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

TL;DR

The paper addresses potential redshift-dependent tensions in ΛCDM by performing a model-agnostic, single-survey reconstruction of BAO and RSD observables from SDSS-IV eBOSS using a Multi-Task Gaussian Process. It jointly infers $D_M/r_d$, $D_H/r_d$, and $fσ_8$ across redshift and translates them into ΛCDM parameter constraints $Ω_m$, $H_0 r_d$, $σ_8$, and $S_8$ at each redshift. The results suggest redshift-dependent trends in these parameters, hinting at possible inconsistencies in ΛCDM and demonstrating a template for future surveys (DESI-3YR, Euclid) to perform model-independent background and perturbation analyses using correlated observables. By leveraging cross-observable correlations and rigorous uncertainty propagation, the approach provides a robust framework for single-survey cosmology with broad applicability.

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 (BAO) and Redshift Space Distortion (RSD) observables as functions of redshift, using measurements from SDSS alone. We apply the Multi-Task Gaussian Process (MTGP) framework to model the interdependencies of cosmological observables $D_M(z)/r_d$, $D_H(z)/r_d$, and $fσ_8(z)$, and track their evolution across different redshifts. Subsequently, we obtain constrained three-dimensional phase space containing $D_M(z)/r_d$, $D_H(z)/r_d$, and $fσ_8(z)$ at different redshifts probed by the SDSS-IV eBOSS survey. Furthermore, assuming the $Λ$CDM model, we obtain constraints on model parameters $Ω_{m}$, $H_{0}r_{d}$, $σ_{8}$ and $S_{8}$ at each redshift probed by SDSS-IV eBOSS. This indicates redshift-dependent trends in $H_0$, $Ω_m$, $σ_8$ and $S_8$ in the $Λ$CDM model, suggesting a possible inconsistency in the $Λ$CDM model. Ours is a template for model-independent extraction of information for both 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.