Search

This paper is dedicated to the study of the geometric composition of a perfect fluid space-time with a conformal Ricci-Bourguignon soliton, which is the extended version of the soliton to the Ricci-Bourguignon flow. Here, we have delineated the conditions for conformal Ricci-Bourguignon soliton to be expanding, steady, or shrinking. We have studied certain curvature conditions on the spacetime that admit conformal Ricci-Bourguignon soliton. We have also discussed conformal Ricci-Bourguignon soliton on some special types of perfect fluid spacetime such as dust fluid, dark fluid, and radiation era. [ABSTRACT FROM AUTHOR]

Abstract We compare supernovae and Baryon Acoustic Oscillations data to the predictions of a cosmological model of interacting dark matter and dark energy. This theoretical model can be derived from the effective field theory of Einstein–Cartan gravity with two scaling exponents δ G and δ Λ , related to the interaction between dark matter and dark energy. We perform a χ 2 fit to the data to compare and contrast it with the standard ΛCDM model. We then explore the range of parameter of the model which gives a better χ 2 than the standard cosmological model. All those results lead to tight constraints on the scaling exponents of the model. Our conclusion is that this class of models, provides a decent alternative to the ΛCDM model. [ABSTRACT FROM AUTHOR]

In this work, under the assumption that the universe is locally rotationally symmetric (LRS) Bianchi type-II and filled with anisotropic dark fluid (dark energy and dark radiation), we investigate whether the generalized second law of thermodynamics (GSLT) is still valid. We also consider another important condition for LRS Bianchi type-II spacetimes whereby the model expansion θ is proportional to the shear σ, which leads to B = An (where n is a constant and A and B are the metric potentials). Based on this, we derive a general circumstance for the GSLT and find that its validity depends on the anisotropies of both the dark uid and the metric potentials. We conclude that the GSLT still holds at all times even if these two anisotropic conditions are removed from the results. [ABSTRACT FROM AUTHOR]

A unification of dark matter and dark energy in terms of a logotropic perfect dark fluid has recently been proposed, where deviations with respect to the standard ΛCDM model are dependent on a single parameter B . In this paper we show that the requirement that the linear growth of cosmic structures on comoving scales larger than 8 h − 1 Mpc is not significantly affected with respect to the standard ΛCDM result provides the strongest limit to date on the model ( B < 6 × 10 − 7 ), an improvement of more than three orders of magnitude over previous upper limits on the value of B . We further show that this limit rules out the logotropic Unified Dark Energy model as a possible solution to the small scale problems of the Λ CDM model, including the cusp problem of Dark Matter halos or the missing satellite problem, as well as the original version of the model where the Planck energy density was taken as one of the two parameters characterizing the logotropic dark fluid. [ABSTRACT FROM AUTHOR]

We propose a new pressure-parametrization model to explain the accelerated expansion of the late-time Universe by considering the baryon matter and dark contents (dark matter and dark energy) as a unified dark fluid. To realize this model more physically, we reconstruct it with the quintessence and phantom scalar fields, respectively. We use the recent cosmological data to constrain this model, distinguish it from the standard cosmological model and find that the value of the Hubble constant H0 = 68.34-0.92+0.53 supports the global measurement by the Planck satellite at the 1s confidence level. [ABSTRACT FROM AUTHOR]

