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. (C) 2000 Published by Elsevier Science B.V.