Unified Dark Energy Research Articles

Phase structure of charged AdS black holes surrounded by exotic fluid with modified Chaplygin equation of state

By considering the concept of the modified Chaplygin gas (MCG) as a single fluid model unifying dark energy and dark matter, we construct a static, spherically charged black hole (BH) solution in the framework of General Relativity. The P–V criticality of the charged anti-de Sitter (AdS) BH with a surrounding MCG is explored in the context of the extended phase space, where the negative cosmological constant operates as a thermodynamical pressure. This critical behavior shows that the small/large BH phase transition is analogous to the van der Waals liquid/gas phase transition. Accordingly, along the P–V phase spaces, we derive the BH equations of state and then numerically evaluate the corresponding critical quantities. Similarly, critical exponents are identified, along with outcomes demonstrating the scaling behavior of thermodynamic quantities near criticality to a universal class. The use of geometrothermodynamic (GT) tools finally offers a new perspective on the discovery of the critical phase transition point. At this stage, we apply a class of GT tools, such as Weinhold, Ruppeiner, HPEM, and Quevedo classes I and II. The findings are therefore non-trivial, as each GT class metric captures at least either the physical limitation point or the phase transition critical point. Overall, this paper provides a detailed study of the critical behavior of the charged AdS BH with surrounding MCG.

Unified dark energy from Chiral-Quintom model with a mixed potential in Friedmann–Lemaître–Robertson–Walker cosmology

For a spatially Friedmann–Lemaître–Robertson–Walker cosmology, we propose a multi-scalar field gravitational model. Specifically, we consider a two-scalar field cosmological model in which the kinetic components of the scalar fields establish a two-dimensional sphere of Lorentzian signature. For our Chiral-Quintom model we choose a mixed potential term Vleft( phi ,psi right) =V_{0}e^{lambda phi }+U_{0}e^{kappa phi }psi ^{frac{1}{sigma } } and we investigate the asymptotic limits of the cosmological parameters. This model for U_{0}=0, provides a generalization of the hyperbolic inflation where the equation of the state parameter can cross the phantom divide line. When U_{0}ne 0 we observe that this cosmological model exhibits asymptotic solutions that encompass accelerated universes, big rip singularities, and dust-like solutions. Hence, this multi-scalar field model it can be regarded as as a dark energy unify model which describes a variety of asymptotic cosmological scenarios.

Development of Matter and Testable Negative Matter as Unified Dark Matter and Dark Energy

Development of Matter and Testable Negative Matter as Unified Dark Matter and Dark Energy

Negative Matter as Unified Dark Matter and Dark ‎Energy: Simplest Model, Theory and Nine Tests

Based on Dirac’s negative energy, we propose and study the negative matter. Bondi’s results are wrong. First, the negative matter can be the simplest model of unified dark matter and dark energy. Next, we discuss various possible theories of the negative matter: some field equations, similar electrodynamics, field equations with non-symmetry, etc. Third, the quantum theory of negative matter is researched. Matter surrounded by dark-negative matter corresponds to an infinitely deep potential trap in quantum mechanics and forms a base of the universal wave-particle duality and quantum mechanics. Fourth, we propose the mechanism of inflation as the origin of positive-negative matters created from nothing. Fifth, assume that dark matter is completely the negative matter, and we may calculate an evolutional ratio between total matter and usual matter from 1 of inflation and the radiation-dominated universe to 7.88 of the present matter-dominated universe. It agrees with the observed value 6.36~7. Sixth, we research the relativity of the negative matter and theory in Lobachevskian geometry. Seventh, we propose a judgment test of the negative matter as dark matter is opposite repulsive lensing and other eight possible tests. Eighty, we propose a figure on the unification of the four basic interactions in three-dimensional space, in which the “running” coupling constants of strong and weak interactions transform each other. The negative matter as a candidate of unification of dark matter and dark energy is not only the simplest, and may explain inflation and be calculated and tested.

