We explore the observational adequacy of a class of unified dark energy and dark matter (UDE/M) models with a fast transition. Our constraints are set using a combination of geometric probes—some low redshift ones and some high redshift ones (CMB-related included). The transition is phenomenologically modeled by two different transition functions corresponding to a fast and to an ultrafast transition, respectively. We find that the key parameters governing the transition can be well constrained, and from the statistical point of view it follows that the models cannot be discarded when compared to LCDM. We find the intriguing result that standard or input parameters such as ${\mathrm{\Omega }}_m$ and ${\mathrm{\Omega }}_b$ are far better constrained than in LCDM, and this is the case for the derived or output parameter measuring the deceleration value at present, $q_0$.

• Received 6 October 2015

DOI:https://doi.org/10.1103/PhysRevD.93.043537