Giovanni Arico tells us about his recent work adding baryons to the modelling of Dark Energy Survey cosmic shear analysis. By adding baryonic modelling he and collaborators are able to use all of the DES data.

The cosmological constraints don’t tighten a lot, because although a lot of data is added, the baryonic effects are largely degenerate with cosmology. However, they are able to obtain a clear lower bound on one of the baryonic parameters, which parameterises the typical halo mass that has ejected 50% of its gas.

Their results are 1.4σ different than DES’ own cosmic shear analysis, and therefore their S₈ measurement is not in tension with Planck. This isn’t mainly due to the baryons though, so there is no clear story that “baryons are responsible for the S₈ tension”. Instead the difference comes roughly half from the methods to include non-linear clustering and half from intrinsic alignment modelling.

This isn’t a death knell for the S₈ tension in any case because DES cosmic shear alone wasn’t in very large tension with Planck, and many other measurements are – however it does highlight how a number of assumptions, each on their own small enough to perhaps not worry about, can ultimately add up to something more substantial.

The future is bright as, with appropriate modelling, the “baryonification” methods used here can be used for all the other large scale structure measurements out there. Also, if the baryon parameters are constrained with additional data sets (tSZ, kSZ, X-ray temperature, stellar mass function, etc) then the degeneracies with cosmological parameters can be broken, allowing full constraining power from these small scales.

Giovanni: https://www.linkedin.com/in/giovanni-aricò-8832684a/

Paper: https://arxiv.org/abs/2303.05537

BACCOemu: https://bitbucket.org/rangulo/baccoemu/src/master/