Some analyzes of the apparent magnitude‐redshift data of type Ia supernovas indicate that the suspected dark energy in the universe cannot be regarded as a cosmological constant of general relativistic origin or as the vacuum energy encountered in quantum field theories. If this is the case, our knowledge of the physical world remains deficient since no tested theory involves such a dark energy. Under this circumstance, an equation of state of the form p = wρ is not well‐motivated and one is unable to use the Einstein equation in this case as well. I argue that the very method of analysing the data by assuming exotic energy densities with strange equations of state itself is misleading and the reasonable remaining option is to make a model‐independent analysis of SNe data, without reference to the energy densities. In this basically kinematic approach, we limit ourselves to the observationally justifiable assumptions of homogeneity and isotropy, i.e., to the assumption that the universe has a RW metric. This cosmographic approach is historically the original one in cosmology. The analysis was performed by expanding the scale factor into a fifth‐order polynomial, an assumption that can be further generalized to any order. The values obtained for the present expansion rates h, q0, r0 etc. are relevant, since any cosmological solution would ultimately need to explain them.
Using this method, we address an important question relevant to cosmology: Was there a decelerating past for the universe? To answer this, the Bayes’s probability theory is employed, which is the most appropriate tool for quantifying our knowledge when it changes through the acquisition of new data. The cosmographic approach helps to sort out models which were always accelerating from those which decelerated for at least some time in the period of interest. Bayesian model comparison technique is used to discriminate these rival hypotheses with the aid of recent releases of supernova data. It is argued that the lessons learned using Bayesian theory are extremely valuable to avoid frequent U‐turns in cosmology.
