Fourier-series expansion of the dark-energy equation of state
  The dark energy component of the universe still remains as 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 Fourier series. This description generalises 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
constraint 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
$\Lambda$CDM, $w$CDM and the standard Taylor approximation. The Fourier series
expansion of $w(z)$ is preferred from $\Lambda$CDM at more than $3\sigma$
significance level based on the improvement in the fit alone. We use the Akaike
criteria to perform the model comparison and found that, even though there are
extra parameters, there is a slight preference of the Fourier series compared
with the $\Lambda$CDM model. The preferred shape of $w(z)$ found here puts in
jeopardy the single scalar field models, as they as they cannot reproduce the
crossing the phantom divide line $w=-1$.
