Addressing Tensions in Λ CDM Cosmology by an Increase in the Optical Depth to Reionization

Recent baryonic acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) are mildly discrepant (2.2σ) with the cosmic microwave background (CMB) when interpreted within ΛCDM. When analyzing these data with extended cosmologies this inconsistency manifests as a ≃3σ preference for subminimal neutrino mass or evolving dark energy. It is known that the preference for subminimal neutrino mass from the suppression of structure growth could be alleviated by increasing the optical depth to reionization τ. We show that, because the CMB-inferred τ is negatively correlated with the matter fraction, a larger optical depth resolves a similar preference from geometric constraints. Optical depths large enough to resolve the neutrino mass tension (τ∼0.09) reduce the preference for evolving dark energy from ≃3σ to ≃1.5σ and increase the CMB-inferred values of n_{s} and H_{0} to 0.968±0.004 and 67.94±0.44 km/s/Mpc, respectively. Conversely, within ΛCDM the combination of DESI BAO, high-ℓ CMB, and CMB lensing yields τ=0.090±0.012, which is in ≃3-5σ tension with Planck low-ℓ polarization data when taken at face value. Essentially all current CMB analyses-including recent results from WMAP + ACT and SPT-adopt the Planck measurement of τ; thus a systematic in large-scale Planck polarization would serve as a "single-point failure" for most modern cosmological analyses that include CMB data. While there is no evidence for systematics in the large-scale Planck data, τ remains the least well-constrained ΛCDM parameter and is far from its cosmic variance limit. This strengthens the case for future large-scale CMB experiments as well as direct probes of the epoch of reionization.