  We study the integrated Sachs-Wolfe (ISW) effect using a model-independent
parameterization of the dark energy equation of state. Cosmic variance severely
restricts the class of models distinguishable from Lambda-CDM. In particular if
the present value of the equation of state satisfies w_Q^o <-0.8, the only
distinguishable models are those with a rapidly varying equation of state,
characterized by a late transition occurring at redshifts z<3. Due to the
degeneracy with other cosmological parameters, models with a slowly varying
behavior cannot be differentiated from each other or from a cosmological
constant using even cosmic variance-limited CMB measurements. This may place a
fundamental limit on our understanding of the origin of the presently observed
acceleration.

Astrophysics;High Energy Physics - Phenomenology;High Energy Physics - Theory