Some Studies About Dark Matter And Dark Energy

Modern cosmology was born in1917. After Einstein wrote relativity field equa-tion of gravity, he used this equation to study cosmology problem. First he assumed that the universe is homogeneous and isotropic, and then he got a group of equations. Solving these equations, he found that steady state cosmology is an unstable solution. He think that is impossible, and he introduced a cosmology constant to make sure that our universe is steady. But in1929, Hubble found that our universe is expanding by measuring the galaxies'red shift.In1940s, George Gamow introduced a new cosmology theory with his students Robert Herman and Ralph Asher Alpher, which is called the big bang theory. In this model, the universe began with an extremely hot state. With the expansion of the universe, the universe become cold, and all kinds of nuclear formed in BBN era. After electron and atomic nucleus combine into atom, photon decoupled and exit in the form of the cosmic microwave background radiation. In1964, Robert Woodrow Wilson and Arno Allan Penzias found the existence of cosmic microwave background radiation. In addition, big bang cosmology model also explain the ratios of light nuclei successfully.In1998, two groups observed that the distant type la supernova looks more dim-mer. It indicated that we live in a accelerating universe. Also there are two different theories to explain this phenomenon. The first one assumes that there exists strange material with negative pressure, which is named dark energy. Another one is modified gravity. In this thesis we mainly discuss the dark energy model.Dark matter was observed more earlier. The earliest evidence is the measurement of galaxies'rotation curves. Since then more evidence was observed, such as gravita-tional lensing, large scale structure, etc. The detection of dark matter is in progress, such as direct detection in underground lab and some satellite cosmic-ray detectors. Although we have made lots of progress, we still do not know the nature of dark matter particle. The most popular dark matter particle candidate is weakly interacting massive particles(WMIP). The dark matter is one of the most important problems nowadays.Now, we know that73%of the total energy in our universe is dark energy,23%of the total energy in our universe is dark matter. Ordinary matter only occupy approx-imately4%.We do some preliminary research on the subjects of dark matter and dark energy. Our work is roughly divided into three parts:In the first part, we develop a new parameterization of dark energy equation of state. It is the modification of Efstathiou's dark energy EoS parameterization. w(z) is a well behaved function for z》1and has same behavior in z at low redshifts with Efstathiou's parameterization. In this parameterization there are two free parameter w0and wa. We discuss the constraints on this model's parameters from current observa-tional data. We find that our parameterization can describe the behavior of dark energy. From the analysis, we can directly see that the quintom model is more favored, but this result is also consistent with the LCDM model in the1σ CL.In the second part, we constrain the parameter of early dark energy model by using current data. Ωe is the parameter which characterized the value of early dark energy, and we find that the best fit of Ωe is0.0129. But Ωe=0still in the1σ CL. This means that there is no enough evidence to prove the existence of early dark energy. In addition, we also use the data to constraint two Ricci dark energy models.In the third part, we analyse the three-year data collected by Fermi Large Area Telescope of globular clusters NGC6388and M15to search for possible DM signals. For NGC6388the detection of γ-ray emission was reported by Fermi collaboration, which is consistent with the emission of a population of millisecond pulsars. The spectral shape of NGC6388is also shown to be consistent with a DM contribution if assuming the annihilation final state is bb. No significant γ-ray emission from M15is observed. We give the upper limits of DM contribution to γ-ray emission in both NGC6388and M15, for annihilation final states bb, W+W-,μ+μ-, τ+τ-and monochromatic line. The constraints are stronger than that derived from observation of dwarf galaxies by Fermi.