CROC Simulations Show Baryon Density Dominates CMB Optical-Depth Power Over Reionization Patchiness

The study decomposes electron-density fields from past lightcones into patchy ionization and density components, then augments the z≈5 simulation endpoint with an analytic low-redshift term. This separation reveals that density-driven fluctuations dominate the high-redshift contribution on scales relevant to upcoming CMB experiments, while accumulated post-reionization gas produces larger total power at all ℓ considered. The result directly challenges assumptions that C_ℓ^ττ primarily encodes reionization morphology.

Prior analytic models and smaller-volume simulations had predicted stronger patchy signals, but CROC’s larger boxes and self-consistent radiation-hydrodynamics capture overlapping ionized bubbles and density correlations more accurately. This aligns with findings from the Thesan and CoDaIII projects that also show density fields imprinting on integrated optical depth. The paper therefore highlights a systematic that must be modeled before τ fluctuations can constrain bubble size distributions or neutral fraction evolution.

Future analyses will require joint modeling of density and ionization fields, likely through cross-correlations with 21 cm intensity mapping or Lyman-α forest data. Experiments such as CMB-S4 and LiteBIRD must incorporate these density templates to avoid biased inferences on reionization timing. Without such separation, constraints on cosmic structure formation during the epoch of reionization risk conflating gravitational and radiative processes.

Improved hydrodynamic runs extending to z<5 and higher-resolution reionization morphology will tighten the separation. Cross-checks against independent codes and larger volumes are needed before the community can treat τ power spectra as clean probes of patchy reionization.