Quintessential Axions

I review the dark energy solutions by a very light pseudoscalar called "quintessential axion". For the explicit breaking terms, we consider both the global anomaly U(1)$_{\rm global}\times$SU(2)$_W^2$ and the potential $\Delta V$. At the field theory level, we will present a strategy for picking up one such pseudoscalar among plenty of pseudoscalars. At string level, numerous antisymmetric tensor fields are pseudoscalars. Including these, we present a confining force example via a $Z_{12-I}$ orbifold compactification of SO(32) heterotic string. In addition, we also aim to draw attention on almost massless mesons of confining nonabelian gauge group, which can be another motivation for introducing an additional confining force, to explain dark energy as the vacuum energy of these mesons.