Of pNGB quiScript Ntessence

We review the pNGB quintessence models, and point out that the reason why the large decay constants fₐ O(1)MPl are really needed is to tame a tachyonic instability present for a wide range of initial $vev$s. Starting very close to potential maxima does not help because quantum fluctuations during early inflation at a scale HI perturb the quintessence $vev$, displacing it from the maxima. This issue is quite interesting for pNGB dark energy in light of the recently discussed difficulties with embedding models with fₐ > MPl in fundamental theory. A possible way around is provided by models with several ultralight pNGBs, which can drive a short burst of very late inflation { together} even if all of their decay constants obey fₐ < MPl. Starting with their vev ~ fₐ, the pNGBs will hold each other up on the potential for a longer time period. Their effective dynamics is captured by a collective mode, containing admixtures of all of the rolling pNGBs, which behaves as an ultralight field with feff > MPl. We point out that there may be potentially observable large scale disturbances in the sea of dark energy in such models.