Studies About Cosmological Models And Dark Energy Related Issues Based On Observational Data

The big bang cosmology is considered to have three observational pillars:Hub-ble expansion, primordial nucleosynthesis and cosmic microwave background(CMB). Basing on the three observational pillars, the standard cosmological model set up by using cosmological principle and general relativity. On the one hand, the standard cos-mological model gives a time-evolution universe which is homogeneous and isotropic. The evolution of the background universe is described by the scale factor, and the evo-lution of the scale factor obeys the Friedmann equation. On the other hand, the modern cosmology holds that the real universe is the one that has perturbation:the space and time of the universe is not perfectly homogeneous, and the clump exists in the distri-bution of matter in the universe. A universe that has tiny fluctuation can be described by the linear theory of cosmological perturbation. The existence of complex structure in the present universe is the result of the evolution of primordial density perturbation due to gravitational instability, and the origin of the primordial perturbation can be ex-plained by inflation theory. According to the inflation theory, the very early universe is highly homogeneous and isotropic. When the energy scale is about 1015GeV, the universe went through a very short process of accelerated expansion. The quantum fluctuation generated during this short time was quickly pulled out of Hubble horizon, and frozen into the classical primordial perturbation on the large scale.With the improvement of observation equipment and technology, now the study of cosmology has entered the precision time. Research on precise cosmology is embodied in the accurate measurement on cosmological parameters by using precise observation-al data. During this stage, the observational cosmology has made a series of progress. Two things are very significant:a. discovering the accelerating expansion of universe by using the data of distant la supernovae (SNIa) in 1998, which implies the existence of dark energy; b. WMAP and Planck, the satellites of two generation that measure the CMB anisotropy after COBE, have precisely given the angular power spectrum of CMB fluctuation, which allows us to carefully study the physical process of the early universe.The current data from traditional cosmological probes, such as CMB, BAO and SNIa, are still supports ACDM model. In the framework of this model, the universe is spatially flat, has a nearly scale-invariant spectrum of primordial scalar fluctuation-s, nearly 70% of the total energy density in the universe is cosmological constant(A) dark energy,25% of the total energy density in the universe is cold dark matter(CDM), and the SM matter only accounts for about 5%. On the other hand, traditional data also support some extended ACDM models, which introduced some extra parameters based on ACDM model. However, due to various degeneracies among these cosmo-logical parameters under the traditional data, even some data that has high precision cannot give very good constraints on some crucial parameters. So we consider a new kind of cosmological probe-the direct measurement on Hubble parameters. Hubble parameters reflect the expansion history of the universe, and it is related to the infor-mation of the various species in the universe, so we hope the observational data from it can help to break the degeneracies and improve the limits. We include the observation-al data from Hubble parameters into the traditional probes, and study the constraints on several extended ACDM models. The results shows it is very helpful to break de-generacies among some cosmological parameters when including the data at different redshift points into the analysis, since the Hubble parameters data could provide the ex-tra information of the expansion rate of universe at late times, thus we can obtain better constraints on these parameters. Additionally, The HST measurement on the Hubble constant, HST Ho prior, can obviously impact on the median values of parameters and also give good constraints on some of them, such as the effective number of neutrinos Neff, due to the degeneracies between these parameters and H0.Since the discovery of the accelerating expansion of the universe from the obser-vations of type la supernovae, dark energy, the mysterious component in our universe which drives the acceleration, remains the hot topic in modern cosmology. There are a variety of assumptions about the nature of dark energy, such as the vacuum energy, and some kinds of scalar field. But none of these physical explanations is compelling. So the determination on the parameterized dark energy model from observational data is very important. For the dark energy, equation of state (EoS) is the key parameter to describe its properties, which determines the dark energy density evolution. Therefore, we adopt the latest observations to study the EoS:we make use of observational data combination of angular power spectrum data from Planck, Pan-STARRS SNIa sample and BAO, and have performed 3 different data fitting by adopting a constant EoS, a time-evolving EoS(w(a)=w0+wa(1-a)) with 2-parameter form and EoS of piece-wise constant bins (bin-w). For the result of bin-w, we do the method of PCA to make our EoS more physical. The result shows that, for the first two forms of param-eterization, the EoS is still consistent with ACDM at 2σ and 1σ respectively; for the bin-w case, the global fitting results are well consistent with ACDM, but EoS may deviate from-1 slightly in some of the redshifts bins after the the model-independent PCA analysis. Considering the relatively larger errors in many bins, we still need more data to confirm this point.