Cosmological Tensions |

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.

PRL: Resolving cosmological tensions with reionization

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

is published in Physical Review Letters ( ). Joint first-authored with Noah Sailer, with Simone Ferraro and Martin White, the paper shows that a moderate $\tau$ increase — within data — can absorb recent $\Lambda$CDM tensions without invoking new physics.

More context on the paper page itself: .

H₀ from the matter–radiation equality scale

Wed, 06 Apr 2022 00:00:00 +0000

The “Hubble tension” — the disagreement between local measurements of the Universe’s expansion rate, $H_0$, and the value inferred from the Cosmic Microwave Background (CMB) and galaxy surveys through observations of Baryon Acoustic Oscillations (BAO) — has been one of cosmology’s most discussed open puzzles. $H_0$ measurements from the CMB and BAO, while done at very different epochs, share a calibration: the scale of the sound horizon at recombination, which depends on early-universe physics. If the tension reflects something missing in our early-universe model, those measurements all move together.

This project develops an independent route to $H_0$ from galaxy surveys that does not rely on the sound horizon at all. It uses the matter–radiation equality scale — a different feature imprinted in the matter power spectrum — as a standard ruler.

What we’ve shown

In a paper led by Oliver Philcox ( , Phys. Rev. D 2020) we applied a preliminary version of our new methodology to galaxy clustering data from BOSS.
(arXiv:2112.10749, Phys. Rev. D 2022) — I further refined the method and led forecasts showing that upcoming spectroscopic galaxy surveys (DESI, Euclid, MegaMapper) can deliver sub-percent precision on $H_0$ from the equality scale alone.
Philcox, Farren, Sherwin, Baxter & Brout ( , Phys. Rev. D 2022) — combining galaxy surveys, CMB lensing, and Type-Ia supernovae to deliver a 3.6% sound-horizon-independent constraint on $H_0$.

The result is an alternative angle on the Hubble tension that future high-redshift spectroscopic data will sharpen further.

Where it has been applied

The method we developed has since been picked up across the field and the Dark Energy Spectroscopic Instrument (DESI) recently reported new percent-level constraints on $H_0$, independent of the sound horizon (Zaborowski et al. , JCAP 2025 & , JCAP 2026).

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.