Lagrangian description of cosmic fluids: Mapping dark energy into unified dark energy

We investigate the appropriateness of the use of different Lagrangians to describe various components of the cosmic energy budget, discussing the degeneracies between them in the absence of nonminimal couplings to gravity or other fields, and clarifying some misconceptions in the literature. We further demonstrate that these degeneracies are generally broken for nonminimal coupled fluids, in which case the identification of the appropriate on-shell Lagrangian may become essential in order characterize the overall dynamics. We then show that models with the same on-shell Lagrangian may have different proper energy densities and use this result to map dark energy models into unified dark energy models in which dark matter and dark energy are described by the same perfect fluid. We determine the correspondence between their equation of state parameters and sound speeds, briefly discussing the linear sound speed problem of unified dark energy models as well as a possible way out associated to the nonlinear dynamics.

Black holes and naked singularities from Anton–Schmidt’s fluids

Abstract Adopting the Tolman–Oppenheimer–Volkoff formalism, we propose a new analytical solution for a static and spherically symmetric black hole where the cosmological constant is generalized to a non-isotropic Anton–Schmidt fluid acting as a unified dark energy–dark matter source. Our novel result can describe a black hole in energetic equilibrium with the surrounding universe. We thus investigate the interplay between the mass of the black hole and the parameters of the Anton–Schmidt equation of state, and physically interpret the solution as a family of space–time metrics that may describe both Schwarzschild–de Sitter black holes and naked singularities. The result opens a new window for further constraining the physical properties of the cosmic fluid using the Event Horizon Telescope results, which complement the analysis relying on classical cosmological observations.

Probing beyond-Horndeski gravity on ultra-large scales

The beyond-Horndeski gravity has recently been reformulated in the dark energy paradigm—which has been dubbed, Unified Dark Energy (UDE). The evolution equations for the given UDE appear to correspond to a non-conservative dark energy scenario, in which the total energy-momentum tensor is not conserved. We investigate both the background cosmology and, the large-scale imprint of the UDE by probing the angular power spectrum of galaxy number counts, on ultra-large scales; taking care to include the full relativistic corrections in the observed overdensity. The background evolution shows that only an effective mass smaller than the Planck mass is needed in the early universe in order for predictions in the given theory to match current observational constraints. We found that the effective mass-evolution-rate parameter, which drives the evolution of the UDE, acts to enhance the observed power spectrum and, hence, relativistic effects (on ultra-large scales) by enlarging the UDE sound horizon. Conversely, both the (beyond) Horndeski parameter and the kineticity act to diminish the observed power spectrum, by decreasing the UDE sound horizon. Our results show that, in a universe with UDE, a multi-tracer analysis will be needed to detect the relativistic effects in the large-scale structure. In the light of a multi-tracer analysis, the various relativistic effects hold the potential to distinguish different gravity models. Moreover, while the Doppler effect will remain significant at all epochs and, thus can not be ignored, the integrated Sachs-Wolfe, the time-delay and the potential (difference) effects, respectively, will only become significant at epochs near z = 3 and beyond, and may be neglected at late epochs. In the same vein, the Doppler effect alone can serve as an effective cosmological probe for the large-scale structure or gravity models, in the angular power spectrum—at all z.

Cosmological solutions in multiscalar field theory

We consider a cosmological model with two scalar fields minimally coupled to gravity which have a mixed kinetic term. Hence, Chiral cosmology is included in our analysis. The coupling function and the potential function, which depend on one of the fields, characterize the model we study. We prove the existence of exact solutions that are of special interest for the cosmological evolution. Furthermore, we provide with a methodology that relates the scale factor behaviour to the free functions characterizing the scalar field kinetic term coupling and potential. We derive the necessary conditions that connect these two functions so that the relative cosmological solutions can be admitted. We find that unified dark matter and dark energy solutions are allowed by the theory in various scenarios involving the aforementioned functions.

