On using the CMB shift parameter in tests of models of dark energy

The so-called shift parameter is related to the position of the first acoustic peak in the power spectrum anisotropies of the cosmic microwave background (CMB) anisotropies. It is an often used quantity in simple tests of dark energy models. However, the shift parameter is not directly measurable from the cosmic microwave background, and its value is usually derived from the data assuming a spatially flat cosmology with dark matter and a cosmological constant. Our aim in this paper is to evaluate the effectiveness of the shift parameter as a constraint on dark energy models, and the potential pitfalls in using it as a test of non-standard models of dark energy. By comparing to full CMB fits, we show that combining the shift parameter with the position of the first acoustic peak in the CMB power spectrum improves the accuracy of the test considerably.