Phenomenology of Hybrid Scenarios of Neutrino Dark Energy
  We study the phenomenology of hybrid scenarios of neutrino dark energy, where in addition to a so-called Mass Varying Neutrino (MaVaN) sector a cosmological constant (from a false vacuum) is driving the accelerated expansion of the universe today. For general power law potentials we calculate the effective equation of state parameter w_{eff}(z) in terms of the neutrino mass scale. Due to the interaction of the dark energy field (acceleron) with the neutrino sector, w_{eff}(z) is predicted to become smaller than -1 for z>0, which could be tested in future cosmological observations. For the considered scenarios, the neutrino mass scale additionally determines which fraction of the dark energy is dynamical, and which originates from the cosmological constant like vacuum energy of the false vacuum. On the other hand, the field value of the acceleron field today as well as the masses of the right-handed neutrinos, which appear in the seesaw-type mechanism for small neutrino masses, are not fixed. This, in principle, allows to realise hybrid scenarios of neutrino dark energy with a high-scale seesaw where the right-handed neutrino masses are close to the GUT scale. We also comment on how MaVaN Hybrid Scenarios with high-scale seesaw might help to resolve stability problems of dark energy models with non-relativistic neutrinos. 
Stefan Antusch (MPI, Munich), Subinoy Das (CCPP, NYU), Koushik Dutta   (MPI, Munich)