https://gerrfarr.github.io/publications/PublicationsHugoBlox Kit (https://hugoblox.com)en-usSun, 01 Feb 2026 00:00:00 +0000https://gerrfarr.github.io/media/icon_hu_83e72c182d80746a.pnghttps://gerrfarr.github.io/publications/Sun, 01 Feb 2026 00:00:00 +0000Thu, 01 Jan 2026 00:00:00 +0000Thu, 01 Jan 2026 00:00:00 +0000https://gerrfarr.github.io/publications/sailer-2025-lxj/Wed, 23 Apr 2025 00:00:00 +0000https://gerrfarr.github.io/publications/sailer-2025-lxj/<p>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.</p> <p>We showed that a <strong>moderate increase in the optical depth to reionization</strong> $\tau$ 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.</p> <h2 id="why-this-matters">Why this matters</h2> <ul> <li>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.</li> <li>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.</li> <li>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.</li> </ul>Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000Wed, 01 Jan 2025 00:00:00 +0000https://gerrfarr.github.io/publications/farren-2024-rla/Tue, 03 Sep 2024 00:00:00 +0000https://gerrfarr.github.io/publications/farren-2024-rla/<p>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.</p>Mon, 01 Jan 2024 00:00:00 +0000Mon, 01 Jan 2024 00:00:00 +0000Mon, 01 Jan 2024 00:00:00 +0000Mon, 01 Jan 2024 00:00:00 +0000Mon, 01 Jan 2024 00:00:00 +0000Mon, 01 Jan 2024 00:00:00 +0000Mon, 01 Jan 2024 00:00:00 +0000Mon, 01 Jan 2024 00:00:00 +0000https://gerrfarr.github.io/publications/farren-2023-yna/Tue, 07 Nov 2023 00:00:00 +0000https://gerrfarr.github.io/publications/farren-2023-yna/<p>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.</p>Sun, 01 Oct 2023 00:00:00 +0000Mon, 11 Sep 2023 00:00:00 +0000<p>Established the unWISE × ACT DR6 CMB lensing cross-correlation pipeline and delivered the first measurement of the structure amplitude at intermediate redshift from this combination. See the project for the broader program.</p>Sat, 01 Jan 2022 00:00:00 +0000Sat, 01 Jan 2022 00:00:00 +0000https://gerrfarr.github.io/publications/farren-2021-grl/Mon, 20 Dec 2021 00:00:00 +0000https://gerrfarr.github.io/publications/farren-2021-grl/<p>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.</p>Wed, 01 Sep 2021 00:00:00 +0000Fri, 01 Jan 2021 00:00:00 +0000Tue, 01 Jan 2019 00:00:00 +0000