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A fast transition between a standard matter-like era and a late $\Lambda$cold dark matter ($\Lambda$CDM)-like epoch (or more in general, a CDM+DE era),generated by a single Unified Dark Matter component, can provide a newinteresting paradigm in the context of general relativity, alternative to$\Lambda$CDM itself or other forms of DE or modified gravity theories invokedto explain the observed acceleration of the Universe. UDM models with a fasttransition have interesting features, leading to measurable predictions, thusthey should be clearly distinguishable from $\Lambda$CDM (and alternatives)through observations. Here we look at different ways of prescribingphenomenological UDM models with fast transition, then focusing on aparticularly simple model. We analyse the viability of this model by studyingfeatures of the background model and properties of the adiabatic UDMperturbations, which depend on the effective speed of sound and the functionalform of the Jeans scale. As a result, theoretical constraints on the parametersof the models are found that allow for a behaviour compatible withobservations.

Phenomenological models for Unified Dark Matter with fast transition