Constraints on the solid dark universe model

If the dark energy is modelled as a relativistic elastic solid then the standard CDM and $\Lambda$CDM models, as well as lattice configurations of cosmic strings or domain walls, are points in the two-dimensional parameter space $(w,c_{\rm s}^2)$. We present a detailed analysis of the best fitting cosmological parameters in this model using data from a range of observations. We find that the $\chi^2$ is improved by $\sim 10$ by including the two parameters and that the $w=-1$ $\Lambda$CDM model is only the best fit to the data when a large number of different datasets are included. Using CMB observations alone we find that $w=-0.38\pm 0.16$ and with the addition of Large-Scale Structure data $w=-0.62\pm 0.15$ and $\log c_{\rm s}=-0.77\pm 0.28$. We conclude that the models based on topological defects provide a good fit to the current data, although $\Lambda$CDM cannot be ruled out.