The FLAMINGO project: revisiting the $S_8$ tension and the role of baryonic physics

TL;DR

This study uses advanced hydrodynamical simulations to investigate the $S_8$ tension in cosmology, finding baryonic physics alone cannot resolve the discrepancy, implying the need for new physics or mechanisms affecting structure growth.

Contribution

The paper provides a comprehensive analysis of baryonic effects on large-scale structure predictions, demonstrating they are insufficient to explain the $S_8$ tension.

Findings

Baryonic effects are too small to resolve the $S_8$ tension.

CMB lensing power spectrum aligns well with the standard model.

Other observables like cosmic shear and tSZ show tension with the model.

Abstract

A number of recent studies have found evidence for a tension between observations of large-scale structure (LSS) and the predictions of the standard model of cosmology with the cosmological parameters fit to the cosmic microwave background (CMB). The origin of this '$S_8$ tension' remains unclear, but possibilities include new physics beyond the standard model, unaccounted for systematic errors in the observational measurements and/or uncertainties in the role that baryons play. Here we carefully examine the latter possibility using the new FLAMINGO suite of large-volume cosmological hydrodynamical simulations. We project the simulations onto observable harmonic space and compare with observational measurements of the power and cross-power spectra of cosmic shear, CMB lensing, and the thermal Sunyaev-Zel'dovich (tSZ) effect. We explore the dependence of the predictions on box size and resolution, cosmological parameters including the neutrino mass, and the efficiency and nature of baryonic 'feedback'. Despite the wide range of astrophysical behaviours simulated, we find that baryonic effects are not sufficiently large to remove the $S_8$ tension. Consistent with recent studies, we find the CMB lensing power spectrum is in excellent agreement with the standard model, whilst the cosmic shear power spectrum, tSZ effect power spectrum, and the cross-spectra between shear, CMB lensing, and the tSZ effect are all in varying degrees of tension with the CMB-specified standard model. These results suggest that some mechanism is required to slow the growth of fluctuations at late times and/or on non-linear scales, but that it is unlikely that baryon physics is driving this modification.