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

TL;DR

This paper presents a model-independent method to reconstruct dark energy from the present to very high redshifts, revealing significant early contributions and improving fit to cosmological data over LCDM.

Contribution

It introduces a flexible, physically motivated binning approach to model dark energy without assuming a specific form, extending the analysis to very high redshifts.

Findings

Dark energy contributes significantly at redshifts around 10^4 to 10^5.

The method improves fit to CMB, BAO, SN, and local H0 data, reducing chi-squared by 41.3.

No strong evidence for late-time deviations from LCDM, but some indication of early dark energy.

Abstract

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.