  We consider the generic scenario of dark energy which arises through the
latent heat of a hidden sector first order cosmological phase transition. This
field could account for the extra radiation degree of freedom suggested by the
CMB. We present the bubble nucleation solution for the viscous limit. The decay
rate of the field is constrained by published KSZ data, and may be an
explanation of current excess ISW correlations. Cross correlation of current
and future surveys can further constrain or test the parameter space. The decay
model is plausibly in the observable range, and avoids anthropic problems. This
class of models is not well constrained by the popular dark energy figure of
merit.
