Phantom Dark Energy Research Articles

Possibility of Cosmic Time Going Back in Some Dark Energy Model Universe

In the course of investigating a universe dominated by phantom dark energy it is found that the idea of negative cosmic time, or in other words, reversing the present arrow of time is valid in such a universe from big rip singularity, thus giving us a realistic universe.

Examining the viability of phantom dark energy

In the standard cosmological framework of the 0th-order FLRW metric and the use of perfect fluids in the stress-energy tensor, dark energy with an equation-of-state parameter $w < -1$ (known as phantom dark energy) implies negative kinetic energy and vacuum instability when modeled as a scalar field. However, the accepted values for present-day $w$ from Planck and WMAP9 include a significant range of values less than $-1$. We find that it is not as obvious as one might think that phantom dark energy has negative kinetic energy categorically. Analogously, we find that field models of quintessence dark energy ($w_\phi>-1$) do not necessarily have positive kinetic energy categorically. Staying within the confines of observational constraints and general relativity, for which there is good experimental validation, we consider a few reasonable departures from the standard 0th-order framework in an attempt to see if negative kinetic energy can be avoided in these settings despite an apparent $w<-1$. We consider a more accurate description of the universe through the perturbing of the isotropic and homogeneous FLRW metric and the components of the stress-energy tensor, and we consider dynamic $w$ and primordial isocurvature and adiabatic perturbations. We find that phantom dark energy does not necessarily have negative kinetic energy for all relevant length scales at all times, and we also find that, by the same token, quintessence dark energy does not necessarily have positive kinetic energy for all relevant length scales at all times.

Apparent horizon and gravitational thermodynamics of the Universe: Solutions to the temperature and entropy confusions and extensions to modified gravity

The thermodynamics of the Universe is restudied by requiring its compatibility with the holographic-style gravitational equations which govern the dynamics of both the cosmological apparent horizon and the entire Universe, and possible solutions are proposed to the existent confusions regarding the apparent-horizon temperature and the cosmic entropy evolution. We start from the generic Lambda Cold Dark Matter ($\Lambda$CDM) cosmology of general relativity (GR) to establish a framework for the gravitational thermodynamics. The Cai-Kim Clausius equation for the isochoric process of an instantaneous apparent horizon indicates that, the Universe and its horizon entropies encode the \emph{positive heat out} thermodynamic sign convention, which encourages us to adjust the traditional positive-heat-in Gibbs equation into the positive-heat-out version $dE_m=-T_mdS_m-P_mdV$. It turns out that the standard and the generalized second laws (GSLs) of nondecreasing entropies are always respected by the event-horizon system as long as the expanding Universe is dominated by nonexotic matter $-1\leq w_m\leq 1$, while for the apparent-horizon simple open system the two second laws hold if $-1\leq w_m<-1/3$; also, the artificial local equilibrium assumption is abandoned in the GSL. All constraints regarding entropy evolution are expressed by the equation of state parameter, which show that from a thermodynamic perspective the phantom dark energy is less favored than the cosmological constant and the quintessence. Finally, the whole framework is extended from GR and $\Lambda$CDM to modified gravities with field equations $R_{\mu\nu}-Rg_{\mu\nu}/2=8\pi G_{\text{eff}} T_{\mu\nu}^{\text{(eff)}}$. Furthermore, this paper argues that the Cai-Kim temperature is more suitable than Hayward, and the Bekenstein-Hawking and Wald entropies cannot unconditionally apply to the event and particle horizons.

Exploring the low redshift universe: two parametric models for effective pressure

Astrophysical observations have put unprecedentedly tight constraints on cosmological theories. The \(\Lambda \)CDM model, mathematically simple and fits observational data sets well, is preferred for explaining the behavior of universe. But many basic features of the dark sectors are still unknown, which leaves room for various nonstandard cosmological hypotheses. As the pressure of the cosmological constant dark energy is unvarying, ignoring contributions from radiation and curvature terms at low redshift, the effective pressure keeps constant. In this paper, we propose two parametric models for a non-constant effective pressure in order to study the tiny deviation from \(\Lambda \)CDM at low redshift. We recover our phenomenological models in the scenarios of quintessence and phantom fields, and we explore the behavior of the scalar field and potential. We constrain our model parameters with SNe Ia and BAO observations, and we detect subtle hints of \(\omega _{\mathrm{de}}<-1\) from the data-fitting results of both models, which indicates possibly a phantom dark energy scenario at present.

