Sign-changeable holographic dark energy in Brans–Dicke theory

We consider the Brans–Dicke (BD) theory of gravity and explore the cosmological implications of the sign-changeable interacting holographic dark energy (HDE) model in the background of a Friedmann–Robertson–Walker (FRW) universe. As the system’s infrared cutoff, we choose the future event horizon, the Granda–Oliveros (GO), and the Ricci cutoffs. For each cutoff, we obtain the density parameter, the equation of state (EoS), and the deceleration parameter of the system. In case of future event horizon, we find out that the EoS parameter, wD, can cross the phantom line; as a result the transition from the deceleration to the acceleration of the Universe expansion can be achieved provided the model parameters are chosen suitably. We also investigate the instability of the sign-changeable interacting HDE model against perturbations in BD theory. For this purpose, we study the squared sound speed [Formula: see text] whose sign determines the stability of the model. When [Formula: see text] the model is unstable against perturbation. For future event horizon, our Universe can be stable ([Formula: see text]) depending on the model parameters. Then, we focus on GO and Ricci cutoffs and find out that although other features of these two cutoffs are consistent with observations, they cannot lead to stable dominated universe, except in a special case with GO cutoff. Our studies confirm that for the sign-changeable HDE model in the setup of BD cosmology, the event horizon is the most suitable horizon that can pass all conditions and leads to a stable dark-energy-dominated universe.

A study of pilgrim dark energy model in Brans–Dicke theory

In this paper, we study the behavior of non-interacting and interacting pilgrim dark energy (DE) for non-flat FRW model in Brans–Dicke (BD) theory. We consider the future event horizon as well as logarithmic form of BD scalar field [Formula: see text], where [Formula: see text], [Formula: see text] and [Formula: see text] is the scale factor. We evaluate some well-known cosmological parameters such as equation of state as well as deceleration parameter and [Formula: see text] plane as well as statefinder parameters. We discuss graphical behavior of these parameters through pilgrim DE parameter [Formula: see text] for both non-interacting as well as interacting case with interacting parameter [Formula: see text]. It is found that the equation of state parameter gives consistent results with the current cosmic behavior while the deceleration parameter represents transition from decelerated to accelerated phase. The cosmological planes represent different DE regions. Finally, we discuss stability of the model through squared speed of sound in both cases.

Cosmic aspects of specific dark energy model in loop quantum cosmology

We consider the interacting scenario of pilgrim dark energy (with future event horizon) with cold dark matter in loop quantum cosmology. We explore various cosmological parameters (Hubble, equation of state and deceleration) and planes ([Formula: see text] and [Formula: see text]–[Formula: see text]) in this framework. We also discuss the validity of generalized second law of thermodynamics and find that it remains valid in the early epoch as well as in late future epoch. Finally, it is remarked that all the above constraints of cosmological parameters show consistency with different observational data like Planck, WP, BAO, [Formula: see text], SNLS and nine-year WMAP.

Analysis of interacting entropy-corrected holographic and new agegraphic dark energies

In the present work, we assume the flat FRW model of the universe is filled with dark matter and dark energy where they are interacting. For dark energy model, we consider the entropy-corrected HDE (ECHDE) model and the entropy-corrected NADE (ECNADE). For entropy-corrected models, we assume logarithmic correction and power law correction. For ECHDE model, length scale [Formula: see text] is assumed to be Hubble horizon and future event horizon. The [Formula: see text]-[Formula: see text] analysis for our different horizons are discussed.

Interacting holographic dark energy model in Brans–Dicke cosmology and coincidence problem

We study the dynamics of interacting holographic dark energy model in Brans–Dicke cosmology for the future event horizon and the Hubble horizon cut-offs. We determine the system of first-order differential equations for the future event horizon and Hubble horizon cut-offs and obtain the corresponding fixed points, attractors, repellers and saddle points. Finally, we investigate the cosmic coincidence problem in this model for the future event horizon and Hubble horizon cut-offs and find that for both cut-offs and for a variety of Brans–Dicke parameters, the coincidence problem is almost resolved.

