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Null test of the cosmic curvature using H(z) and supernovae data

Rong-Gen Cai; Zong-Kuan Guo; Tao YangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We introduce a model-independent approach to the null test of the cosmic curvature which is geometrically related to the Hubble parameter H(z) and luminosity distance dL(z). Combining the independent observations of H(z) and dL(z), we use the model-independent smoothing technique,Gaussian processes, to reconstruct them and determine the cosmic curvature (0)K in the null test relation. The null test is totally geometrical and does not assume any cosmological model. We show that the cosmic curvature (0)K = 0 is consistent with current observational data sets, falling within the 1#27; limit. To demonstrate the effect on the precision of the null test, we produce a series of simulated data of the models with different (0) K . Future observations in better quality can provide a greater improvement to constrain or refute the flat universe with (0)K = 0. |

Principal component analysis of the reionization history from Planck 2015 data

Wei-Ming Dai; Zong-Kuan Guo; Rong-Gen CaiSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

The simple assumption of an instantaneous reionization of the Universe may bias estimates of cosmological parameters. In this paper a model-independent principal component method for the reionization history is applied to give constraints on the cosmological parameters from recent Planck 2015 data. We find that the Universe are not completely reionized at redshifts z #21; 8.5 at 95% CL. Both the reionization optical depth and the matter fluctuation amplitude are higher than but consistent with those obtained in the standard instantaneous reionization scheme. The high estimated value of the matter fluctuation amplitude strengthens the tension between Planck CMB observations and some astrophysical data, such as cluster counts and weak lensing. The tension can significantly be relieved if the neutrino masses are allowed to vary. Thanks to a high scalar spectral index, the low-scale spontaneously broken SUSY inflationary model can fit the data well, which is marginally disfavored at 95% CL in the Planck analysis. |

Reconstructing the interaction between dark energy and dark matter using Gaussian processes

Tao Yang ; Zong-Kuan Guo; Rong-Gen CaiSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We present a nonparametric approach to reconstruct the interaction between dark energy and dark matter directly from SNIa Union 2.1 data using Gaussian processes, which is a fully Bayesian approach for smoothing data. In this method, once the equation of state (w) of dark energy is specified, the interaction can be reconstructed as a function of redshift. For the decaying vacuum energy case with w = ?1, the reconstructed interaction is consistent with the standard #3;CDM model, namely, there is no evidence for the interaction. This also holds for the constant w cases from ?0.9 to ?1.1 and for the Chevallier-Polarski-Linder (CPL) parametrization case. If the equation of state deviates obviously from ?1, the reconstructed interaction exists at 95% confidence level. This shows the degeneracy between the interaction and the equation of state of dark energy when they get constraints from the observational data. |

Reconstruction of the primordial power spectra with Planck and BICEP2

Bin Hu; Jian-Wei Hu; Zong-Kuan Guo; Rong-Gen CaiSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

By using the cubic spline interpolation method, we reconstruct the shape of the primordial scalar and tensor power spectra from the recently released Planck temperature and BICEP2 polarization cosmic microwave background data. We find that the vanishing scalar index running (dns/d ln k) model is strongly disfavored at more than 3σ confidence level on the k = 0.0002 Mpc?1 scale. Furthermore, the power-law parameterization gives a blue-tilt tensor spectrum, no matter using only the first 5 bandpowers nt = 1.20+0.56 (95%CL) or the full 9 bandpowers nt = 1.24+0.51 (95%CL) of?0.64 ?0.58BICEP2 data sets. Unlike the large tensor-to-scalar ratio value (r 0.20) under the scale-invariant tensor spectrum assumption, our interpolation approach gives r0.002 < 0.060 (95%CL) by using the first 5 bandpowers of BICEP2 data. After comparing the results with/without BICEP2 data, we find that Planck temperature with small tensor amplitude signals and BICEP2 polarization data with large tensor amplitude signals dominate the tensor spectrum reconstruction on the large and small scales, respectively. Hence, the resulting blue tensor tilt actually reflects the tension between Planck and BICEP2 data. |

Nucleosynthesis constraint on Lorentz invariance violation in the neutrino sector

Zong-Kuan Guo; Jian-Wei HuSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We investigate the nucleosynthesis constraint on Lorentz invariance violation in the neutrino sector which influences the formation of light elements by altering the energy density of the Universe and weak reaction rates prior to and during the big-bang nucleosynthesis epoch. We derive the weak reaction rates in the Lorentz-violating extension of the standard model. Using measurements of the primordial helium-4 and deuterium abundances, we give a tighter constraint on the deformed parameter than that derived from measurements of the cosmic microwave background anisotropies. |

Cosmological constraints on Lorentz invariance violation in the neutrino sector

Zong-Kuan Guo; Qing-Guo Huang; Rong-Gen Cai,; Yuan-Zhong ZhangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We derive the Boltzmann equation in the synchronous gauge for massive neutrinos with a deformed dispersion relation. Combining the 7-year WMAP data with lower-redshift measurements of the expansion rate, we give constraints on the deformation parameter and find that the deformation parameter is strong degenerate with the physical dark matter density rather than the neutrino mass. Our results show that there is no evidence for Lorentz invariant violation in the neutrino sector. The ongoing Planck experiment could provide improved constraints on the deformation parameter. |

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