Model-agnostic Cosmological Inference with SDSS-IV eBOSS: Simultaneous Probing for the Background and Perturbed Universe

Abstract

Here, we explore certain subtle features imprinted in data from the completed Sloan Digital Sky Survey IV (SDSS-IV) extended Baryon Oscillation Spectroscopic Survey (eBOSS) as a combined probe for the background and perturbed universe. We reconstruct the baryon acoustic oscillation and redshift space distortion observables as functions of redshift, using measurements from SDSS alone. We apply the multi-task Gaussian process framework to model the interdependencies of cosmological observables DM(z)/rd, DH(z)/rd, and fσ8(z), and track their evolution across different redshifts. Subsequently, we obtain a constrained three-dimensional phase space containing DM(z)/rd, DH(z)/rd, and fσ8(z) at different redshifts probed by the SDSS-IV eBOSS survey. Furthermore, assuming the Λ-cold dark matter (ΛCDM) model, we obtain constraints on model parameters Ωm, H0rd, σ8, and S8 at each redshift probed by SDSS-IV eBOSS. This indicates redshift-dependent trends in H0, Ωm, σ8, and S8 in the ΛCDM model, suggesting a possible inconsistency in the ΛCDM model. Ours is a template for model-independent extraction of information for both a background and perturbed universe using a single galaxy survey, taking into account all the existing correlations between background and perturbed observables and this can be easily extended to future DESI-3YR as well as Euclid results.