A kinematical approach to dark energy studies

We present and employ a new kinematical approach to cosmological `dark energy' studies. We construct models in terms of the dimensionless second and third derivatives of the scale factor a(t) with respect to cosmic time t, namely the present-day value of the deceleration parameter q_0 and the cosmic jerk parameter, j(t). An elegant feature of this parameterization is that all LCDM models have j(t)=1 (constant), which facilitates simple tests for departures from the LCDM paradigm. Applying our model to the three best available sets of redshift-independent distance measurements, from type Ia supernovae and X-ray cluster gas mass fraction measurements, we obtain clear statistical evidence for a late time transition from a decelerating to an accelerating phase. For a flat model with constant jerk, j(t)=j, we measure q_0=-0.81+-0.14 and j=2.16+0.81-0.75, results that are consistent with LCDM at about the 1sigma confidence level. A standard `dynamical' analysis of the same data, employing the Friedmann equations and modeling the dark energy as a fluid with an equation of state parameter, w (constant), gives Omega_m=0.306+0.042-0.040 and w=-1.15+0.14-0.18, also consistent with LCDM at about the 1sigma level. In comparison to dynamical analyses, the kinematical approach uses a different model set and employs a minimum of prior information, being independent of any particular gravity theory. The results obtained with this new approach therefore provide important additional information and we argue that both kinematical and dynamical techniques should be employed in future dark energy studies, where possible. Our results provide further interesting support for the concordance LCDM paradigm.