Higuchi ghosts and gradient instabilities in bimetric gravity

Bimetric gravity theories allow for many different types of cosmological solutions, but not all of them are theoretically allowed. In this work we discuss the conditions to satisfy the Higuchi bound and to avoid gradient instabilities in the scalar sector at the linear level. We find that in expanding universes the ratio of the scale factors of the reference and observable metric has to increase at all times. This automatically implies a ghost-free helicity-2 and helicity-0 sector and enforces a phantom dark energy. Furthermore, the condition for the absence of gradient instabilities in the scalar sector will be analyzed. Finally, we discuss whether cosmological solutions can exist, including exotic evolutions like bouncing cosmologies, in which both the Higuchi ghost and scalar instabilities are absent at all times.

Emerging the dark sector from thermodynamics of cosmological systems with constant pressure

We investigate the thermodynamics of general fluids that have the constriction that their pressure is constant. For example, this happens in the case of pure dust matter, for which the pressure vanishes and also in the case of standard dark matter phenomenology. Assuming a finite non-zero pressure, the corresponding dynamics is richer than one naively would expect. In particular, it can be considered as a unified description of dark energy and dark matter. We first consider the more general thermodynamic properties of this class of fluids finding the important result that for them adiabatic and isothermal processes should coincide. We therefore study their behaviors in curved space-times where local thermal equilibrium can be appealed. Thus, we show that this dark fluid degenerates with the dark sector of the LCDM model only in the case of adiabatic evolution. We demonstrate that, adding dissipative processes, a phantom behavior can occur and finally we further highlight that an arbitrary decomposition of the dark sector, into ad hoc dark matter and dark energy terms, may give rise to phantom dark energy, whereas the whole dark sector remains non-phantom.

Entropy-corrected new agegraphic dark energy model in the context of Chern–Simons modified gravity

Within the framework of Chern–Simons (CS) modified gravity, we studied dark energy models. The new agegraphic dark energy (NADE) model, entropy-corrected new agegraphic dark energy (ECNADE) model and NADE model with generalized uncertainty principle (GUP) are investigated. For these models, we studied the evolution of scale factor a, Hubble parameter H and deceleration parameter q. On meantime, we studied the state finder parameters s and r. These models show some similar behavior with modified Chaplygin gas model in some regions, while in other regions some similarity with phantom and quintessence dark energy is noticed.

Cosmological perturbations in a mimetic matter model

We investigate the cosmological evolution of mimetic matter model with arbitrary scalar potential. The cosmological reconstruction, which is the way to construct a model for arbitrary evolutions of the scale factor, is explicitly done for different choices of potential. The cases that mimetic matter model shows the evolution as Cold Dark Matter(CDM), wCDM model, dark matter and dark energy with dynamical $Om(z)$, where $Om(z) \equiv [ (H(z)/H_0)^2-1 ]/[(1+z)^3 -1],$ or phantom dark energy with phantom-non-phantom crossing are presented in detail. The cosmological perturbations for such evolution are studied in mimetic matter model. For instance, the evolution behavior of the matter density contrast which is different from usual one, i.e. $\ddot \delta + 2 H \dot \delta - \kappa ^2 \rho \delta /2 = 0$ is investigated. The possibility of peculiar evolution of $\delta$ in the model under consideration is shown. Special attention is paid to the behavior of matter density contrast near to future singularity where decay of perturbations may occur much earlier the singularity.

