Abstract
Recent baryonic acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) are mildly discrepant ($2.2\sigma$) with the cosmic microwave background (CMB) when interpreted within $\Lambda$CDM. When analyzing these data with extended cosmologies this inconsistency manifests as a $\simeq3\sigma$ 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 $\tau$. We show that, because the CMB-inferred $\tau$ 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 ($\tau\sim0.09$) reduce the preference for evolving dark energy from $\simeq3\sigma$ to $\simeq1.5\sigma$ 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 $\Lambda$CDM the combination of DESI BAO, high-$\ell$ CMB and CMB lensing yields $\tau= 0.090 \pm 0.012$, which is in $\simeq3$–$5\sigma$ 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 $\tau$: 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, $\tau$ remains the least well-constrained $\Lambda$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.
Type
Publication
Physical Review Letters 136, 081002
publicationsRecent measurements of the expansion history of the universe, primarily from Baryon Acoustic Oscillations from the Dark Energy Spectroscopic Instrument (DESI), have hinted at some tension with results from the Cosmic Microwave Background (CMB). The natural reaction is to invoke new physics. Together with Noah Sailer (joint first author), Simone Ferraro, and Martin White, we took a different angle.
We showed that a moderate increase in the optical depth to reionization $\tau$ could absorb these tensions without modifying the standard $\Lambda$CDM model. The optical depth is primarily constrained by observations of the largest scales in the CMB; a measurement that is notoriously difficult to perform and susceptible to yet-undiscovered systematics that might explain the tension.
Why this matters
- It reframes the data: what looks like a tension with the cosmological constant model may be reinterpreted as a shift in $\tau$. The former would imply new physics; the latter is arguably the least fundamental of the parameters of the $\Lambda$CDM model.
- It identifies a concrete, testable handle: precision measurements of $\tau$ from upcoming experiments (e.g. LiteBIRD) will discriminate between this scenario and a true breakdown of $\Lambda$CDM.
- It demonstrates that, to convince the community of any claim of new physics, we need to critically examine the role different pieces of data play in the analysis.
