Interacting dark energy: Dynamical system analysis

We investigate the impacts of interaction between dark matter (DM) and dark energy (DE) in the context of two DE models, holographic (HDE) and ghost dark energy (GDE). In fact, using the dynamical system analysis, we obtain the cosmological consequence of several interactions, considering all relevant component of universe, i.e. matter (dark and luminous), radiation and DE. Studying the phase space for all interactions in detail, we show the existence of unstable matter-dominated and stable DE-dominated phases. We also show that linear interactions suffer from the absence of standard radiation-dominated epoch. Interestingly, this failure resolved by adding the nonlinear interactions to the models. We find an upper bound for the value of the coupling constant of the interaction between DM and DE as [Formula: see text][Formula: see text]in the case of holographic model, and [Formula: see text] in the case of GDE model, to result in a cosmological viable matter-dominated epoch. More specifically, this bound is vital to satisfy instability and deceleration of matter-dominated epoch.

Dark Energy as a Cosmological Consequence of Existence of the Dirac Scalar Field in Nature

The solution of the field equations of the conformal theory of gravitation with Dirac scalar field in Cartan-Weyl spacetime at the very early Universe is obtained. In this theory dark energy (described by an effective cosmological constant) is a function of the Dirac scalar field β. This solution describes the exponential decreasing of β at the inflation stage and has a limit to a constant value of the dark energy at large time. This can give a way to solving the fundamental cosmological constant problem as a consequence of the fields dynamics in the early Universe.

COSMIC ACCELERATION IN A MODEL OF SCALAR–TENSOR GRAVITATION

In this paper we consider a model of the scalar–tensor theory of gravitation, in which the scalar field ϕ determines the gravitational coupling G and has a Lagrangian of the form [Formula: see text]. We study the cosmological consequence of this theory in the matter-dominated era and show that it leads to a transition from an initial decelerated expansion to an accelerated expansion phase at the present epoch. Using observational constraints, we see that the effective equation of state today for the scalar field turns out to be pϕ = wϕρϕ, with wϕ = -0.88, and that the transition to an accelerated phase happened at a redshift of about 0.3.

NEUTRINO DARK MATTER WITH A GALACTIC-RANGE NEW FORCE

We propose a new force of the galactic range acting among massive neutrinos to weaken the phase space constraint for the neutrino dark halo. As a consequence we find that the neutrino with mass 10 eV becomes a candidate for the galaxy dark halo to explain the observed flat rotation curves. Cosmological consequence of the new force is briefly discussed. It is, furthermore, pointed out that the new force can be naturally incorporated into the see-saw model for the neutrino mass.