Reconstruction of f(T) Gravity with Interacting Variable-Generalised Chaplygin Gas and the Thermodynamics with Corrected Entropies

This article reports a study on variable-generalised Chaplygin gas (VGCG) interacting with pressureless dark matter (DM) with interaction term Q chosen in the form Q=3HδρΛ, where ρΛ denotes the density of the VGCG. Detailed cosmology of the interacting VGCG has been studied and a quintom behaviour of the equation of state (EoS) parameter has been observed. A statefinder analysis has shown attainment of ΛCDM fixed point by the interacting VGCG. Subsequently, a reconstruction scheme for f(T) gravity has been presented based on the interacting VGCG with power-law form of scale factor. The EoS parameter corresponding to the reconstructed f(T) has shown quintom behaviour. Finally, we have studied the generalised second law (GSL) of thermodynamics in reconstructed f(T) cosmology considering the universe as a closed bounded system with future event horizon as the cosmological boundary. We have associated two different entropies with the cosmological horizons with a logarithmic correction term and a power-law correction term. We have studied the validity of the GSL for both of these corrections.

Holographic dark energy with the sign-changeable interaction term

We use three infrared (IR) cutoffs, including the future event horizon, the Hubble and Granda–Oliveros (GO) cutoffs, to construct three holographic models of dark energy (DE). Additionally, we consider a Friedmann–Robertson–Walker (FRW) universe filled by a dark matter (DM) and a DE that interact with each other through a mutual sign-changeable interaction with positive coupling constant. Thereinafter, we address the evolution of the some cosmological parameters, such as the equation of state (EoS) and dimensionless density parameters of DE as well as the deceleration parameter, during the cosmic evolution from the matter-dominated era until the late-time acceleration. We observe that a holographic dark energy (HDE) model with Hubble cutoff interacting with DM cannot be in line with the current universe. Our study shows that models with the future event horizon as the IR cutoff or the GO cutoff are in good agreement with the observational data. In fact, we find out that these two recent models can predict the universe transition from a deceleration phase to the acceleration one in a compatible way with observations. The three obtained models may also allow the EoS parameter to cross the phantom line, a result which depends on the values of the system’s constants such as the value of the interaction coupling constant.

Holographic dark energy from minimal supergravity

We embed models of holographic dark energy (HDE) coupled to dark matter (DM) in minimal supergravity plus matter, with one chiral superfield. We analyze two cases. The first one has the Hubble radius as the infrared (IR) cutoff and the interaction between the two fluids is proportional to the energy density of the DE. The second case has the future event horizon as IR cutoff while the interaction is proportional to the energy density of both components of the dark sector.

Semiclassical approach for the evaporating black hole revisited

A recent calculation shows that the observed energy density in the Unruh state at the future event horizon as seen by a freely falling observer is finite if the observer is released from rest at any positive distance outside the horizon; however, it is getting larger and larger so that it is negatively divergent at the horizon in the limit that the observer starts falling from rest at the horizon, which corresponds to the infinite boost with respect to the freely falling observer at a finite distance from the horizon. In order to resolve some conflicts between the recent calculation and the conventional ones in the well-known literatures, the calculation of the free-fall energy density is revisited and some differences are pointed out.

A dynamical system analysis of holographic dark energy models with different IR cutoff

The paper deals with a dynamical system analysis of the cosmological evolution of an holographic dark energy (HDE) model interacting with dark matter (DM) which is chosen in the form of dust. The infrared cutoff of the holographic model is considered as future event horizon or Ricci length scale. The interaction term between dark energy (DE) and DM is chosen of following three types: (i) proportional to the sum of the energy densities of the two dark components, (ii) proportional to the product of the matter energy densities and (iii) proportional to DE density. The dynamical equations are reduced to an autonomous system for the three cases and corresponding phase space is analyzed.