A model-independent reconstruction of dark energy to very high redshift

We provide a model-independent reconstruction of dark energy from $z=0$ to $\gtrsim {10}^5$. We parameterise the model by a perfect fluid with a series of physically well-motivated bins in energy-density, such that the equation of state is always $-1\le w\le 1$. Our method is capable of describing a range of theoretical models with smooth modifications to the expansion history. Combining the latest CMB, BAO, SN and local $H_0$ measurements, we obtain a large improvement of $\mathrm{\Delta }{\chi }^2=41.3$ over LCDM, at the expense of 33 additional parameters in the fit, with dark energy contributing significantly between $z\sim {10}^4-{10}^5$, and intriguingly with a sound speed $c_s^2 \sim 1/3$\mathrm{.}Asignificantpartofthe$\Delta \chi^2$ improvement comes from \Planck\

• Atacama Cosmology Telescope ($\text{textsc}Act$) data, alleviating tension between them within LCDM. We apply a correlation prior to penalise models with unnecessary degrees of freedom and find no preference for deviations from LCDM at late-times, but moderate Bayesian evidence of an early dark energy (EDE) component. Although the model has a large amount of freedom, it is unable to reduce $S_8\equiv {\sigma }_8({\mathrm{\Omega }}_{\mathrm{c}}\mathrm{/}0.3{)}^{0.5}$ below that of \lcdm, to bring about full concordance with large-scale structure data.