Dark Energy Fluid Research Articles

Diffusive dark matter and dark energy scenario and k -essence in the context of Supernova Ia observations

We consider a unified model of interacting dark matter and dark energy to account for coincidence of present day dark energy and dark matter densities. We assume dark energy to be represented by a homogeneous scalar field $\phi$ whose dynamics is driven by a (non-canonical) $k$-essence Lagrangian with constant potential and the particles of dark matter fluid undergoing velocity diffusion in background medium of the $k-$essence scalar field $\phi$. This results in a transfer of energy from the fluid of dark matter to that of dark energy. This effect shows up as a source term in the continuity equation for dark matter and dark energy fluids. The source term involves a diffusion coefficient which is a measure of average energy transferred per unit time due to diffusion. We use time evolutions of the scale factor of background FRW spacetime, energy density and pressure of the dark fluid obtained from analysis of Supernova Ia data to obtain bounds on the diffusion parameter. For a constant potential in the $k$-essence Lagrangian, the temporal behaviour of a homogeneous $k$-essence field $\phi$ is obtained for different values of the diffusion parameter. The obtained temporal behaviour may be expressed as $\phi(t/t_0) = \phi_0 + \varepsilon_1 (t/t_0 - 1) + \varepsilon_2 (t/t_0 -1)^2$, where $t_0$ is the time corresponding to present epoch. The coefficients $\varepsilon_1$ and $\varepsilon_2$ have been found and obtained as linear functions of diffusion parameter.

The unphysical character of minimally coupled dark energy fluids

It is well known that, in the context of general relativity, an unknown kind of matter that must violate the strong energy condition is required to explain the current accelerated phase of expansion of the Universe. This unknown component is called dark energy and is characterized by an equation of state parameter w=p/rho with w_0<-1/3. For minimally coupled dark energy fluids, thermodynamic stability requires that 3w-dln |w|/dln age 0 and positiveness of entropy that wge -1. In this paper we proof that we cannot obtain a differentiable function w(a) to represent the dark energy that satisfies these conditions during the entire history of the Universe.

Finite scale factor and future singularities

The main characteristic of the dark energy is its negative pressure. In a homogeneous and isotropic FRW background, we consider several models for the dark energy fluid, which lead to finite time future singularities of the type I-IV, by introducing the pressure density as a function of the scale factor. This approach gives acceptable behavior of the dark energy equation of state. We give various numerical examples of models with type I-IV singularities, that show very similar late time behavior, making it difficult to determine the type of singularity that would take place in the future.

Interacting models of generalized Chaplygin gas and modified Chaplygin gas with barotropic fluid

In this paper, we consider two different models of our present universe. We choose the models which consist of different sets of two separate fluids. The first one of each set tries to justify the late time acceleration and the second one is barotropic fluid. The former model considers our present time universe to be homogeneously filled up by Generalized Chaplygin Gas which is interacting with barotropic fluid. On the other hand, the latter model considers that the cosmic acceleration is generated by Modified Chaplygin Gas which is interacting with matter depicted by barotropic equation of state (EoS). For both the models, we consider the interaction term to vary proportionally with Hubble’s parameter as well as with the exotic matter/dark energy’s energy density. We find an explicit function form of the energy density of the cosmos which is found to depend on different cosmological parameters like scale factor, dark energy and barotropic fluid’s EoS parameters and other constants, like interacting constants, etc. We draw curves of effective EoS-s, different cosmological parameters like deceleration parameter q, statefinder parameters r and s with respect to the redshift z (for different values of dark energy and barotropic fluid parameters) and study them thoroughly. We compare two models as well as the nature of dependencies on these models’ interaction coefficients. We point out the particular redshift for which the universe may transit from a deceleration to acceleration phase. We tally all these values with different observational data. Here, we also analyze how this value of particular redshift does change for different values of interaction coefficients and different dark energy models.