Extensions of modified Chaplygin gas from Geometrothermodynamics

We derive modified classes of Chaplygin gas by using the formalism of Geometrothermodynamics. In particular, our strategy gives us extended versions of Chaplygin gas, providing a novel thermodynamic explanation. Thus, we show that our models correspond to systems with internal thermodynamic interaction. Bearing this in mind, we find new free parameters which are derived from thermodynamics and we give them an interpretation. To this end, we predict the range of values that every term can take in the context of homogeneous and isotropic universe. We also show that our new versions of modified Chaplygin gas can be interpreted as unified dark energy models, independently from the introduction of our new additional terms. Finally, we compare our theoretical scenarios through a fit on a grid based on the Union 2.1 compilation and we evaluate the growth factor of small perturbations. In this respect, we show that our model better adapts to the theoretical varLambda CDM value, namely gamma _{varLambda CDM}=frac{6}{11}, than previous versions of modified Cgaplygin gas. We show numerical constraints at late and early redshift domains, which turn out to be compatible with previous results on standard versions of Chaplygin gas models.

Astronomical bounds on the modified Chaplygin gas as a unified dark fluid model

The modified Chaplygin gas could be considered to abide by the unified dark fluid model because the model might describe the past decelerating matter dominated era and at present time it provides an accelerating expansion of the Universe. In this paper, we have employed the Planck 2015 cosmic microwave background anisotropy, type-Ia supernovae, observed Hubble parameter data sets to measure the full parameter space of the modified Chaplygin gas as a unified dark matter and dark energy model. The model parameters Bs, α, and B determine the evolutional history of this unified dark fluid model by influencing the energy density ρMCG = ρMCG0[Bs + (1 − Bs)a−3(1 + B)(1 + α)]1/(1 + α). We assumed the pure adiabatic perturbation of unified modified Chaplygin gas in the linear perturbation theory. In the light of Markov chain Monte Carlo method, we find that Bs = 0.727+0.040+0.075−0.039−0.079, α = −0.0156+0.0982+0.2346−0.1380−0.2180, B = 0.0009+0.0018+0.0030−0.0017−0.0030 at 2σ level. The model parameters α and B are very close to zero and the nature of unified dark energy and dark matter model is very similar to cosmological standard model ΛCDM.

Observational constraints on EoS parameters of various modified Chaplygin gas models

Cosmological models of a class of modified Chaplygin gas as a candidate of unified dark matter and energy are studied to determine observational constraints on its EoS parameters using the background data. These data consist of $$H(z)-z$$ (OHD) data, baryonic acoustic oscillations peak parameter data, CMB shift parameter and SN Ia (Union 2.1) data. Best-fit value of present Hubble parameter ( $$H_{0}$$ ), present matter density ( $$\Omega _{m0}$$ ) and present age of the Universe ( $$t_{0}$$ ) has been determined in all these models. The acceptable range of values of the EoS parameters is determined in the analysis. Variations of EoS parameter ( $$\omega$$ ), squared sound speed ( $$c^2_{s}$$ ) and deceleration parameter (q) with redshift are also studied here. Akaike information criteria and Bayesian information criterion are used to check the suitability of the models. Density perturbation and CMB temperature anisotropy have also been studied.

Constraint on the generalized Chaplygin gas as an unified dark fluid model after Planck 2015

Abstract The generalized Chaplygin gas could be considered as the unified dark fluid model because it might describe the past decelerating matter dominated era and at present time it provides an accelerating expansion of the Universe. In this paper, we employed the Planck 2015 cosmic microwave background anisotropy, type-Ia supernovae, observed Hubble parameter data sets to measure the full parameter space of the generalized Chaplygin gas as an unified dark matter and dark energy model. The model parameters B s and α determine the evolutional history of this unified dark fluid model by influencing the energy density ρ G C G = ρ G C G 0 [ B s + ( 1 − B s ) a − 3 ( 1 + α ) ] 1 ∕ ( 1 + α ) . We assume the pure adiabatic perturbation of unified generalized Chaplygin gas. In the light of Markov Chain Monte Carlo method, we found that B s = 0 . 75 9 − 0 . 032 − 0 . 046 + 0 . 020 + 0 . 051 and α = 0 . 080 1 − 0 . 0801 − 0 . 0801 + 0 . 0208 + 0 . 1087 at 2 σ level. The model parameter α is very close to zero, the nature of GCG model is very similar to cosmological standard model Λ CDM.