Wormholes with a barotropic equation of state admitting a one-parameter group of conformal motions

The theoretical construction of a traversable wormhole proposed by Morris and Thorne maintains complete control over the geometry by assigning both the shape and redshift functions, thereby leaving open the determination of the stress–energy tensor. This paper examines the effect of introducing the linear barotropic equation of state pr=ωρ on the theoretical construction. If either the energy density or the closely related shape function is known, then the Einstein field equations do not ordinarily yield a finite redshift function. If, however, the wormhole admits a one-parameter group of conformal motions, then both the redshift and shape functions exist provided that −3<ω<−1. In a cosmological setting, the equation of state p=ωρ, ω<−1, is associated with phantom dark energy, which is known to support traversable wormholes. The restriction −3<ω<−1 that arises in the present wormhole setting can be attributed to the assumption of conformal symmetry.

Dynamics of dark energy in the gravitational fields of matter inhomogeneities

We study the dynamical properties and space distribution of dark energy in the weak and strong gravitational fields caused by inhomogeneities of matter in the static world of galaxies and clusters. We show that the dark energy in the weak gravitational fields of matter density perturbations can condense or dilute, but amplitudes of its perturbations remain very small on all scales. We illustrate also how the ``accretion'' of the phantom dark energy onto the matter overdensity forms the dark energy underdensity. We analyze the behavior of dark energy in the gravitational fields of stars and black holes with the Schwarzschild metric. It is shown that, in the case of stars, the static solution of the differential equations for energy-momentum conservation exists and describes the distribution of density of dark energy inside and outside of a star. We have found that for stars and galaxies its value differs slightly from the average and is a bit higher for the quintessential scalar field as dark energy and a bit lower for the phantom one. The difference grows with the decrease of the effective sound speed of dark energy and is large in the neighborhood of neutron stars. We obtain and analyze also the solutions of equations that describe the stationary accretion of the dark energy as a test component onto the Schwarzschild black hole. It is shown that the rate of change of mass of the dark energy is positive in the case of quintessential dark energy and is negative in the case of the phantom one.

Parametrizing the transition to the phantom epoch with supernovae Ia and standard rulers

The reconstruction of a (non)canonical scalar field Lagrangian from the dark energy equation of state (EOS) parameter is studied, where it is shown that any EOS parametrization can be well reconstructed in terms of scalar fields. Several examples of EOS parameters are studied and the particular scalar field Lagrangian is reconstructed. Then, we propose some new parametrizations that may present a (fast) transition to a phantom dark energy EOS (where ${w}_{\mathrm{DE}}&lt;\ensuremath{-}1$), and the scalar field Lagrangian is also reconstructed numerically. Furthermore, the properties of these parametrizations of the dark energy EOS are studied by using supernovae Ia data (HST Cluster Supernova Survey) combined with standard ruler data sets [cosmic microwave background (CMB) and baryon acoustic oscillations (BAO)] and its comparison with the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model is analyzed. Then, the best fit of the models is obtained, which provides some information about whether a phantom transition may be supported by the observations. In this regard, the crossing of the phantom barrier is allowed statistically but the occurrence of a future singularity seems unlikely.

Confronting dark energy anisotropic stress

We use the currently available cosmic observations to probe and constrain an imperfect dark energy fluid which is characterized by a constant equation of state $w$ and a constant speed of viscosity $c^2_{vis}$. The model parameter space was scanned by using Markov chain Monte Carlo method. The results show that the speed of viscosity $c^2_{vis}$ was not well constrained when currently available date sets, which include the cosmic microwave background radiation from {\it Planck}2013, type Ia supernovae and baryon acoustic oscillations, are used. But the cosmic data sets favor phantom dark energy with a negative speed of viscosity $c^2_{vis}$ slightly.

Galaxy clusters and structure formation in quintessence versus phantom dark energy universe

A self-gravitating gas in the Newtonian limit is studied in the presence of dark energy with a linear and constant equation of state. Entropy extremization associates to the isothermal Boltzmann distribution an effective density that includes ``dark energy particles''---which either strengthen or weaken mutual gravitational attraction in the case of quintessence or phantom dark energy, respectively---that satisfy a linear equation of state. Stability is studied for microcanonical (fixed-energy) and canonical (fixed-temperature) ensembles. Compared to the previously studied cosmological constant case, in the present work it is found that quintessence increases the instability domain under gravitational collapse, while phantom dark energy decreases it. Thus, structures are more easily formed in a quintessence- rather than in a phantom-dominated universe. Assuming that galaxy clusters are spherical, nearly isothermal, and in hydrostatic equilibrium, we find that dark energy with a linear and constant equation of state (for fixed radius, mass, and temperature) steepens their total density profile. In the case of a cosmological constant, this effect accounts for a 1.5% increase in the density contrast, that is, the center to edge density ratio of the cluster. We also propose a method to constrain phantom dark energy.