F(R,\xa0T) models applied to baryogenesis

This paper is devoted to the reproduction of the gravitational baryogenesis epoch in the context of f(R, T) theory of gravity, where R and T are respectively the curvature scalar and the trace of the energy-momentum tensor, respectively. It is assumed a minimal coupling between matter and gravity. In particular we consider the following two models, f(R,T) = R +alpha T + beta T^2 and f(R,T) = R+ mu R^2 + lambda T, with the assumption that the universe is filled by dark energy and perfect fluid where the baryon to entropy ratio during a radiation domination era is non-zero. We constrain the models with the cosmological gravitational baryogenesis scenario, highlighting the appropriate values of model’s parameters compatible with the observation data of the baryon-entropy ratio.

Model-independent constraints on dark energy evolution from low-redshift observations

Knowing the late time evolution of the Universe and finding out the causes for this evolution are the important challenges of modern cosmology. In this work, we adopt a model-independent cosmographic approach and approximate the Hubble parameter considering the Pade approximation which works better than the standard Taylor series approximation for $z>1$. With this, we constrain the late time evolution of the Universe considering low-redshift observations coming from SNIa, BAO, $H(z)$, $H_{0}$ , strong-lensing time-delay as well as the Megamaser observations for angular diameter distances. We confirm the tensions with $\Lambda$CDM model for low-redshifts observations. The present value of the equation of state for the dark energy has to be phantom-like and for other redshifts, it has to be either phantom or should have a phantom crossing. For lower values of $\Omega_{m0}$, multiple phantom crossings are expected. This poses serious challenges for single, non-interacting scalar field models for dark energy. We derive constraints on the {\it statefinders} $(r,s)$ and these constraints show that a single dark energy model cannot fit data for the whole redshift range $0\leq z\leq 2$: in other words, we need multiple dark energy behaviors for different redshift ranges. Moreover, the constraint on sound speed for the total fluid of the Universe, and for the dark energy fluid (assuming them being barotropic), rules out the possibility of a barotropic fluid model for unified dark sector and barotropic fluid model for dark energy, as fluctuations in these fluids are unstable as $c_{s}^2 < 0$ due to constraints from low-redshift observations.

Dynamical Anisotropic Dark Energy Cosmological Model in General Relativity

ABSTRACTWe have investigated an anisotropic dark energy cosmological model in the presence of anisotropic dark energy fluid coupled with mass less scalar field in KantowskiSachs space-time in general theory of relativity. A physically viable dark energy cosmological model is presented using (i) a hybrid expansion law proposed by Akarsu et al. (JCAP. 01, 022: 2014) and (ii) a relation between the metric potential. The dark energy density, the equation of state (EoS) parameter and the skewness parameters of the model are determined and their physical siguificance is pointed out. The deceleration parameter and the jerk parameter of the model show that there is a smooth transition of the uuiverse from early deceleration to late time acceleration which confirms the observations modem cosmology.

Bulk viscous embedded hybrid dark energy models

In this paper, we have constructed the cosmological model of the universe in a two-fluid environment with a newly developed mathematical formalism. In order to construct the model, Binachi type V (BV) space time is considered with a time varying deceleration parameter. Both the fluids, the viscous fluid and the dark energy (DE) fluid have shown their dominance respectively in early time and late time of the cosmic evolution. The scale factor that simulates the cosmic transition based on the value of the bulk viscous coefficient. Within the developed formalism, a general form of the skewness parameters is also obtained as a functional form of the scale factor. The physical parameter of the model such as equation of state (EoS) parameter is also derived and analysed. The state finder diagnostic pair is also obtained to understand the geometrical nature of the model.

Gravity in mimetic scalar-tensor theories after GW170817

We derive the most general mimetic scalar-tensor theory assuming a healthy “seed” action and accounting for the constraints on the speed of gravitational-wave propagation arising from the GW170817 event. By analysing linear perturbations around a flat FLRW background in this model, we obtain a suitable form of the Poisson equation, which allows us to calculate the effective gravitational constant felt by “ordinary” matter. By restricting to a minimally coupled model, such an effective gravitational constant is equivalent to that obtained within General Relativity, with cold dark matter plus a perfect fluid dark energy component, with vanishing sound speed. Assuming, further, a ΛCDM background, the effective gravitational constant cannot be distinguished from that of the standard ΛCDM model, at linear order. For the full non-minimally coupled mimetic gravity model we obtain a non-vanishing gravitational slip and an effective gravitational constant which always differs from that of standard ΛCDM .

