  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.