A transition between bouncing hyper-inflation to ΛCDM from diffusive scalar fields

We consider the history of the universe from a possible big bang or a bounce into a late period of a unified interacting dark energy–dark matter model. The model is based on the Two Measures Theories (TMT) which introduces a metric independent volume element and this allows us to construct a unification of dark energy and dark matter. A generalization of the Two Measures Theories gives a diffusive nonconservative stress-energy–momentum tensor in addition to the conserved stress-energy tensor which appear in Einstein equations. These leads to a formulation of interacting DE–DM dust models in the form of a diffusive-type interacting Unified Dark Energy and Dark Matter scenario. The deviation from [Formula: see text]CDM is determined by the diffusion constant [Formula: see text]. For [Formula: see text] the model is indistinguishable from [Formula: see text]CDM. Numerical solutions of the theories show that in some [Formula: see text] the evolution of the early universe is governed by Stiff equation of state or the universe bounces to hyper-inflation. But all of those solutions have a final transition to [Formula: see text]CDM as a stable fixed point for the late universe.

Unified dark energy and dark matter from dynamical spacetime

A unification of dark matter and dark energy based on a dynamical space time theory is suggested. By introducing a dynamical space time vector field $\chi_\mu$ as a Lagrange multiplier, a conservation of an energy momentum tensor $T^{\mu\nu}_{(\chi)}$ is implemented. This Lagrangian generalizes the "Unified dark energy and dark matter from a scalar field different from quintessence" [Phys.RevD 81, 043520 (2010)] which did not consider a Lagrangian formulation. This generalization allows the solutions which were found previously, but in addition to that also non singular bouncing solutions that rapidly approach to the $\Lambda$CDM model. The dynamical time vector field exactly coincides with the cosmic time for the a $\Lambda$CDM solution and suffers a slight shift (advances slower) with respect to the cosmic time in the region close to the bounce for the bouncing non singular solutions. In addition we introduced some exponential potential which could enter into the $T^{\mu\nu}_{(\chi)}$ stress energy tensor or coupled directly to the measure $\sqrt{-g}$, gives a possible interaction between DE and DM and could explain the coincidence problem.

Cosmographic Thermodynamics of Dark Energy

Dark energy’s thermodynamics is here revised giving particular attention to the role played by specific heats and entropy in a flat Friedmann-Robertson-Walker universe. Under the hypothesis of adiabatic heat exchanges, we rewrite the specific heats through cosmographic, model-independent quantities and we trace their evolutions in terms of z. We demonstrate that dark energy may be modeled as perfect gas, only as the Mayer relation is preserved. In particular, we find that the Mayer relation holds if j − q > 1 2 . The former result turns out to be general so that, even at the transition time, the jerk parameter j cannot violate the condition: j t r > 1 2 . This outcome rules out those models which predict opposite cases, whereas it turns out to be compatible with the concordance paradigm. We thus compare our bounds with the Λ CDM model, highlighting that a constant dark energy term seems to be compatible with the so-obtained specific heat thermodynamics, after a precise redshift domain. In our treatment, we show the degeneracy between unified dark energy models with zero sound speed and the concordance paradigm. Under this scheme, we suggest that the cosmological constant may be viewed as an effective approach to dark energy either at small or high redshift domains. Last but not least, we discuss how to reconstruct dark energy’s entropy from specific heats and we finally compute both entropy and specific heats into the luminosity distance d L , in order to fix constraints over them through cosmic data.