Dark energy with zero pressure: Accelerated expansion and large scale structure in action-dependent Lagrangian theories

We develop a cosmological model based on action-dependent Lagrangian theories. The main feature here is the nonconservation of the energy momentum tensor due to the nontrivial geometrical construction of the theory. We provide the basic set of equations necessary to study both the cosmological background expansion as well as the linear matter perturbation growth. We show that the simplest realization of the Universe as described by only one component is not viable as expected from the existing correspondence between this model and the case of viscous cosmological fluids. However, modeling the energy content of the Universe as composed by two pressureless fluids, i.e., one a typical cold dark matter fluid and the other a pressureless dark energy fluid which is responsible for driving the late-time acceleration expansion, is qualitatively compatible with observational data.

Dynamical systems perspective of cosmological finite-time singularities in f(R) gravity and interacting multifluid cosmology

In this work we shall investigate the occurrence of future cosmological finite-time singularities in the dynamical system corresponding to two cosmological theories, namely that of vacuum $f(R)$ gravity and that of three fluids. The vacuum $f(R)$ gravity is an example for which the variables we will choose to quantify the phase space dynamics, do not necessarily blow-up near a cosmological singularity. After appropriately choosing the variables, we shall investigate the behavior of the corresponding dynamical system near some types of cosmological finite-time singularities, for some limiting cases in which we can produce analytic solutions for the dynamical variables. The most interesting case from both a mathematical and physical point of view, is the Big Rip case, and particularly in the limiting case of a very strong singularity. The physically appealing outcome is that the resulting non-autonomous dynamical system is attracted asymptotically to an accelerating attractor solution, with equation of state parameter $w_{eff}=-1$. Our analytic results, show that an extremely strong Big Rip singularity in vacuum $f(R)$ gravity theories is always related to an accelerating solution, or tends to acceleration. The converse statement though may not be true. The second cosmology we shall study is a multifluid cosmology, consisting of three fluids, the interacting dark matter and dark energy fluids, and the baryonic fluid. By appropriately choosing the variables, we will show that the dynamical system can become an autonomous polynomial dynamical system, in which case, by using a dominant balance analysis, we shall investigate the occurrence of finite-time singularities in this system.

Study of finite-time singularities of loop quantum cosmology interacting multifluids

In this work we study the occurrence of finite-time cosmological singularities in a cosmological system comprising from three fluids. Particularly, the system contains two dark fluids, namely that of dark energy and dark matter, which are interacting, and of a non-interacting baryonic fluid. For the study we adopt the phase space approach by constructing the cosmological dynamical system in such a way so that it rendered to be an autonomous polynomial dynamical system, and in order to achieve this, we appropriately choose the variables of the dynamical system. By employing a rigid mathematical framework, that of dominant balances analysis, we demonstrate that there exist non-singular solutions of the dynamical system, which correspond to a general set of initial conditions, which proves that no Big Rip or Type III finite-time singularities occur in this LQC multifluid dynamical system. Thus the new feature of this work is that we are able to do this using an analytic technique instead of adopting a numerical approach. In addition, we perform a fixed point analysis of the cosmological dynamical system, and we examine the behavior of the total effective equation of state parameter, at the fixed points, as a function of the free parameters of the system. Finally, we investigate the phenomenological implications of the dark energy equation of state which we assumed that it governs the dark energy fluid.

