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A number of recent studies have found evidence for a tension betweenobservations of large-scale structure (LSS) and the predictions of the standardmodel of cosmology with the cosmological parameters fit to the cosmic microwavebackground (CMB). The origin of this '$S_8$ tension' remains unclear, butpossibilities include new physics beyond the standard model, unaccounted forsystematic errors in the observational measurements and/or uncertainties in therole that baryons play. Here we carefully examine the latter possibility usingthe new FLAMINGO suite of large-volume cosmological hydrodynamical simulations.We project the simulations onto observable harmonic space and compare withobservational measurements of the power and cross-power spectra of cosmicshear, CMB lensing, and the thermal Sunyaev-Zel'dovich (tSZ) effect. We explorethe dependence of the predictions on box size and resolution, cosmologicalparameters including the neutrino mass, and the efficiency and nature ofbaryonic 'feedback'. Despite the wide range of astrophysical behaviourssimulated, we find that baryonic effects are not sufficiently large to removethe $S_8$ tension. Consistent with recent studies, we find the CMB lensingpower spectrum is in excellent agreement with the standard model, whilst thecosmic shear power spectrum, tSZ effect power spectrum, and the cross-spectrabetween shear, CMB lensing, and the tSZ effect are all in varying degrees oftension with the CMB-specified standard model. These results suggest that somemechanism is required to slow the growth of fluctuations at late times and/oron non-linear scales, but that it is unlikely that baryon physics is drivingthis modification.

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