Phantom Phase Research Articles

Can holographic dark energy increase the mass of the wormhole?

In this work, we have studied accretion of dark energy (DE) onto Morris- Thorne wormhole with three different forms, namely, holographic dark energy, holographic Ricci dark energy and modified holographic Ricci dark energy . Considering the scale factor in power-law form we have observed that as the holographic dark energy accretes onto wormhole, the mass of the wormhole is decreasing. In the next phase we considered three parameterization schemes that are able to get hold of quintessence as well as phantom phases. Without any choice of scale factor we reconstructed Hubble parameter from conservation equation and dark energy densities and subsequently got the mass of the wormhole separately for accretion of the three dark energy candidates. It was observed that if these dark energies accrete onto the wormhole, then for quintessence stage, wormhole mass decreases up to a certain finite value and then again increases to aggressively during phantom phase of the universe.

Modified f(R,T) gravity theory and scalar field cosmology

In this paper, we explore the behaviors of scalar field in modified f(R,T) gravity theory within the framework of a flat Friedmann-Robertson-Walker cosmological model. The universe is assumed to be filled with two non-interacting matter sources, scalar field (normal or phantom) with scalar potential and matter contribution due to f(R,T) action. We first explore a model where the potential is a constant, and the universe evolves as a de Sitter type. This model is compatible with phantom scalar field only which gives fine tuning with the recent observations. The model exhibits a wide variety of early time physical phenomena that eventually behaves like a cosmological constant at late times. The model shows transition from decelerated to accelerated expansion of the universe. We also explore a model where the scalar field potential and the scale factor evolve exponentially as a scalar field. This model is compatible with normal scalar field only and describes transition from inflationary to the decelerated phase at early times and quintessence to phantom phase at late times. We constraint our results with the recent observational data and find that some values of parameters are consistent with SNe Ia and H(z)+SNe Ia data to describe accelerated expansion only whereas some one give decelerated and accelerated expansions with H(z), WMAP7 and WMAP7+BAO+H(z) observational data.

MO‐G‐18C‐02: Flow Quantification Quality Assurance for Multiple MR Scanners Using Power Injector

Purpose:To evaluate a flow quantification QA method on multiple MR scanners with different magnetic field strength from different manufacturer.Methods:Our previous study proposed a low‐cost, reliable and convenient MRI flow quantification QA method based on a simple and compact flow contrast phantom and a power injector. Five different MR scanners (Siemens MAGNETOM Espree 1.5T, Aera 1.5T and Skyra 3T; GE Optima MR450w 1.5T and Discovery MR750w 3T) were used to test the reproducibility and sensitivity of the QA method. Flow phantom phase contrast images were acquired at magnetic field isocenter for all the MR scanners. Eight repeated acquisitions and measurements were performed on each scanner. In addition, off‐center effects were tested on Siemens Espree and two GE scanners.Results:The positive flow rates measured at isocenter of five MR scanners range from +1.84± 0.01 mL/s to +2.01± 0.02 mL/s, within 8% difference from the 2mL/s flow rate provided by the power injector. The negative flow rate measured at isocenter range from −1.89± 0.01 mL/s to ‐2.14± 0.03 mL/s, within 7% difference from 2mL/s flow rate. Flow rate measures were most affected by off‐center effect when the flow phantom was shifted 12 cm transversely and 12 cm longitudinally from the isocenter. For GE Optima scanner, positive flow of +1.53± 0.01 mL/s was measured, which is 24.5% difference from power injector flow rate. For GE Discovery scanner, negative flow of −2.68± 0.02 mL/s was measured, which is 34% difference from 2mL/s flow rate.Conclusion:This QA method was sensitive enough to detect flow quantification difference between scanners. Flow quantification errors at locations away from magnetic field isocenter due to the worse gradient fidelity can be detected by our flow quantification QA method. In conclusion, flow phantom coupled with power injector is presented to be a widely available, consistent and low cost flow quantification method.

