Cosmokinetics
  Our fundamental lack of understanding of the acceleration of the Universe suggests that we consider a kinematic description. The simplest formalism involves the third derivative of the scale factor through a jerk parameter. A new approach is presented for describing the results of astronomical observations in terms of the contemporary jerk parameter and this is related to the equation of state approach. Simple perturbative expansions about $\Lambda$CDM are given. 
R. D. Blandford, M. Amin, E. A. Baltz, K. Mandel & P. J. Marshall