Of pNGB qui𝒩tessence

We review the pNGB quintessence models, and point out that the reason why the large decay constants are really needed is to tame a tachyonic instability present for a wide range of initial vevs. 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.