Cosmological investigations of (extended) nonlinear massive gravity schemes with nonminimal coupling

In this paper we investigate the case of nonminimal coupling in the (extended) nonlinear massive gravity theories. We first consider massive gravity in the Brans-Dicke background such that the graviton mass is replaced by ${A}^{2}(\ensuremath{\sigma})m$ where $\ensuremath{\sigma}$ is the Brans-Dicke field and $A(\ensuremath{\sigma})$ is the conformal coupling and show that there is no viable thermal history of the Universe in this case. We then invoke a cubic Galileon term as the nonlinear completion of the $\ensuremath{\sigma}$ Lagrangian and show that there is a stable de Sitter solution in this case. However, the de Sitter is blocked by the matter phase, which is also a simultaneous attractor of the dynamics. The de Sitter phase can, however, be realized by invoking unnatural fine-tunings. We next investigate the cosmology of quasidilaton nonlinear massive gravity with nonminimal coupling. As a generic feature of the nonminimal coupling, we show that the model exhibits a transient phantom phase that is otherwise impossible. While performing the observational data analysis on the models, we find that a small value of the coupling constant is allowed for quasidilaton nonlinear massive gravity. For both of the cases under consideration, it is observed that we have an effective pressure of matter that comes from the constraint equation. For mass-varying nonlinear massive gravity in the Brans-Dicke background, the effective pressure of matter is nonzero, which affects the evolution of the Hubble parameter, thereby spoiling the consistency of the model with data. As for the quasidilaton nonlinear massive gravity, the effective pressure of matter can be kept around zero by controlling the coupling constant, and the model is shown to be fit well with observations.

Late-time acceleration in the coupled cubic Galileon models

We investigate the linearly and quadratically coupled cubic Galileon models that include linear potentials. These models may explain the late-time acceleration. In these cases, we need two equations of state parameter named the native and effective equations of state to test whether the universe is accelerating or not because there is coupling between the cold dark matter and Galileon. It turns out that there is no transition from accelerating phase to phantom phase in the future.

Effective equation of state for running vacuum: 'mirage' quintessence and phantom dark energy

Past analyses of the equation of state (EoS) of the Dark Energy (DE) were not incompatible with a phantom phase near our time. This has been the case in the years of WMAP observations, in combination with the remaining cosmological observables. Such situation did not completely disappear from the data collected from the Planck satellite mission. In it the EoS analysis may still be interpreted as suggesting w<-1, and so a mildly evolving DE cannot be discarded. In our opinion the usual ansatzs made on the structure of the EoS for dynamical DE models (e.g. quintessence and the like) are too simplified. In this work we examine in detail some of these issues and suggest that a general class of models with a dynamical vacuum energy density could explain the persistent phantom anomaly, despite there is no trace of real phantom behavior in them. The spurious or "mirage" effect is caused by an attempt to describe them as if the DE would be caused by fundamental phantom scalar fields. Remarkably, the effective DE behavior can also appear as quintessence in transit to phantom, or vice versa.

(m, n)-TYPE HOLOGRAPHIC DARK ENERGY MODELS

We construct (m, n)-type holographic dark energy models at a phenomenological level, which can be viewed as a generalization of agegraphic models with the conformal-like age as the holographic characteristic size. For some values of (m, n) the holographic dark energy can automatically evolve across ω = -1 into a phantom phase even without introducing an interaction between the dark energy and background matter. Our construction is also applicable to the holographic dark energy with generalized future event horizon as the characteristic size. Finally, we address the issue on the stability of our model and show that they are generally stable under the scalar perturbation.

A Generalized Semi-Holographic Universe

We study the semi-holographic idea in the context of decaying dark components. The energy flow between dark energy and the compensating dark matter is thermodynamically generalized to involve a particle number variable dark component with non-zero chemical potential. It is found that, unlike the original semi-holographic model, no cosmological constant is needed for a dynamical evolution of the universe. A transient phantom phase appears while a non-trivial dark energy-dark matter scaling solution stays at a later time, which evades the big-rip and helps to resolve the coincidence problem. For reasonable parameters, the deceleration parameter is well consistent with current observations. The original semi-holographic model is extended and it also suggests that the concordance model may be reconstructed from the semi-holographic idea.