Implications of an extended dark energy cosmology with massive neutrinos for cosmological tensions

We perform a comprehensive analysis of the most common early- and late-Universe solutions to the $H_0$, Ly-$\alpha$, and $S_8$ discrepancies. When considered on their own, massive neutrinos provide a natural solution to the $S_8$ discrepancy at the expense of increasing the $H_0$ tension. If all extensions are considered simultaneously, the best-fit solution has a neutrino mass sum of $\sim 0.4$ eV, a dark energy equation of state close to that of a cosmological constant, and no additional relativistic degrees of freedom. However, the $H_0$ tension, while weakened, remains unresolved. Motivated by this result, we perform a non-parametric reconstruction of the evolution of the dark energy fluid density (allowing for negative energy densities), together with massive neutrinos. When all datasets are included, there exists a residual $\sim1.9\sigma$ tension with $H_0$. If this residual tension remains in the future, it will indicate that it is not possible to solve the $H_0$ tension solely with a modification of the late-Universe dynamics within standard general relativity. However, we do find that it is possible to resolve the tension if either galaxy BAO or JLA supernovae data are omitted. We find that \textit{negative} dark energy densities are favored near redshift $z\sim2.35$ when including the Ly-$\alpha$ BAO measurement (at $\sim 2\sigma$). This behavior may point to a negative curvature, but it is most likely indicative of systematics or at least an underestimated covariance matrix. Quite remarkably, we find that in the extended cosmologies considered in this work, the neutrino mass sum is always close to $0.4$ eV regardless of the choice of external datasets, as long as the $H_0$ tension is solved or significantly decreased.

Kerr\u2013Newman-AdS black hole surrounded by perfect fluid matter in Rastall gravity

The Rastall gravity is the modified Einstein general relativity, in which the energy-momentum conservation law is generalized to T^{mu nu }_{~~;mu }=lambda R^{,nu }. In this work, we derive the Kerr–Newman-AdS (KN-AdS) black hole solutions surrounded by the perfect fluid matter in the Rastall gravity using the Newman–Janis method and Mathematica package. We then discuss the black hole properties surrounded by two kinds of specific perfect fluid matter, the dark energy (omega =-,2/3) and the perfect fluid dark matter (omega =-,1/3). Firstly, the Rastall parameter kappa lambda could be constrained by the weak energy condition and strong energy condition. Secondly, by analyzing the number of roots in the horizon equation, we get the range of the perfect fluid matter intensity alpha , which depends on the black hole mass M and the Rastall parameter kappa lambda . Thirdly, we study the influence of the perfect fluid dark matter and dark energy on the ergosphere. We find that the perfect fluid dark matter has significant effects on the ergosphere size, while the dark energy has smaller effects. Finally, we find that the perfect fluid matter does not change the singularity of the black hole. Furthermore, we investigate the rotation velocity in the equatorial plane for the KN-AdS black hole with dark energy and perfect fluid dark matter. We propose that the rotation curve diversity in Low Surface Brightness galaxies could be explained in the framework of the Rastall gravity when both the perfect fluid dark matter halo and the baryon disk are taken into account.

Interaction of anisotropic dark energy fluid with perfect fluid in the presence of cosmological term Λ

Considering the locally rotationally symmetric (LRS) Bianchi type-I metric with cosmological constant [Formula: see text], Einstein’s field equations are discussed based on the background of anisotropic fluid. We assumed the condition A = B[Formula: see text] for the metric potentials A and B, where m is a positive constant to obtain the viable model of the Universe. It is found that [Formula: see text] is positive and inversely proportional to time. The values of matter-energy density [Formula: see text], dark energy density [Formula: see text] and deceleration parameter q are found to be consistent with the values of WMAP observations. State finder parameters and anisotropic deviation parameter are also investigated. It is also observed that the derived model is an accelerating, shearing and non-rotating Universe. Some of the asymptotic and geometrical behaviors of the derived models are investigated with the age of the Universe.

Effect of dark energy perturbation on cosmic voids formation

In this paper, we present the effects of dark energy perturbation on the formation and abundance of cosmic voids. We consider dark energy to be a fluid with a negative pressure characterised by a constant equation of state w and speed of sound $c^2_s$. By solving fluid equations for two components, namely, dark matter and dark energy fluids, we quantify the effects of dark energy perturbation on the sizes of top-hat voids. We also explore the effects on the size distribution of voids based on the excursion set theory. We confirm that dark energy perturbation negligibly affects the size evolution of voids;$c^2_s = 0$ varies the size only by 0.1% as compared to the homogeneous dark energy model. We also confirm that dark energy perturbation suppresses the void size when w < -1 and enhances the void size when $w > -1$ (Basse et al. 2011). In contrast to the negligible impact on the size, we find that the size distribution function on scales larger than 10 Mpc/h highly depends on dark energy perturbation; compared to the homogeneous dark energy model, the number of large voids of radius 30Mpc is 25% larger for the model with $w = -0.9$ and $c^2_s = 0$ while they are 20% less abundant for the model with $w = -1.3$ and $c^2_s = 0$.

