Model-independent constraints on $Ω_m$ and $H(z)$ from the link between geometry and growth

Abstract: We constrain the expansion history of the Universe and the cosmological matter density fraction in a model-independent way by exclusively making use of the relationship between background and perturbations under a minimal set of assumptions. We do so by employing a Gaussian process to model the expansion history of the Universe from present time to the recombination era. The expansion history and the cosmological matter density are then constrained using recent measurements from cosmic chronometers, Type-Ia supernovae, baryon acoustic oscillations, and redshift-space distortion data. Our results show that the evolution in the reconstructed expansion history is compatible with the \textit{Planck} 2018 prediction at all redshifts. The current data considered in this study can constrain a Gaussian process on $H(z)$ to an average $9.4 \%$ precision across redshift. We find $\Omega_m = 0.224 \pm 0.066$, lower but statistically compatible with the \textit{Planck} 2018 cosmology. Finally, the combination of future DESI measurements with the CMB measurement considered in this work holds the promise of $8 \%$ average constraints on a model-independent expansion history as well as a five-fold tighter $\Omega_m$ constraint using the methodology developed in this work.