Constraining late-time transitions in the dark energy equation of state

One of the most compelling goals of observational cosmology is the characterisation of the properties of the dark energy component thought to be responsible for the recent acceleration of the universe, including its possible dynamics. In this work we study phenomenological but physically motivated classes of models in which the dark energy equation of state can undergo a rapid transition at low redshifts, perhaps associated with the onset of the acceleration phase itself. Through a standard statistical analysis we have used low-redshift cosmological data, coming from Type Ia supernova and Hubble parameter measurements, to set constraints on the steepness of these possible transitions as well as on the present-day values of the dark energy equation of state and in the asymptotic past in these models. We have also studied the way in which these constraints depend on the specific parametrisation being used. Our results confirm that such late-time transitions are strongly constrained. If one demands a matter-like pre-transition behaviour, then the transition is constrained to occur at high redshifts (effectively in the matter era), while if the pre-transition equation of state is a free parameter then it is constrained to be close to that of a cosmological constant. In any case, the value of dark energy equation of state near the present day must also be very similar to that of a cosmological constant. The overall conclusion is that any significant deviations from this behaviour can only occur in the deep matter era, so there is no evidence for a transition associated with the onset of acceleration. Observational tools capable of probing the dynamics of the universe in the deep matter era are therefore particularly important.