Yun-He Li

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.

Large-scale stable interacting dark energy model: Cosmological perturbations and observational constraints

Dark energy might interact with cold dark matter in a direct, nongravitational way. However, the usual interacting dark energy models (with constant w ) suffer from some catastrophic difficulties. For example, the Q ∝ ρ c model leads to an early-time large-scale instability, and the Q ∝ ρ de model gives rise to the future unphysical result for cold dark matter density (in the case of a positive coupling). In order to overcome these fatal flaws, we propose in this paper an interacting dark energy model (with constant w ) in which the interaction term is carefully designed to realize that Q ∝ ρ de at the early times and Q ∝ ρ c in the future, simultaneously solving the early-time superhorizon instability and future unphysical ρ c problems. The concrete form of the interaction term in this model is Q = 3 β H ρ de ρ c ρ de + ρ c , where β is the dimensionless coupling constant. We show that this model is actually equivalent to the decomposed new generalized Chaplygin gas (NGCG) model, with the relation β = - α w . We calculate the cosmological perturbations in this model in a gauge-invariant way and show that the cosmological perturbations are stable during the whole expansion history provided that β > 0 . Furthermore, we use the Planck data in conjunction with other astrophysical data to place stringent constraints on this model (with eight parameters), and we find that indeed β > 0 is supported by the joint constraint at more than 1 σ level. The excellent theoretical features and the support from observations all indicate that the decomposed NGCG model deserves more attention and further investigation.

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

Running coupling: does the coupling between dark energy and dark matter change sign during the cosmological evolution?

Q=3\beta H\rho_{de}

Cosmological constraints on neutrinos after BICEP2

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

Testing models of vacuum energy interacting with cold dark matter

\beta\sim\mathcal{O}(10^{-3})

Interacting model of new agegraphic dark energy:observational constraints and age problem

) can survive in current RSD data. However, all these analyses had to assume

A global fit study on the new agegraphic dark energy model

w>-1

Neutrinos in the holographic dark energy model: constraints from latest measurements of expansion history and growth of structure

and