Jing-Fei Zhang

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.

Sterile neutrinos help reconcile the observational results of primordial gravitational waves from Planck and BICEP2

Abstract We show that involving a sterile neutrino species in the Λ CDM + r model can help relieve the tension about the tensor-to-scalar ratio r between the Planck temperature data and the BICEP2 B-mode polarization data. Such a model is called the Λ CDM + r + ν s model in this paper. Compared to the Λ CDM + r model, there are two extra parameters, N eff and m ν , sterile eff , in the Λ CDM + r + ν s model. We show that in this model the tension between Planck and BICEP2 can be greatly relieved at the cost of the increase of n s . However, comparing with the Λ CDM + r + d n s / d ln ⁡ k model that can significantly reduce the tension between Planck and BICEP2 but also makes trouble to inflation due to the large running of the spectral index of the order 10 − 2 produced, the Λ CDM + r + ν s model is much better for inflation. By including a sterile neutrino species in the standard cosmology, besides the tension with BICEP2, the other tensions of Planck with other astrophysical data, such as the H 0 direct measurement, the Sunyaev–Zeldovich cluster counts, and the galaxy shear data, can all be significantly relieved. So, this model seems to be an economical choice. Combining the Planck temperature data, the WMAP-9 polarization data, and the baryon acoustic oscillation data with all these astrophysical data (including BICEP2), we find that in the Λ CDM + r + ν s model n s = 0.999 ± 0.011 , r = 0.21 − 0.05 + 0.04 , N eff = 3.95 ± 0.33 and m ν , sterile eff = 0.51 − 0.13 + 0.12 eV . Thus, our results prefer Δ N eff > 0 at the 2.7 σ level and a nonzero mass of sterile neutrino at the 3.9 σ level.

INTERACTING MODEL OF NEW AGEGRAPHIC DARK ENERGY: COSMOLOGICAL EVOLUTION AND STATEFINDER DIAGNOSTIC

The statefinder diagnostic is a useful method for distinguishing different dark energy models. In this paper, we investigate the new agegraphic dark energy model with interaction between dark energy and matter component by using statefinder parameter pair {r, s} and study its cosmological evolution. We plot the trajectories of the new agegraphic dark energy model for different interaction cases in the statefinder plane. As a result, the influence of the interaction on the evolution of the universe is shown in the statefinder diagrams.

Sandage-Loeb test for the new agegraphic and Ricci dark energy models

Abstract The Sandage–Loeb (SL) test is a unique method to explore dark energy at the “redshift desert” ( 2 ≲ z ≲ 5 ), an era not covered by any other dark energy probes, by directly measuring the temporal variation of the redshift of quasar (QSO) Lyman-α absorption lines. In this Letter, we study the prospects for constraining the new agegraphic dark energy (NADE) model and the Ricci dark energy (RDE) model with the SL test. We show that, assuming only a ten-year survey, the SL test can constrain these two models with high significance.

Parametrized Post-Friedmann Framework for Interacting Dark Energy

Dark energy might directly interact with cold dark matter. However, in such a scenario, an early-time large-scale instability occurs occasionally, which may be due to the incorrect treatment for the pressure perturbation of dark energy as a nonadiabatic fluid. To avoid this nonphysical instability, we establish a new framework to correctly calculate the cosmological perturbations in the interacting dark energy models. Inspired by the well-known parametrized post-Friedmann approach, the condition of the dark energy pressure perturbation is replaced with the relationship between the momentum density of dark energy and that of other components on large scales. By reconciling the perturbation evolutions on the large and small scales, one can complete the perturbation equations system. The large-scale instability can be successfully avoided and the well-behaved density and metric perturbations are obtained within this framework. Our test results show that this new framework works very well and is applicable to all the interacting dark energy models.

Exploring the full parameter space for an interacting dark energy model with recent observations including redshift-space distortions: Application of the parametrized post-Friedmann approach

Dark energy can modify the dynamics of dark matter if there exists a direct interaction between them. Thus a measurement of the structure growth, e.g., redshift-space distortions (RSD), can provide a powerful tool to constrain the interacting dark energy (IDE) models. For the widely studied

Cosmological constraints on neutrinos after BICEP2

Q=3\beta H\rho_{de}

Testing models of vacuum energy interacting with cold dark matter

model, previous works showed that only a very small coupling (