Environmental scatter and the origin of the negative patchy–density correlation

Extend the CROC lightcone approach to a larger simulation ensemble to bound environmental scatter in the patchy and density optical-depth power spectra, resolve the origin of the negative patchy–density cross-correlation, and connect this decomposition to forecasts for cosmic microwave background observables sourced by optical-depth fluctuations.

Background

The analysis uses five CROC simulation volumes, including three independent realizations and two large-scale DC-mode variants. The limited ensemble prevents a definitive quantification of volume-to-volume and large-scale-environment sample variance in the patchy and density components of the optical-depth power spectrum.

The results also reveal a negative patchy–density cross-correlation at sufficiently high multipoles, plausibly associated with enhanced recombination and self-shielding in overdense gas, but the physical origin is not established. The authors explicitly identify larger simulation ensembles, investigation of this correlation, and connections to CMB-observable forecasts as directions requiring future work.