Reconstruction of modified $$f(R,T)$$ f ( R , T ) gravity with perfect fluid cosmological models

In this paper we present the cosmological viability of reconstruction of an alternative gravitational theory, namely, the modified \(f(R,T)\) gravity, where \(R\) is the Ricci scalar curvature and \(T\) the trace of stress energy momentum tensor. A functional form of \(f(R,T)= R + 2f(T)\) is chosen for the reconstruction in perfect fluid flat Friedmann–Robertson–Walker model. The gravitational field equations contain two fluid sources, one is perfect fluid and other is due to modified \(f(R,T)\) gravity which is to be considered as an exotic fluid. This allows us for derivation and analysis of a set of new cosmological solutions for \(f(R,T)\) gravity by considering these two fluids as a non-interacting. Two known forms of scale factor (de Sitter and power-law) are considered for the explicit and successful reconstruction. The equation of state parameter (EoS) of exotic matter and the effective EoS parameter have been discussed. In de Sitter solution we find that the \(f(R,T)\) fluid behaves as phantom dark energy when the usual matter (perfect fluid) shows the behavior between decelerated phase to accelerated phase. In the absence of usual matter it behaves as a cosmological constant. In case of power -law cosmology two different cases are discussed and analyzed the behavior of different phases of the universe accordingly through the equation of state and density parameters.

Chasing the phantom: A closer look at type Ia supernovae and the dark energy equation of state

Some recent observations provide $> 2\sigma$ evidence for phantom dark energy -- a value of the dark energy equation of state less than the cosmological-constant value of $-1$. We focus on constraining the equation of state by combining current data from the most mature geometrical probes of dark energy: Type Ia supernovae (SNe Ia) from the Supernova Legacy Survey (SNLS3), the Supernova Cosmology Project (Union2.1), and the Pan-STARRS1 survey (PS1); cosmic microwave background measurements from Planck and WMAP9; and a combination of measurements of baryon acoustic oscillations. The combined data are consistent with $w = -1$ for the Union2.1 sample, though they present moderate ($\sim 1.9\sigma$) evidence for a phantom value when either the SNLS3 or PS1 sample is used instead. We study the dependence of the constraints on the redshift, stretch, color, and host galaxy stellar mass of SNe, but we find no unusual trends. In contrast, the constraints strongly depend on any external $H_0$ prior: a higher adopted value for the direct measurement of the Hubble constant ($H_0 \gtrsim 71~\text{km/s/Mpc}$) leads to $\gtrsim 2\sigma$ evidence for phantom dark energy. Given Planck data, we can therefore make the following statement at $2\sigma$ confidence: either the SNLS3 and PS1 data have systematics that remain unaccounted for or the Hubble constant is below 71 km/s/Mpc; else the dark energy equation of state is indeed phantom.

Gravitational stability of dark energy in galaxies and clusters of galaxies

We analyze the behavior of the scalar field as dark energy of the Universe in a static world of galaxies and clusters of galaxies. We find the analytical solutions of evolution equations of the density and velocity perturbations of dark matter and dark energy, which interact only gravitationally, along with the perturbations of metric in a static world with background Minkowski metric. It was shown that quintessential and phantom dark energy in the static world of galaxies and clusters of galaxies is gravitationally stable and can only oscillate by the influence of self-gravity. In the gravitational field of dark matter perturbations, it is able to condense monotonically, but the amplitude of density and velocity perturbations on all scales remains small. It was also illustrated that the “accretion” of phantom dark energy in the region of dark matter overdensities causes formation of dark energy underdensities-the regions with negative amplitude of density perturbations of dark energy.

