A model of the Universe including Dark Energy accounted for by both a   Quintessence Field and a (negative) Cosmological Constant
  In this work 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 future collapsing universe (for a given region of the parameter space), which is necessary for a consistent formulation of string theory and quantum field theory. We have also reproduced the measurements of modulus distance from supernovae with good accuracy. 
Rolando Cardenas, Tame Gonzalez, Yoelsy Leiva, Osmel Martin and Israel   Quiros