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

Wed, 23 Apr 2025 00:00:00 +0000

Recent 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.

The Atacama Cosmology Telescope: Multi-probe cosmology with unWISE galaxies and ACT DR6 CMB lensing

Tue, 03 Sep 2024 00:00:00 +0000

A joint multi-probe analysis combining the unWISE galaxy auto-correlation, the galaxy–CMB-lensing cross-correlation, and the CMB lensing auto-spectrum to tighten constraints on structure growth across two redshift bins. See the project for the broader program.

Detection of the CMB lensing – galaxy bispectrum

Tue, 07 Nov 2023 00:00:00 +0000

A first detection of the three-point cross-correlation between CMB lensing and galaxies, opening a new non-Gaussian probe of large-scale structure beyond what two-point analyses can access. See the project for context.

Determining the Hubble constant without the sound horizon: Perspectives with future galaxy surveys

Mon, 20 Dec 2021 00:00:00 +0000

Forecasts showing that upcoming spectroscopic galaxy surveys (DESI, Euclid, MegaMapper) can deliver sub-percent precision on $H_0$ from the matter–radiation equality scale, independent of the sound horizon. See the project for the broader program.