Narrow Results

We consider a model of interacting cosmological constant/quintessence, where dark matter and dark energy behave as, respectively, two coexisting phases of a fluid, a thermally excited Bose component and a condensate, respectively. In a simple phenomenological model for the dark components interaction we find that their energy density evolution is strongly coupled during the universe evolution. This feature provides a possible way out for the coincidence problem affecting many quintessence models.

We investigate the cosmology of ELKO spinor model when minimal coupling with torsion is included. Unlike the Dirac spinor which interacts only with the totally anti-symmetric components of torsion tensor, we find the interaction of the time component of the trace of torsion tensor with ELKO spinor provides different aspects of cosmology with fermions. We discuss a couple of cases with given potentials of ELKO spinor which can result in interesting cosmological consequences. Especially, we show that there exists an exact cosmological solution in which the universe began its acceleration only recently and this solution is an attractor. This corresponds to specific forms of torsion and potential with a mild fine-tuning which can address the coincidence problem.

We discuss the possible solution of "coincidence" problem in the modified holographic dark energy (MHDE) model with an interacting term Q = 3γρ_m. Theoretically, the phase-space analytical results of the automatic system of the equation of state (EoS) parameter ω_d and its evolution ω′_d show a scaling solution F_M appears in the interacting case which hints a possibility of alleviating the "coincidence" problem. On the observational side, the results from the combined data Supernova Legacy Survey (SNLS)+Cosmic Microwave Background (CMB)/Baryon Acoustic Oscillations (BAO)+H₀ slightly favor a positive interaction which meets the existing and unstable requirements of the scaling solution.

We are building a cosmological model that has small anisotropy and curvature. For this purpose, the Bianchi type-V (BV) model with dark matter (DM) and anisotropic dark energy (DE) is considered. Assuming a law for deviation from the equation of state (Eos) of DE is linear, we study the transition from decelerated to late accelerated expansion of the Universe. In the proposed model, the linear interaction of the two components alleviates the coincidence problem of the present Universe. Examples of model parameter values compatible with observational data are given.

In this paper, we investigate anisotropic fluid cosmology in a situation where the space–time metric back-reacts in a local, time-dependent way to the presence of inhomogeneities. We derive exact solutions to the Einstein field equations describing Friedmann–Lemaítre–Robertson–Walker (FLRW) large-scale cosmological evolution in the presence of local inhomogeneities and time-dependent backreaction. We use our derivation to tackle the cosmological constant problem. A cosmological constant emerges by averaging the backreaction term on spatial scales of the order of 100 Mpc, at which our universe begins to appear homogeneous and isotropic. We find that the order of magnitude of the “emerged” cosmological constant agrees with astrophysical observations and is related in a natural way to baryonic matter density. Thus, there is no coincidence problem in our framework.

This paper reviews a systematic dynamical analysis on a general form of scalar field as Dark Energy (DE) with dark matter (DM) to sort out the “cosmic coincidence” problem. Here the autonomous system of differential equations is two-dimensional (2D) as well as nonlinear. So we have utilized nonlinear dynamical theory to explain various cosmological implications of this model. Nowadays, we have noted that some works are undertaking this nonlinear systems theory. Although we have seen that most of the works are simplifying the underlying nonlinear dynamical systems similar to a linear one, that can lead to flawed conclusions about the evolution of the universe. Since an important theorem, Poincare–Bendixson theorem asserts linearization of the nonlinear system and does not give “global” stability, unlike the linear one if the dimension is more than two. Anyway, our work is different from others in this regard. Here the dimension of the system is two, and we have obtained some interesting stuffs also. We have applied the above theorem of nonlinear dynamical systems and others to find the “global” stability. This theorem offers completely different stable solutions, contrary to the prediction of linear analysis. As a result, we have obtained two fixed points; one of them is a stable “attractor” (it is attracting “node” actually), and thereafter, we have analyzed the stability. To investigate the dynamical system behavior, we have drawn different figures. These figures include vector field and a new plotting strategy (explained later). These investigations suggest a way out of the coincidence problem (or, precisely speaking, what should be the mathematical form of the term “$C$”, which indicates interaction between DE and DM to reduce coincidence). In this scenario, if the equation of state (EoS) of DE and DM obeys ${\omega }_d<{\omega }_m$, then coincidence problem may be avoided.

Using the quantum corrected Friedmann equation, obtained from the quantum Raychaudhuri equation, and assuming a small mass of the graviton (but consistent with observations and theory), we propose a resolution of the smallness problem (why is observed vacuum energy so small?) and the coincidence problem (why does it constitute most of the universe, about 70%, in the current epoch?).

We investigate the interacting holographic dark energy (HDE) with Granda–Oliveros (GO) infrared (IR)-cutoff in the framework of Brans–Dicke (BD) cosmology. We obtain the equation of state (EoS) parameter of HDE, w_D, the effective EoS parameter w_eff, the deceleration parameter q and the squared of sound speed in a flat Friedmann–Robertson–Walker (FRW) universe. We show that at late-time the cosmic coincidence problem can be alleviated. Also we show that for noninteracting case, HDE can give a unified dark matter–dark energy (DM–DE) profile in BD cosmology, except that it cannot solve the coincidence problem in the future. By studying the EoS parameter, we see that the phantom divide may be crossed. Using the latest observational data, we calculate the best values of the parameters for interacting HDE in BD framework. Computing the deceleration parameter implies that the transition from deceleration to the acceleration phase occurred for redshift z ≥ 0.5. Finally, we investigate the sound stability of the model, and find that HDE with Granda–Oliveros (GO)-cutoff in the framework of BD cosmology can lead to a stable DE-dominated universe favored by observations, provided we take β = 0.44 and b² < 0.35. This is in contrast to HDE model in Einstein gravity which does not lead to a stable DE-dominated universe.

In the six-dimensional (6D) Einstein gravity with a negative cosmological constant, we determine the structure of warped spacetimes bounded by 4-branes. We find an accelerating Universe solution with the induced dark energy, from the 4-brane obtained by orbifolding an external space, and suggest a possibility of addressing problems related to the cosmological constant.

The novel idea is that the undergoing accelerated expansion of the universe happens due to infrared quantum gravity modifications at intermediate astrophysical scales of galaxies or galaxy clusters, within the framework of Asymptotically Safe gravity. The reason is that structures of matter are associated with a scale-dependent positive cosmological constant of quantum origin. In this context, no extra unproven energy scales or fine-tuning are used. Furthermore, this model was confronted with the most recent observational data from a variety of probes, and with the aid of Bayesian analysis, the most probable values of the free parameters were extracted. Finally, the model proved to be statistically equivalent with $\mathrm{\Lambda }$CDM, and thus being able to resolve naturally the concept of dark energy and its associated cosmic coincidence problem.