No way ou$\ddot{I}„$: Epoch of Reionization Observations Do not Support Large Values of the Optical Depth to Reionization

Abstract

Recent cosmological analyses combining high-redshift cosmic microwave background (CMB) measurements with low-redshift baryon acoustic oscillation (BAO) data have reported a preference for dynamical dark energy, with the cosmological constant scenario ($\hat{I}›$CDM) disfavored at the $\sim 3\ddot{I}ƒ$ level. These analyses, however, typically rely on large-scale CMB polarization measurements to constrain the optical depth to reionization $\ddot{I}{„}_{\mathrm{reio}}\sim 0.06$, raising the question of whether potential systematics in this dataset could influence the inferred cosmological preference. Excluding large-scale polarization data substantially weakens the tension with $\hat{I}›$CDM to the $\lesssim 2\ddot{I}ƒ$ level, nevertheless at a price of increasing $\ddot{I}{„}_{\mathrm{reio}}$ significantly to $\sim 0.09$. Here, we use a physically motivated Gompertzian reionization framework to perform a self-consistent Bayesian analysis combining CMB (excluding large-scale polarization data), BAO, and independent measurements of the neutral hydrogen fraction evolution from quasar damping wing observations and dark pixel constraints. We derive $\ddot{I}{„}_{\mathrm{reio}}=0.067\pm 0.011$ (dynamical dark energy scenario) in good agreement with cosmological analyses that would include large-scale CMB polarization data, while the inferred reionization history is consistent with multiple observational constraints. Our analysis recovers a preference for dynamical dark energy at the $⪆2\ddot{I}ƒ$ level. These results demonstrate that astrophysical probes of reionization independently recover the optical depth required by CMB polarization measurements, suggesting that potential systematics in large-scale polarization alone are unlikely to fully explain the emerging preference for dynamical dark energy.

Comments welcome! 8 pages, 5 figures. New version replaces the LambdaT model for the main model used in Ref. [5]

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