Phantom dark energy from nonlocal infrared modifications of general relativity

We discuss the cosmological consequences of a model based on a nonlocal infrared modification of Einstein equations. We find that the model generates a dynamical dark energy that can account for the presently observed value of ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{DE}}$, without introducing a cosmological constant. Tuning a free mass parameter $m$ to a value $m\ensuremath{\simeq}0.67{H}_{0}$, we reproduce the observed value ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{DE}}\ensuremath{\simeq}0.68$. This leaves us with no free parameter, and we then get a pure prediction for the equation of state parameter of dark energy. Writing ${w}_{\mathrm{DE}}(a)={w}_{0}+(1\ensuremath{-}a){w}_{a}$, we find ${w}_{0}\ensuremath{\simeq}\ensuremath{-}1.04$ and ${w}_{a}\ensuremath{\simeq}\ensuremath{-}0.02$, consistent with the Planck data and on the phantom side. We also argue that nonlocal equations of the type that we propose must be understood as purely classical effective equations, such as those derived in semiclassical gravity for the in-in matrix elements of the metric. As such, any apparent ghost instability in such equations only affects the classical dynamics, but there is no propagating degree of freedom associated to the ghost and no issue of ghost-induced quantum vacuum decay.

Evolution of the Universe with Time Varying Constants

In this paper, we discuss the evolution of the universe by considering Λ and G as time varying constants with different choices of Λ. We choose the polytropic equation of state as the total energy content and find approximate values of the constant parameters for the cold dark matter and radiation components of the fluid of the universe. We also mention different possibilities for the constant parameters in order to interpret quintessence, vacuum, and phantom dark energy phases of the universe. Also, we find the conditions on the different constant parameters under which the behavior of G and Λ is consistent with the present observations. Finally, we calculate the statefinder parameters in this scenario.

A one-parameter formula for testing slow-roll dark energy: observational prospects

Numerous upcoming observations, such as Wide-Field Infrared Survey Telescope, Baryon Oscillation Spectroscopic Survey (BOSS), BigBOSS, Large Synoptic Survey Telescope, Euclid and Planck, will constrain dark energy (DE)'s equation of state with great precision. They may well find that the ratio of pressure to energy density, w, is −1, meaning DE is equivalent to a cosmological constant. However, many time-varying DE models have also been proposed. A single parametrization to test a broad class of them and that is itself motivated by a physical picture is therefore desirable. We suggest that the simplest model of DE has the same mechanism as inflation, likely a scalar field slowly rolling down its potential. If this is so, DE will have a generic equation of state and the Universe will have a generic dependence of the Hubble constant on redshift, independent of the potential's starting value and shape. This equation of state and expression for the Hubble constant offer the desired model-independent but physically motivated parametrization, because they will hold for most of the standard scalar field models of DE such as quintessence and phantom DE. Up until now two-parameter descriptions of w have been available, but this work finds an additional approximation that leads to a single-parameter model. Using it, we conduct a χ2 analysis and find that experiments in the next seven years should be able to distinguish any of these time-varying DE models on the one hand from a cosmological constant on the other to 73 per cent confidence if w today differs from −1 by 3.5 per cent. In the limit of perfectly accurate measurements of Ωm and H0, this confidence would rise to 96 per cent. We also include discussion of the current status of DE experiments, a table compiling the techniques each will use and tables of the precisions of the experiments for which this information was available at the time of publication.

Phantom dark energy as an effect of bulk viscosity

In a homogeneous and isotropic universe bulk viscosity is the unique viscous effect capable to modify the background dynamics. Effects like shear viscosity or heat conduction can only change the evolution of the perturbations. The existence of a bulk viscous pressure in a fluid, which in order to obey to the second law of thermodynamics is negative, reduces its effective pressure. We discuss in this study the degeneracy in bulk viscous cosmologies and address the possibility that phantom dark energy cosmology could be caused by the existence of non-equilibrium pressure in any cosmic component. We establish the conditions under which either matter or radiation viscous cosmologies can be mapped into the phantom dark energy scenario with constraints from multiple observational data-sets