I have read that the current expansion of the universe could be due to a bare positive cosmological constant along with a negative cosmological constant due to the false vacuum of the higgs field. Currently this negative CC is not enough to cancel the positive cc but with time this can be changed via electroweak tunnelling. Now whether this is the actual case or not is one issue but I want to understand the argument. Firstly the usual value of the cc due ro a dynamic scalar field (quintessence or inflaton) is due to a negative pressure which is equal to the negative of the energy density of the potential that is slowly rolled over. Note that the energy density is NOT negative here although the pressure is. Now if what I've read is correct then the implication is that there are potential curves for the higgs field which entail the energy density going negative. I guess this means the true vacuum is below zero energy and the rim of the "mexican hat" is negative or below zero. I don't really know anything about particle physics so I need some help here - Usually the datum for potential energy is a matter of convenience so I am not sure how to understand negative vacuum energies - how are they manifest and what fixes the zero level of energy density anyway. Can anyone help me to look at this the right way. I have a research degree in cosmology but my background in particle physics is bad so any help would be gratefully recieved.