Hongya Liu

Agegraphic dark energy as a quintessence

Recently, a dark energy model characterized by the age of the universe, dubbed “agegraphic dark energy”, was proposed by Cai. In this paper, a connection between the quintessence scalar-field and the agegraphic dark energy is established, and accordingly, the potential of the agegraphic quintessence field is constructed.

Holographic tachyon model

We propose in this Letter a holographic model of tachyon dark energy. A connection between the tachyon scalar-field and the holographic dark energy is established, and accordingly, the potential of the holographic tachyon field is constructed. We show that the holographic evolution of the universe with c >= I can be described completely by the resulting tachyon model in a certain way. (c) 2007 Elsevier B.V. All rights reserved.

Holographic dark energy in a cyclic universe

In this paper we study the cosmological evolution of the holographic dark energy in a cyclic universe, generalizing the model of holographic dark energy proposed by Li. The holographic dark energy with c<1 can realize a quintom behavior; namely, it evolves from a quintessence-like component to a phantom-like one. The holographic phantom energy density grows rapidly and dominates the late-time expanding phase, helping to realize a cyclic universe scenario in which the high energy regime is modified by the effects of quantum gravity, causing a turn-around (and a bounce) of the universe. The dynamical evolution of holographic dark energy in the regimes of low energy and high energy is governed by two differential equations, respectively. It is of importance to link the two regimes for this scenario. We propose a link condition giving rise to a complete picture of holographic evolution of a cyclic universe.

Constraints on modified Chaplygin gas from recent observations and a comparison of its status with other models

Abstract In this Letter, a modified Chaplygin gas (MCG) model of unifying dark energy and dark matter with the exotic equation of state p MCG = B ρ MCG − A ρ MCG α is constrained from recently observed data: the 182 Gold SNe Ia, the 3-year WMAP and the SDSS baryon acoustic peak. It is shown that the best fit value of the three parameters ( B , B s , α ) in MCG model are ( − 0.085 , 0.822 , 1.724 ). Furthermore, we find the best fit w ( z ) crosses −1 in the past and the present best fit value w ( 0 ) = − 1.114 − 1 , and the 1σ confidence level of w ( 0 ) is − 0.946 ⩽ w ( 0 ) ⩽ − 1.282 . Finally, we find that the MCG model has the smallest χ min 2 value in all eight given models. According to the Akaike Information Criterion (AIC) of model selection, we conclude that recent observational data support the MCG model as well as other popular models.

Fifth force from fifth dimension

Abstract If the world has more than four dimensions, but is still described by a theory like general relativity, there are (small) modifications to the conservation laws and dynamics. Using a novel coordinate system, we examine a 5D space and isolate a fifth force which in principle is measurable. In general, the detection of departures from conventional dynamics is a way of testing dimensionality.

CONSTRAINTS TO DECELERATION PARAMETERS BY RECENT COSMIC OBSERVATIONS

In this paper, two simple parametrized deceleration parameters and areconstrained by using the latest SNe Ia Gold dataset, observational Hubble data and their combination. It is found that the transition redshift from decelerated expansion to accelerated expansion zT and current decelerated parameter values q0 are consistent with each other in 1σ region by only using SNe Ia Gold dataset and observational Hubble data in three parametrizations respectively. By combining the SNe Ia Gold dataset and observational Hubble data together, a tight constraint is obtained. With this combined constraint, zT is and with 1σ error in the parametrizations respectively.

Exact global solutions of brane universes and big bounce

Abstract Exact global solutions of five-dimensional cosmological models compactified on a S 1 / Z 2 orbifold with two 3-branes are presented, and evolution of a simple model is studied. It is found that on all 4D spacetime hypersurfaces, except a singular one, the expanding universe was started not from a big bang but from a big bounce, and before the bounce the universe was in a deflationary contracting phase. It is also found that whether the energy density on the second brane is positive, zero, or negative depends on the size of the fifth dimension.

INDUCED PHANTOM AND 5D ATTRACTOR SOLUTION IN SPACETIME–MATTER THEORY

In Spacetime–Matter theory we assume that the 4D induced matter of the 5D Ricci-flat bouncing cosmological solutions contains a perfect fluid as well as an induced scalar field. Then we show that the conventional 4D quintessence and phantom models of dark energy could be recovered from the 5D cosmological solutions. By using the phase-plane analysis to study the stability of evolution of the 5D models, we find that the conventional 4D late-time attractor solution is also recovered. This attractor solution shows that the scale factors of the phantom dominated universes in both the 4D and 5D theories will reach infinity in a finite time and the universes will be ended at a new kind of spacetime singularity at which everything will be annihilated. We also find that the repulsive force of the phantom may provide us with a mechanics to explain the bounce.

CONSTRAINTS ON TRANSITION REDSHIFT AND DECELERATION PARAMETER FROM RECENT OBSERVATIONS

In this paper, the 1σ confidence level of transition redshift and present deceleration parameter are studied using modified Chaplygin gas (MCG) model from recent observations: the 182 Gold SNe Ia, three-year WMAP, SDSS baryon acoustic peak and X-ray gas mass fraction in clusters. Concretely, the best fit values of the transition redshift and present deceleration parameter with the 1σ confidence level are

Waves and particles in Kaluza–Klein theory

z_{T}=0.45_{-0.09}^{+0.19}