Addressing Tensions in ΛCDM Cosmology by an Increase in the Optical Depth to Reionization.

Area of Science:

• Cosmology
• Astrophysics
• Particle Physics

Background:

• Recent baryonic acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) present a mild discrepancy (2.2σ) with cosmic microwave background (CMB) data within the standard ΛCDM cosmological model.
• This inconsistency suggests potential issues with the ΛCDM model, manifesting as a preference for non-minimal neutrino mass or evolving dark energy when extended cosmologies are considered.

Purpose of the Study:

• To investigate the impact of optical depth to reionization (τ) on resolving cosmological tensions between DESI BAO and CMB data.
• To explore the implications of increased τ for neutrino mass, dark energy evolution, and other cosmological parameters within extended models.

Main Methods:

• Analysis of DESI BAO measurements in conjunction with CMB data (high-ℓ CMB, CMB lensing) within extended cosmological models.
• Investigating the correlation between CMB-inferred τ and matter fraction to assess the impact of τ on geometric constraints.
• Comparing results with existing Planck low-ℓ polarization data and considering potential systematic errors.

Main Results:

• Increasing optical depth to reionization (τ) to approximately 0.09 alleviates the tension related to subminimal neutrino mass and reduces the preference for evolving dark energy from ≃3σ to ≃1.5σ.
• Higher τ values increase CMB-inferred values for the scalar spectral index (n_{s}) and Hubble constant (H_{0}) to 0.968±0.004 and 67.94±0.44 km/s/Mpc, respectively.
• The combination of DESI BAO, high-ℓ CMB, and CMB lensing yields τ=0.090±0.012, which is in significant tension (≃3-5σ) with Planck low-ℓ polarization data, suggesting a potential systematic error in Planck data.

Conclusions:

• A significant optical depth to reionization (τ∼0.09), consistent with DESI BAO and CMB data, could resolve tensions within the ΛCDM model but highlights a potential systematic issue in Planck low-ℓ polarization data.
• The optical depth (τ) remains the least constrained ΛCDM parameter, underscoring the need for future large-scale CMB experiments and direct probes of the epoch of reionization.
• Systematic errors in large-scale Planck polarization measurements could be a critical failure point for modern cosmological analyses relying on CMB data.