Title: How Well Do Dark Energy Parametrizations Match Physical Models?

Abstract: 

  In modeling dark energy, we are faced not with a lack of models, but with an excess.  Scalar fields, barotropic models, and modifications to gravity all compete with the standard cosmological constant in a very crowded marketplace of ideas.  To reduce this complexity, we often resort to simple parametrizations of the equation of state parameter, w (the ratio of the dark energy pressure to its density).  The most popular of these is the Chevallier-Polarski-Linder (CPL) parametrization, in which w is taken to be a linear function of the scale factor, a. The CPL parametrization has become the de facto gold standard, but how well does it describe physical models?  I will show how the CPL parametrization maps onto several classes of dark energy models.  The results suggest that the CPL parametrization may not be optimal for distinguishing physical dark energy models.  I will also discuss work in progress on other dark energy parametrizations, other physical models for dark energy, and attempts to understand what happens if you try to shoehorn nonlinear functional forms for w(a) into the CPL parametrization.