. The “best” oscillating models are compared with ΛCDM using different dimensionally consistent and Bayesian-based information criteria; the conclusion is reached that at present, data cannot discriminate between a cosmological constant and oscillating equation of state.     

The virial equation of fluid state and non-classical criticality

The analysis of fluid criticality in the framework of the “virial” approach leads, usually, to pure classical results. In the present paper we propose a consistent procedure allowing us to find the non-classical critical parameters of real fluid starting from the representation of the pressure near the critical point as the sum of a regular (a few of the first terms of the virial series) and a singular (non-analytical “remainder” of the series) part. The critical temperature, density, and the singular contribution to the fluid pressure are found self-consistently using not two(by van der Waals) but three(as in the fluctuation theory) conditions on the density partial derivatives of the pressure at the critical point. The calculated critical parameters converge rather rapidly to some limiting values as the number of terms in the regular part of the representation of the pressure is increased. Our calculations (when taking account of the virial terms up to the sixth one, inclusively) are in accordance, on the whole, with the numerical “experiments” data for the Lennard-Jones fluid, although we predict a somewhat greater (approximately 10%) value of the critical density. The refinement of the obtained results can be achieved when using more precise values of the higher (i.e. the seventh onwards) virial coefficients.