Proposal for a Constant Cosmological Constant

It is proposed that the apparent positive acceleration of the cosmological scale factor is due to the vacuum energy of an incomplete chiral phase transition in a hidden SU(2) sector. Constraints from primordial nucleosynthesis imply that the present metastable phase is in a substantially supercooled state. It is argued that massless chiral condensates can substantially enhance the possibility of supercooling, and a linear sigma model exhibiting scale invariance broken only at the quantum level is shown to accommodate the required supercooling with a reasonable choice of quartic couplings. The extensive supercooling can in principle be confirmed or rejected on the basis of interface tension measurements in lattice simulations with dynamical fermions.