Cosmological evolution, future singularities, Little Rip and Pseudo-Rip in viable f(R) theories and their scalar–tensor counterpart

Modified f(R) gravity is one of the most promising candidates for dark energy, and even for the unification of the whole cosmological evolution, including the inflationary phase. Within this class of theories, the so-called viable modified gravities represent realistic theories that are capable of reproducing late-time acceleration and satisfy strong constraints at local scales, where general relativity is recovered. This paper deals with the analysis of the cosmological evolution for some of these models, which indicates that the evolution may enter into a phantom phase, but the behavior may be asymptotically stable. Furthermore, the scalar–tensor equivalence of f(R) gravity is considered, which provides useful information about the possibility of the occurrence of a future singularity. The so-called Little Rip and Pseudo-Rip are also studied in the framework of this class of modified gravities.

Notes on teleparallel cosmology with nonminimally coupled scalar field

We consider the spatially flat Friedmann-Lemaitre-Robertson-Walker space time in the teleparallel model of gravity and assume that the universe is filled nearly by cold dark matter and a nonminimally coupled scalar field with a power-law potential as dark energy. We investigate the possibility that the universe undergoes a transition from quintessence to phantom phase. An analytical solution for the scalar field is obtained and necessary conditions required for such a transition are discussed.

Particle creation in flat Friedmann–Robertson–Walker (FRW) universe in the framework of f(T) gravity

We study particle production in a flat Friedmann–Robertson–Walker universe in the framework of f(T) gravity. An exact power-law solution is obtained by solving the Friedmann equations and assuming that matter is minimally coupled with gravitation. The torsion scalar, T, appears to plays the same role as the curvature (Ricci scalar) in general relativity (GR) and its modified theories, f(R). Particularly, in the phantom phase, we observe that the vacuum state corresponds to a vanishing torsion scalar and particle production becomes important as the torsion scalar diverges. This aspect not only provides the equivalence between teleparallel gravity and GR, but also between their respective modified versions, f(T) and f(R), in the view of massless particle production phenomenon when matter is minimally coupled with gravity. However, when the gravitational and scalar fields are not minimally coupled, it appears that this similarity between the teleparallel gravity and GR may break down, because the torsion scalar no longer has the same time-dependent expression as the Ricci scalar.

Periodic Cosmological Evolutions of Equation of State for Dark Energy

We demonstrate two periodic or quasi-periodic generalizations of the Chaplygin gas (CG) type models to explain the origins of dark energy as well as dark matter by using the Weierstrass ξ(t), σ(t) and ζ (t) functions with two periods being infinite. If the universe can evolve periodically, a non-singular universe can be realized. Furthermore, we examine the cosmological evolution and nature of the equation of state (EoS) of dark energy in the Friedmann–Lemaître–Robertson–Walker cosmology. It is explicitly illustrated that there exist three type models in which the universe always stays in the non-phantom (quintessence) phase, whereas it always evolves in the phantom phase, or the crossing of the phantom divide can be realized. The scalar fields and the corresponding potentials are also analyzed for different types of models.

MASSIVE GRAVITONS DARK MATTER SCENARIO REVISITED

We reexamine the massive graviton dark matter scenario (MGCDM) which was recently considered as an alternative to dark energy models. When introducing the native and effective equations of state (EoS), it is shown that there is no phantom phase in the evolution toward the far past. Also we show that the past accelerating phase arises from the interaction between massive graviton and cold dark matter.

Power-law solutions in f (T) gravity

We have considered an action of the form $T+f(T)+L_m$ describing Einstein's gravity plus a function of the torsion scalar. By considering an exact power-law solution we have obtained the Friedmann equation as a differential equation for the function $f(T)$ in spatially flat universe and obtained the real valued solutions of this equation for some power-law solutions. We have also studied the power-law solutions when the universe enters a Phantom phase and shown that such solutions may exist for some f(T) solutions.

