Fourier-series expansion of the dark-energy equation of state

Abstract

The dark-energy component of the Universe still remains a mystery; however, several papers based on observational data have shown that its equation of state may have an oscillatory behaviour. In this paper, we provide a general description for the dark-energy equation of state w(z) in the form of a Fourier series. This description generalizes some previous dynamical dark-energy models and is in agreement with the w(z) reconstructions. We make use of a modified version of a simple and fast Markov chain Monte Carlo code to constrain the model parameters. For the analysis we use data from supernovae type Ia, baryon acoustic oscillations, H(z) measurements and cosmic microwave background. We provide a comparison of the proposed model with ΛCDM, wCDM and the standard Taylor approximation. The Fourierseries expansion of w(z) is preferred from ΛCDM at more than the 3σ significance level based on the improvement in the fit alone. We use the Akaike criterion to perform the model comparison and find that, even though there are extra parameters, there is a slight preference for the Fourier series compared with the ΛCDM model. The preferred shape of w(z) found here puts in jeopardy the single scalar field models, as they cannot reproduce the crossing of the phantom divide line w =-1.