We propose a heuristic unification of dark matter and dark energy in terms of a single “dark fluid” with a logotropic equation of state P = A ln ⁡ ( ρ / ρ P ) , where ρ is the rest-mass density, ρ P = 5.16 × 10 99 g m − 3 is the Planck density, and A is the logotropic temperature. The energy density ϵ is the sum of a rest-mass energy term ρ c 2 ∝ a − 3 mimicking dark matter and an internal energy term u ( ρ ) = − P ( ρ ) − A = 3 A ln ⁡ a + C mimicking dark energy ( a is the scale factor). The logotropic temperature is approximately given by A ≃ ρ Λ c 2 / ln ⁡ ( ρ P / ρ Λ ) ≃ ρ Λ c 2 / [ 123 ln ⁡ ( 10 ) ] , where ρ Λ = 6.72 × 10 − 24 g m − 3 is the cosmological density and 123 is the famous number appearing in the ratio ρ P / ρ Λ ∼ 10 123 between the Planck density and the cosmological density. More precisely, we obtain A = 2.13 × 10 − 9 g m − 1 s − 2 that we interpret as a fundamental constant. At the cosmological scale, our model fulfills the same observational constraints as the ΛCDM model (they will differ in about 25 Gyrs when the logotropic universe becomes phantom). However, the logotropic dark fluid has a nonzero speed of sound and a nonzero Jeans length which, at the beginning of the matter era, is about λ J = 40.4 pc , in agreement with the minimum size of the dark matter halos observed in the universe. The existence of a nonzero Jeans length may solve the missing satellite problem. At the galactic scale, the logotropic pressure balances the gravitational attraction, providing halo cores instead of cusps. This may solve the cusp problem. The logotropic equation of state generates a universal rotation curve that agrees with the empirical Burkert profile of dark matter halos up to the halo radius. In addition, it implies that all the dark matter halos have the same surface density Σ 0 = ρ 0 r h = 141 M ⊙ / pc 2 and that the mass of dwarf galaxies enclosed within a sphere of fixed radius r u = 300 pc has the same value M 300 = 1.93 × 10 7 M ⊙ , in remarkable agreement with the observations [Donato et al. [10] , Strigari et al. [13] ]. It also implies the Tully–Fisher relation M b / v h 4 = 44 M ⊙ km − 4 s 4 . We stress that our model has no free parameter. [ABSTRACT FROM AUTHOR]

Cosmic Dark Fluid is considered as a non-stationary medium, in which electromagnetic waves propagate, and magneto-electric field structures emerge and evolve. A medium-type representation of the Dark Fluid allows us to involve in its analysis the concepts and mathematical formalism elaborated in the framework of classical covariant electrodynamics of continua, and to distinguish dark analogs of well-known medium-effects, such as optical activity, pyro-electricity, piezo-magnetism, electro- and magneto-striction and dynamo-optical activity. The Dark Fluid is assumed to be formed by a duet of a Dark Matter (a pseudoscalar axionic constituent) and Dark Energy (a scalar element); respectively, we distinguish electrodynamic effects induced by these two constituents of the Dark Fluid. The review contains discussions of 10 models, which describe electrodynamic effects induced by Dark Matter and/or Dark Energy. The models are accompanied by examples of exact solutions to the master equations, correspondingly extended; applications are considered for cosmology and space-times with spherical and pp-wave symmetries. In these applications we focused the attention on three main electromagnetic phenomena induced by the Dark Fluid: first, emergence of Longitudinal Magneto-Electric Clusters; second, generation of anomalous electromagnetic responses; third, formation of Dark Epochs in the Universe history. [ABSTRACT FROM AUTHOR]

In this work, we model galactic haloes describing the dark matter as a non-zero pressure fluid and derive, not impose, a dark matter equation of state by using observational data of the rotation curves of galaxies. In order to reach hydrostatic equilibrium, as expected for the halo, it is mandatory that dark fluid's pressure should not be zero. The equation of state is obtained by solving the matter-geometry system of equations assuming different dark matter density or rotational velocity profiles. The resulting equations of state are, in general, different to a barotropic equation of state. The free parameters of the equation of state are fixed by fitting the observed rotational velocities of a set of galaxies. [ABSTRACT FROM AUTHOR]

Anisotropic dark energy cosmological models are constructed in the frame work of generalised Brans-Dicke theory with a self-interacting potential. A unified dark fluid characterised by a linear equation of state is considered as the source of dark energy. The shear scalar is considered to be proportional to the expansion scalar simulating an anisotropic relationship among the directional expansion rates. The dynamics of the universe in the presence of a unified dark fluid in anisotropic background have been discussed. The presence of an evolving scalar field makes it possible to get an accelerating phase of expansion even for a linear relationship among the directional Hubble rates. It is found that the anisotropy in expansion rates does not affect the scalar field, the self-interacting potential, but it controls the non-evolving part of the Brans-Dicke parameter. [ABSTRACT FROM AUTHOR]

