Modelling dark energy with quintessence and a cosmological constant
  In this talk we present a model of the universe in which dark energy is modelled explicitely with both a dynamical quintessence field and a cosmological constant. Our results confirm the possibility of a collapsing universe (for a given region of the parameter space), which is advantageous for an adequate formulation of both perturbative quantum field and string theories. We have also reproduced the measurements of modulus distance from supernovae with good accuracy. 
Rolando Cardenas, Tame Gonzalez, Osmel Martin and Israel Quiros