Je-An Gu

Inhomogeneity-induced cosmic acceleration in a dust universe

It is the common consensus that the expansion of a universe always slows down if the gravity provided by the energy sources therein is attractive and accordingly one needs to invoke dark energy as a source of anti-gravity for understanding the cosmic acceleration. To examine this point we find counterexamples for a spherically symmetric dust fluid described by the Lemaitre–Tolman–Bondi solution without singularity. Thus, the validity of this naive consensus is indeed doubtful and the effects of inhomogeneities should be restudied. These counter-intuitive examples open a new perspective on the understanding of the evolution of our universe.

Can the quintessence be a complex scalar field

Abstract In light of the recent observations of type Ia supernovae suggesting an accelerating expansion of the Universe, we wish in this Letter to point out the possibility of using a complex scalar field as the quintessence to account for the acceleration. In particular, we extend the idea of Huterer and Turner in deriving the reconstruction equations for the complex quintessence, showing the feasibility of making use of a complex scalar field (instead of a real scalar field) while maintaining the uniqueness feature of the reconstruction for two possible situations, respectively. We discuss very briefly how future observations may help to distinguish the different quintessence scenarios, including the scenario with a positive cosmological constant.

Teleparallel dark energy with purely non-minimal coupling to gravity

Abstract We propose the simplest model of teleparallel dark energy with purely a non-minimal coupling to gravity but no self-potential, a single model possessing various interesting features: simplicity, self-potential-free, the guaranteed late-time cosmic acceleration driven by the non-minimal coupling to gravity, tracker behavior of the dark energy equation of state at earlier times, a crossing of the phantom divide at a late time, and the existence of a finite-time future singularity. We find the analytic solutions of the dark-energy scalar field respectively in the radiation, matter, and dark energy dominated eras, thereby revealing the above features. We further illustrate possible cosmic evolution patterns and present the observational constraint of this model obtained by numerical analysis and data fitting.

Singularity problem in teleparallel dark energy models

We study future singularity in teleparallel dark energy models, particularly its behavior and its (non)occurrence in the observationally viable models. For the models with a general self-potential of the scalar field, we point out that both at early times and in the future near the singularity the behavior of dark energy can be described by the analytic solutions of the scalar field we obtained for the model with no self-potential. As to the (non)occurrence in the viable models, we consider a natural binding-type self-potential, the quadratic potential, when fitting observational data, and illustrate the constraining region up to the

CONSISTENCY TEST OF DARK ENERGY MODELS

3\ensuremath{\sigma}

Accelerating universe as from the evolution of extra dimensions

confidence level as well as the region where a singularity will occur. As a result, the singularity region is outside the

Constraints on the phase plane of the dark energy equation of state

3\ensuremath{\sigma}

Evolution of the universe with flat extra dimensions

constraint. Thus, although the future singularity problem potentially exists in teleparallel dark energy models, the observationally viable models may not suffer this problem.

Stress-energy tensor induced by a bulk Dirac spinor in the Randall-Sundrum model

Recently we proposed a new approach to test dark energy models based on the observational data. In that work we focused particularly on quintessence models for demonstration and invoked a widely used parametrization of the dark energy equation of state. In this paper we take the more recent SN Ia, CMB and BAO data, invoke the same parametrization, and apply this method of consistency test to five dark energy models, including the ΛCDM model, the generalized Chaplygin gas, and three quintessence models: exponential, power-law and inverse-exponential potentials. We find that the exponential potential of quintessence is ruled out at the 95.4% confidence level, while the other four models are consistent with data. This consistency test can be efficiently performed since for all models it requires the constraint of only a single parameter space that by choice can be easily accessed.

Fate of an accelerating universe

In this paper we propose that the accelerating expansion of the present matter-dominated universe, as suggested by the recent distance measurements of type Ia supernovae, is generated along with the evolution of space in extra dimensions. The Einstein equations are first analyzed qualitatively and then solved numerically, so as to exhibit explicitly these patterns of the accelerating expansion in this scenario. A fine-tuning problem associated with such a scenario is also described and discussed.