A five dimensional Kaluza-Klein space time is considered with wet dark fluid (WDF) source in the framework of f(R, T) gravity, where R is the Ricci scalar and T is the trace of the energy-momentum tensor proposed by Harko et al. (Phys. Rev. D 84, 024020, (2011)). A new equation of state in the form of WDF has been used for dark energy (DE) component of the universe. It is modeled on the equation of state p = ω(ρ - ρ*) which can be describing a liquid, for example water. The exact solutions to the corresponding field equations are obtained for power law and exponential law of the volumetric expansion. The geometrical and physical parameters for both the models are studied. The model obtained here may represent the inflationary era in the early universe and the very late time of the universe. This model obtained here shows that even in the presence of wet dark fluid, the universe indicates accelerated expansion of the universe. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the five-dimensional Kaluza-Klein space time with wet dark fluid (WDF), which is a candidate for dark energy (DE), in the framework of f(R, T) gravity. R and T denote the Ricci scalar and the trace of the energy-momentum tensor, respectively (Harko et al. Phys. Rev. D, 84, 024020 (2011)). We have used equation of state in the form of WDF for the DE component of the universe. It is modeled on the equation of state p =ω(p-p*). With the help of the power law and exponential law of volumetric expansion, we have derived the exact solutions of the corresponding field equations. The geometrical and physical parameters for both the models are studied. The model obtained here may represent the inflationary era in the early universe and very late time of the universe. It is concluded that the model obtained here shows that even in the presence of WDF, the universe indicates accelerated expansion of the universe. [ABSTRACT FROM AUTHOR]

In this paper, we test the spherical collapse of a viscous unified dark fluid (VUDF) which has constant adiabatic sound speed and show the nonlinear collapse for VUDF, baryons, and dark matter, which are important in forming the large-scale structure of our Universe. By varying the values of the model parameters $$\alpha $$ and $$\zeta _{0}$$ , we discuss their effects on the nonlinear collapse of the VUDF model, and we compare its result to the $$\Lambda $$ CDM model. The results of the analysis show that, within the spherical top-hat collapse framework, larger values of $$\alpha $$ and smaller values of $$\zeta _{0}$$ make the structure formation earlier and faster, and the other collapse curves are almost distinguished with the curve of $$\Lambda $$ CDM model if the bulk viscosity coefficient $$\zeta _{0}$$ is less than $$10^{-3}$$ . [ABSTRACT FROM AUTHOR]

In this paper, we continue our previous work of studying viscous generalized Chaplygin gas as a unified dark fluid but the bulk viscosity perturbations. By using the currently available cosmic observational data from SNLS3, BAO, HST, and recently released Planck, we obtain a constraint on the bulk viscosity coefficient: $$\zeta _0=0.0000138_{- 0.0000105- 0.0000138- 0.0000138}^{+ 0.00000614+ 0.0000145+ 0.0000212}$$ in $$1, 2, 3\sigma $$ regions, respectively, via the Markov Chain Monte Carlo method. The result shows that when considering perturbations of the bulk viscosity, the current cosmic observations favor a smaller bulk viscosity coefficient. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate the role of wet dark fluid (WDF) in Kantowski-Sachs space-time in the general theory of relativity. In this theory, we solved the field equations for the case where ρWDF = -2pWDF . Various physical and geometrical properties of the model are also discussed. [ABSTRACT FROM AUTHOR]

In this paper, we test the spherical collapse of a unified dark fluid (UDF) which has constant adiabatic sound speed. By choosing the different values of model parameters B and α, we show the non-linear collapse for UDF and baryons which are considered for their formation of the large scale structure of our Universe. The analyzed results show that larger values of α and B make the structure formation faster and earlier. [ABSTRACT FROM AUTHOR]

We examine the evolution of an inhomogeneous mixture of non-relativistic pressureless cold dark matter (CDM), coupled to dark energy (DE) characterised by the equation of state parameter w<-1/3, with the interaction term proportional to the DE density. This coupled mixture is the source of a spherically symmetric Lemaître-Tolman-Bondi (LTB) metric admitting an asymptotic Friedman-Lemaître-Robertson-Walker (FLRW) background. Einstein’s equations reduce to a 5-dimensional autonomous dynamical system involving quasi-local variables related to suitable averages of covariant scalars and their fluctuations. The phase space evolution around the critical points (past/future attractors and five saddles) is examined in detail. For all parameter values and both directions of energy flow (CDM to DE and DE to CDM) the phase space trajectories are compatible with a physically plausible early cosmic times behaviour near the past attractor. This result compares favourably with mixtures with interaction driven by the CDM density, whose past evolution is unphysical for DE to CDM energy flow. Numerical examples are provided describing the evolution of an initial profile that can be associated with idealised structure formation scenarios. [ABSTRACT FROM AUTHOR]