Interacting diffusive unified dark energy and dark matter from scalar fields

Here we generalize ideas of unified dark matter–dark energy in the context of two measure theories and of dynamical space time theories. In two measure theories one uses metric independent volume elements and this allows one to construct unified dark matter–dark energy, where the cosmological constant appears as an integration constant associated with the equation of motion of the measure fields. The dynamical space-time theories generalize the two measure theories by introducing a vector field whose equation of motion guarantees the conservation of a certain Energy Momentum tensor, which may be related, but in general is not the same as the gravitational Energy Momentum tensor. We propose two formulations of this idea: (I) by demanding that this vector field be the gradient of a scalar, (II) by considering the dynamical space field appearing in another part of the action. Then the dynamical space time theory becomes a theory of Diffusive Unified dark energy and dark matter. These generalizations produce non-conserved energy momentum tensors instead of conserved energy momentum tensors which leads at the end to a formulation of interacting DE–DM dust models in the form of a diffusive type interacting Unified dark energy and dark matter scenario. We solved analytically the theories for perturbative solution and asymptotic solution, and we show that the Lambda CDM is a fixed point of these theories at large times. Also a preliminary argument as regards the good behavior of the theory at the quantum level is proposed for both theories.

New limit on logotropic unified dark energy models

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 8h−1Mpc 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 ΛCDM 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.

Recent observational constraints on generalized Chaplygin gas in UDME scenario

Recent observational predictions suggest that our Universe is passing through an accelerating phase in the recent past. This acceleration may be realized with the negatively pressured dark energy. Generalized Chaplygin gas may be suitable to describe the evolution of the Universe as a candidate of unified dark matter energy (UDME) model. Its EoS parameters are constrained using (i) dimensionless age parameter (H 0 t 0) and (ii) the observed Hubble (H(z)−z) data (OHD) + baryon acoustic oscillation (BAO) data + cosmic microwave background (CMB) shift data + supernovae (Union2.1) data. Dimensionless age parameter puts loose bounds on the EoS parameters. Best-fit values of the EoS parameters H 0, A s and α (A s and α are defined in the energy density for generalized Chaplygin gas (GCG) and in EoS) are then determined from OHD + BAO + CMB + Union2.1 data and contours are drawn to obtain their allowed range of values. The present age of the Universe (t 0) and the present Hubble parameter (H 0) have been estimated with 1σ confidence level. Best-fit values of deceleration parameter (q), squared sound speed ( $c_{\mathrm {s}}^{2}$ ) and EoS parameter (ω) of this model are then determined. It is seen that GCG satisfactorily accommodates an accelerating phase and structure formation phase.

Cosmological aspects of a unified dark energy and dust dark matter model

Recently, a model of modified gravity plus single scalar field was proposed, in which the scalar couples both to the standard Riemannian volume form given by the square root of the determinant of the Riemannian metric, as well as to another non-Riemannian volume form given in terms of an auxiliary maximal rank antisymmetric tensor gauge field. This model provides an exact unified description of both dark energy (via dynamically generated cosmological constant) and dark matter (as a “dust” fluid due to a hidden nonlinear Noether symmetry). In this paper, we test the model against Supernovae type Ia experimental data and investigate the future Universe evolution which follows from it. Our results show that this model has very interesting features allowing various scenarios of Universe evolution and in the same time perfectly fits contemporary observational data. It can describe exponentially expanding or finite expanding Universe and moreover, a Universe with phase transition of first kind. The phase transition occurs to a new, emerging at some time ground state with lower energy density, which affects significantly the Universe evolution.

Cosmological constraints on fast transition unified dark energy and dark matter models

We explore the observational adequacy of a class of Unified Dark Energy/Matter (UDE/M) models with a fast transition. Our constraints are set using a combination of geometric probes, some low redshift ones, and some high redshift ones (CMB related included). The transition is phenomenologically modeled by two different transition functions corresponding to a fast and to an ultra-fast transition respectively. We find that the key parameters governing the transition can be well constrained, and from the statistical point of view it follows that the models cannot be discarded when compared to $\Lambda$CDM. We find the intriguing result that standard/input parameters such as $\Omega_m$ and $\Omega_b$ are far better constrained than in $\Lambda$CDM, and this is the case for the derived/output parameter measuring the deceleration value at present, $q_0$.