  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.
