Bayesian Analysis and Constraints on Kinematic Models from Union SNIa
  The kinematic expansion history of the universe is investigated by using the
307 supernovae type Ia from the Union Compilation set. Three simple model
parameterizations for the deceleration parameter (constant, linear and abrupt
transition) and two different models that are explicitly parametrized by the
cosmic jerk parameter (constant and variable) are considered. Likelihood and
Bayesian analyses are employed to find best fit parameters and compare models
among themselves and with the flat $\Lambda$CDM model. Analytical expressions
and estimates for the deceleration and cosmic jerk parameters today ($q_0$ and
$j_0$) and for the transition redshift ($z_t$) between a past phase of cosmic
deceleration to a current phase of acceleration are given. All models
characterize an accelerated expansion for the universe today and largely
indicate that it was decelerating in the past, having a transition redshift
around 0.5. The cosmic jerk is not strongly constrained by the present
supernovae data. For the most realistic kinematic models the $1\sigma$
confidence limits imply the following ranges of values: $q_0\in[-0.96,-0.46]$,
$j_0\in[-3.2,-0.3]$ and $z_t\in[0.36,0.84]$, which are compatible with the
$\Lambda$CDM predictions, $q_0=-0.57\pm0.04$, $j_0=-1$ and $z_t=0.71\pm0.08$.
We find that even very simple kinematic models are equally good to describe the
data compared to the concordance $\Lambda$CDM model, and that the current
observations are not powerful enough to discriminate among all of them.
