The Applications Of Cosmic Distance-Duality Relation And Constraints On Cosmic Neutrinos

Observational cosmology has embarked upon plenty of great progress in recent decades. With lots of novel techniques, methods and cosmological probes being developed, a number of scientific discoveries come into our lives and revolutionize our understand-ing of the universe. This thesis is heavily based on the applications of cosmological probes, which promises this kind of phenomenological study connecting theories and observations.The first chapter briefly introduces the research backgrounds and motivations for our two works; the first one is using the cosmic distance-duality relation to test galaxy cluster gas mass fraction measurements, and the second one observational constraints on cosmic neutrino properties along with dark energy equation of state from various cosmological probes.In the second chapter, we propose a consistency test of some recent X-ray gas mass fraction (fgas) measurements in galaxy clusters, using the cosmic distance-duality relation,ηthcory=DL(1+z)-2/DA, with luminosity distance (DL) data from the Union2compilation of type la supernovae. We set ηtheory≡1, instead of assigning any redshift parameterizations to it, and constrain the cosmological information preferred by fg data along with supernova observations. We adopt a new binning method in the reduc-tion of the Union2data, in order to minimize the statistical errors. Four data sets of X-ray gas mass fraction, which are reported by Allen et al.(2samples), LaRoque et al. and Ettori et al., are detailedly analyzed against two theoretical modelings of fgas. The results from the analysis of Allen et al.’s samples prove the feasibility of our method. It is found that the preferred cosmology by LaRoque et al.’s sample is consistent with its reference cosmology within1-σ confidence level. However, for Ettori et al.’s fgas sample, the inconsistency can reach more than3-σ confidence level and this dataset shows special preference to an ΩΛ=0cosmology.The third chapter is highly associated with our work on cosmological constraints on neutrino properties. Using several cosmological observations, i.e. the cosmic mi-crowave background anisotropies (WMAP7yr), the weak gravitational lensing (CFHTL-S3yr), the measurements of baryon acoustic oscillations (SDSS+WiggleZ), the most recent observational Hubble parameter data, the Union2.1compilation of type la su-pernovae, and the HST prior, we impose constraints on the sum of neutrino masses (∑mν), the effective number of neutrino species (Neff) and dark energy equation of s-tate (w), individually and collectively. We find that a tight upper limit on∑mν can be extracted from the full data combination, if Neff and w are fixed. However this upper bound is severely weakened if Ncff and w are allowed to vary. This result naturally raises questions on the robustness of previous strict upper bounds on∑mν, ever re-ported in the literature. The best-fit values from our most generalized constraint read∑mν=0.556+0.231-0.288eV,Neff=3.839±0.452, and w=-1.058±0.088at68%confi-dence level, which shows a firm lower limit on total neutrino mass, favors an extra light degree of freedom, and supports the cosmological constant model. The constraining ability of current weak lensing data is indeed helpful when w=-1yet is of little help once w is freed. The dataset of Hubble parameter gains numerous advantages over su-pernovae, when w is fixed, particularly its illuminating power in constraining Neff. As long as w is included as a free parameter, it is still the standardizable candles of type la supernovae that play the most dominant role in the parameter constraints.Finally, we present a brief summary of our research works included in this thesis, and deliver a prospect for further studies.