Examining the cosmic acceleration with the latest Union2 supernova data
  In this Letter, by reconstructing the $Om$ diagnostic and the deceleration
parameter $q$ from the latest Union2 Type Ia supernova sample with and without
the systematic error along with the baryon acoustic oscillation (BAO) and the
cosmic microwave background (CMB), we study the cosmic expanding history, using
the Chevallier-Polarski-Linder (CPL) parametrization. We obtain that Union2+BAO
favor an expansion with a decreasing of the acceleration at $z<0.3$. However,
once the CMB data is added in the analysis, the cosmic acceleration is found to
be still increasing, indicating a tension between low redshift data and high
redshift one. In order to reduce this tension significantly, two different
methods are considered and thus two different subsamples of Union2 are
selected. We then find that two different subsamples+BAO+CMB give completely
different results on the cosmic expanding history when the systematic error is
ignored, with one suggesting a decreasing cosmic acceleration, the other just
the opposite, although both of them alone with BAO support that the cosmic
acceleration is slowing down. However, once the systematic error is considered,
two different subsamples of Union2 along with BAO and CMB all favor an
increasing of the present cosmic acceleration. Therefore a clear-cut answer on
whether the cosmic acceleration is slowing down calls for more consistent data
and more reliable methods to analyze them.
