Dispu$τ$able: the high cost of a low optical depth
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2025
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| author | Sailer, Noah Farren, Gerrit S. Ferraro, Simone White, Martin |
| author_facet | Sailer, Noah Farren, Gerrit S. Ferraro, Simone White, Martin |
| contents | 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 $Λ$CDM. When analyzing these data with extended cosmologies this inconsistency manifests as a $\simeq3σ$ preference for sub-minimal neutrino mass or evolving dark energy. It is known that the preference for sub-minimal 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 ($τ\sim0.09)$ reduce the preference for evolving dark energy from $\simeq3σ$ to $\simeq1.5σ$ and increase the CMB-inferred values of $n_s$ and $H_0$ to $0.968\pm0.004$ and $67.94\pm0.44$ km/s/Mpc, respectively. Conversely, within $Λ$CDM the combination of DESI BAO, high-$\ell$ CMB and CMB lensing yields $τ= 0.090 \pm 0.012$, which is in $\simeq3-5σ$ tension with Planck low-$\ell$ 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 $Λ$CDM 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2504_16932 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Dispu$τ$able: the high cost of a low optical depth Sailer, Noah Farren, Gerrit S. Ferraro, Simone White, Martin Cosmology and Nongalactic Astrophysics 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 $Λ$CDM. When analyzing these data with extended cosmologies this inconsistency manifests as a $\simeq3σ$ preference for sub-minimal neutrino mass or evolving dark energy. It is known that the preference for sub-minimal 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 ($τ\sim0.09)$ reduce the preference for evolving dark energy from $\simeq3σ$ to $\simeq1.5σ$ and increase the CMB-inferred values of $n_s$ and $H_0$ to $0.968\pm0.004$ and $67.94\pm0.44$ km/s/Mpc, respectively. Conversely, within $Λ$CDM the combination of DESI BAO, high-$\ell$ CMB and CMB lensing yields $τ= 0.090 \pm 0.012$, which is in $\simeq3-5σ$ tension with Planck low-$\ell$ 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 $Λ$CDM 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. |
| title | Dispu$τ$able: the high cost of a low optical depth |
| topic | Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2504.16932 |