Solvable k-essence cosmologies and modified Chaplygin gas unified models of dark energy and dark matter

This paper is devoted to the investigation of the modified Chaplygin gas model in the context of solvable k-essence cosmologies. For this purpose, we construct equations of state parameter of this model for some particular values of the parameter n. The graphical behavior of these equations are also discussed by using power law form of scalar field. The relationship between k-essence and the modified Chaplygin gas model shows viable results in the dark energy scenario. We conclude that the universe behaves as a cosmological constant, quintessence phase or phantom phase depending upon n.

Stability analysis of holographic dark energy in Brans-Dicke cosmology

Stability analysis of agegraphic dark energy in Brans-Dicke theory is presented in this paper. We constrain the model parameters with the observational data and thus the results become broadly consistent with those expected from experiment. Stability analysis of the model without best fitting shows that universe may begin from an unstable state passing a saddle point and finally become stable in future. However, with the best fitted model, There is no saddle intermediate state. The agegraphic dark energy in the model by itself exhibits a phantom behavior. However, contribution of cold dark matter on the effective energy density modifies the state of the universe from phantom phase to quintessence one. The statefinder diagnosis also indicates that the universe leaves an unstable state in the past, passes the LCDM state and finally approaches the sable state in future.

DARK ENERGY FROM SCALAR FIELD WITH GAUSS–BONNET AND NON-MINIMAL KINETIC COUPLING

We consider a model of scalar field with non-minimal kinetic couplings to the curvature, and additional coupling to the Gauss–Bonnet four-dimensional invariant. The model presents rich cosmological dynamics and some of its solutions are analyzed. A variety of scalar fields and potentials giving rise to power-law expansion have been found. Two solutions with dynamical equation of state are considered. The first solution unifies early time power-law behavior with late time cosmological constant dominance. The second solution is able to describe a universe in the phantom phase, and depending on the parameters may describe essentially dark energy behavior, or may contain the decelerated and accelerated phases.

Kantowski–Sachs string cosmological model in f(R) theory of gravity

The exact solutions of the field equations for a Kantowski–Sachs space–time filled with cosmic strings in the framework of f(R) gravity are derived. The physical behavior of the model is studied. The function f(R) of the Ricci scalar is also evaluated for the model. This model represents a continuously expanding shearing universe (from the start of the big bang) currently entering the phantom phase.

Stability analysis of agegraphic dark energy in Brans–Dicke cosmology

Stability analysis of agegraphic dark energy in Brans-Dicke theory is presented in this paper. We constrain the model parameters with the observational data and thus the results become broadly consistent with those expected from experiment. Stability analysis of the model without best fitting shows that universe may begin from an unstable state passing a saddle point and finally become stable in future. However, with the best fitted model, There is no saddle intermediate state. The agegraphic dark energy in the model by itself exhibits a phantom behavior. However, contribution of cold dark matter on the effective energy density modifies the state of the universe from phantom phase to quintessence one. The statefinder diagnosis also indicates that the universe leaves an unstable state in the past, passes the LCDM state and finally approaches the sable state in future.

Interacting new agegraphic dark energy in a cyclic universe

The main goal of this work is investigation of NADE in the cyclic universe scenario. Since, cyclic universe is explained by a phantom phase ($\omega<-1$), it is shown when there is no interaction between matter and dark energy, ADE and NADE do not produce a phantom phase, then can not describe cyclic universe. Therefore, we study interacting models of ADE and NADE in the modified Friedmann equation. We find out that, in the high energy regime, which it is a necessary part of cyclic universe evolution, only NADE can describe this phantom phase era for cyclic universe. Considering deceleration parameter tells us that the universe has a deceleration phase after an acceleration phase, and NADE is able to produce a cyclic universe. Also it is found valuable to study generalized second law of thermodynamics. Since the loop quantum correction is taken account in high energy regime, it may not be suitable to use standard treatment of thermodynamics, so we turn our attention to the result of \citep{29}, which the authors have studied thermodynamics in loop quantum gravity, and we show that which condition can satisfy generalized second law of thermodynamics.