The Essence of Quintessence and the Cost of Compression

Standard two-parameter compressions of the infinite dimensional dark energy model space show crippling limitations even with current Type Ia supernova (SN Ia) data unless strong priors are imposed. First, they cannot cope with rapid evolutionour best fit to the latest SN Ia data shows late and very rapid evolution to w0 = -2.85. However, all of the standard parameterizations (incorrectly) claim that this best fit is ruled out at more than 2 primarily because they track it well only at very low redshift, z 0.2. Furthermore, they incorrectly rule out the observationally compatible region w -1 for z 1. Second, the parameterizations give wildly different estimates for the redshift of acceleration, which vary from zacc = 0.14 to zacc = 0.59. Although these failings are largely cured by including higher order terms (3 parameters), this results in new degeneracies and opens up large regions of previously ruled out parameter space. All of this casts serious doubt on the usefulness of the standard two-parameter compressions in the coming era of high-precision dark energy cosmology and emphasizes the need for decorrelated compressions with at least three parameters.