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 $\Delta \chi^2=41.3$ 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 $c_s^2 \sim
1/3$. A significant part of the $\Delta \chi^2$ improvement comes from \Planck\
+ Atacama Cosmology Telescope (\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 $S_8 \equiv \sigma_8 (\Omega_\mathrm{c} / 0.3)^{0.5}$ below that of
\lcdm, to bring about full concordance with large-scale structure data.
