Examining Cosmic Acceleration With Cosmological Observations

Various observations, including: Type Ia supernova (SNIa), baryonic acoustic oscil-lation (BAO), cosmic microwave background (CMB) and so on, have shown the fact thatour universe is undergoing an accelerating expansion. In order to explain this observedscenario, usually, it is assumed that in our universe there exists an exotic energy com-ponent with negative pressure: dark energy. Up to now many dark energy models havebeen proposed. Most of them support the phenomenon that the acceleration of cosmicexpansion is speeding up. While, recently, using the low redshift SNIa and BAO datasets with the CPL parametrization for the equation of state of dark energy, expandinguniverse, it has been found that the acceleration of cosmic expansion might be slowingdown in the near past. Once the high redshift CMB data is included, the results turnout to be still supporting that the universe is undergoing an accelerating expansion withan increasing acceleration. Thus, there appears some tension between the low redshiftdata (SNIa+BAO) and the high redshif (CMB) one. By incorporating the curvature kas a free parameter, it has been found that this tension can be ameliorated, and bothSNIa+BAO and SNIa+BAO+CMB favor that the university has entered an epoch thatexpansion is slowing down in the sign at the small redshift.In this paper, with the Monte Carlo Markov chain method, we re-examine the evo-lutionary behavior of q(z) using the latest observational data including the Union2SNIa,BAO, and CMB data (R(z), lA(z), z) from Wilkinson Microwave Anisotropy Probe7-yr (WMAP7). For the BAO data, four diferent data sets obtained from the Two-DegreeField Galaxy Survey, the combination of SDSS and2dFGRS, the WiggleZ Dark Ener-gy Survey and the Baryonic Oscillation Spectroscopic Survey, are used. Except for thespatially fat case constrained by SNIa+the WiggleZ BAO data, both SNIa and otherBAO+SNIa favor that the present cosmic acceleration is slowing down irrespective ofwhether the spatial curvature is included or not. However, once the WMAP7CMB datais included, observations favor strongly the Lambda cold dark matter model, a spatiallyfat universe, and a speeding-up of the cosmic acceleration. Therefore, the inclusion ofspatial curvature seems to has no efect on alleviating the tension between SNIa+BAOand CMB in clear contrast to the previous work in the literature.