Imprint of thawing scalar fields on the large scale galaxy overdensity

We investigate the observed galaxy power spectrum for the thawing class of scalar field models taking into account various general relativistic corrections that occur on very large scales. We consider the full general relativistic perturbation equations for the matter as well as the dark energy fluid. We form a single autonomous system of equations containing both the background and the perturbed equations of motion which we subsequently solve for different scalar field potentials. First we study the percentage deviation from the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model for different cosmological parameters as well as in the observed galaxy power spectra on different scales in scalar field models for various choices of scalar field potentials. Interestingly the difference in background expansion results from the enhancement of power from $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ on small scales, whereas the inclusion of general relativistic (GR) corrections results in the suppression of power from $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ on large scales. This can be useful to distinguish scalar field models from $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ with future optical/radio surveys. We also compare the observed galaxy power spectra for tracking and thawing types of scalar field using some particular choices for the scalar field potentials. We show that thawing and tracking models can have large differences in observed galaxy power spectra on large scales and for smaller redshifts due to different GR effects. But on smaller scales and for larger redshifts, the difference is small and is mainly due to the difference in background expansion.

Bianchi-V string cosmological model with dark energy anisotropy

The role of anisotropic components on the dark energy and the dynamics of the universe is investigated. An anisotropic dark energy fluid with different pressures along different spatial directions is assumed to incorporate the effect of anisotropy. One dimensional cosmic strings aligned along x-direction supplement some kind of anisotropy. Anisotropy in the dark energy pressure is found to evolve with cosmic expansion at least at late times. At an early phase, the anisotropic effect due to the cosmic strings substantially affect the dynamics of the accelerating universe.

Light bending, static dark energy, and related uniqueness of Schwarzschild–de Sitter spacetime

Since the Schwarzschild-de Sitter spacetime is static inside the cosmological event horizon, if the dark energy state parameter is sufficiently close to $-1$, apparently one could still expect an effectively static geometry, in the attraction dominated region inside the maximum turn around radius, $R_{\rm TA, max}$, of a cosmic structure. We take the first order metric derived recently assuming a static and ideal dark energy fluid with equation of state $P(r)=\alpha\rho(r)$ as a source in Ref. [1], which reproduced the expression for $R_{\rm TA, max}$ found earlier in the cosmological McVittie spacetime. Here we show that the equality originates from the equivalence of geodesic motion in these two backgrounds, in the non-relativistic regime. We extend this metric up to the third order and compute the bending of light using the Rindler-Ishak method. For $ \alpha\neq -1$, a dark energy dependent term appears in the bending equation, unlike the case of the cosmological constant, $\alpha=-1$. Due to this new term in particular, existing data for the light bending at galactic scales yields, $(1+\alpha)\lesssim {\cal O}(10^{-14})$, thereby practically ruling out any such static and inhomogeneous dark energy fluid we started with. Implication of this result pertaining the uniqueness of the Schwarzschild-de Sitter spacetime in such inhomogeneous dark energy background is discussed.

Anisotropic Cosmological Models with Two Fluids

Anisotropic dark energy cosmological models have been constructed in a Bianchi V space-time, with the energy momentum tensor consisting of two noninteracting fluids, namely, bulk viscous fluid and dark energy fluid. Two different models are constructed based on the power law cosmology and de Sitter universe. The constructed model was also embedded with different pressure gradients along different spatial directions. The variable equation of state (EoS) parameter and skewness parameters for both models are obtained and analysed. The physical properties of the models obtained with the use of scale factors of power law and de Sitter law are also presented.