Preprint typeset using L ATEX style emulateapj v. 6/22/04 THE ESSENCE OF QUINTESSENCE AND THE COST OF COMPRESSION
Bruce A. Bassett, Pier Stefano Corasaniti, Martin Kunz · 2008
Standard two-parameter compressions of the infinite dimensional dark energy model space show crippling limitations even with current SN-Ia data. Firstly they cannot cope with rapid evolution-our best-fit to the latest SN-Ia data shows late and very rapid evolution to w0 = −2.85. However all of the standard parametrisations (incorrectly) claim that this best-fit is ruled out at more than 2σ primarily because they track it well only at very low redshift, z ≤ 0.2. Further they incorrectly rule out the observationally compatible region w ≪ −1 for z> 1. Secondly the parametrisations 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. Finally we test the parametrisations against a suite of theoretical quintessence models. The widely used linear expansion in z is generally the worst, with errors of up to 10 % at z = 1 and 20 % at z ≥ 2. 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 emphasises the need for decorrelated compressions with at least three parameters. Subject headings: 1.