Addressing Tensions in ΛCDM Cosmology by an Increase in the Optical Depth to Reionization
Joint first-author Phys. Rev. Lett. arguing that an upward shift in τ can absorb the DESI BAO vs CMB tension without new physics.
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Gerrit S. Farren
(he/him)
Cosmologist & Data Scientist | Mapping the Universe at Scale
Lawrence Berkeley National Laboratory, Physics Division
University of California, Berkeley, Berkeley Center for Cosmological Physics
Cosmologist working on multi-probe analyses of the largest cosmological datasets. I build large scale analysis pipelines that combine Bayesian inference, distributed/HPC computing, and careful systematics control to turn modern survey data into robust physical insight.
Experience
Chamberlain Postdoctoral Fellow
Lawrence Berkeley National Laboratory, CA
Sep 2024 – Present
PhD Researcher
University of Cambridge, UK
Oct 2020 – Jul 2024
Education
PhD Theoretical Physics & Applied Mathematics
University of Cambridge, UK
Oct 2020 – July 2024
BSc Physics
Haverford College, PA
Aug 2016 – May 2020
Interests
Multi-probe cosmology
Big Data
Machine Learning
Bayesian Inference
My Research
I work on cosmology — extracting insights about fundamental physics from the largest datasets we have about the Universe. My focus is cross-correlating observations from different surveys, leveraging complementary information to mitigate biases that any single survey would carry alone. In practice, that means combining gravitational lensing of the Cosmic Microwave Background with galaxy surveys to map structure across cosmic time.
Day-to-day, my work sits at the intersection of statistics, scientific computing, and large data: Bayesian inference at scale, distributed and high-performance computing, and end-to-end analysis pipelines that thread careful systematics control from raw observations to published constraints. The same toolkit transfers naturally beyond cosmology — to any setting that demands rigorous inference, robust software engineering, and the ability to extract subtle signals from massive, noisy data.
I lead and contribute to analyses across the Simons Observatory (SO), Dark Energy Spectroscopic Instrument (DESI), and the Atacama Cosmology Telescope (ACT) collaborations, and I am also a member of LSST DESC.
Featured Publications
The Atacama Cosmology Telescope: Multi-probe cosmology with unWISE galaxies and ACT DR6 CMB lensing
Multi-probe extension of the unWISE × ACT DR6 lensing pipeline; joint fit of auto- and cross-spectra.
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Detection of the CMB lensing – galaxy bispectrum
First detection of the CMB lensing–galaxy bispectrum. Submitted to PRL.
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Determining the Hubble constant without the sound horizon: Perspectives with future galaxy surveys
A novel route to H₀ via the matter–radiation equality scale, bypassing the sound horizon.
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Recent Publications
(2026).
The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes.
JCAP.
(2026).
Unified and Consistent Structure Growth Measurements from Joint ACT, SPT, and Planck CMB Lensing.
Phys. Rev. Lett..
(2025).
Addressing Tensions in ΛCDM Cosmology by an Increase in the Optical Depth to Reionization.
Physical Review Letters 136, 081002.
(2025).
An iterative CMB lensing estimator minimizing instrumental noise bias.
Phys. Rev. D.
Recent News
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Missing baryons recovered with kSZ + CMB lensing
Our paper measuring the gas fraction in galaxies and groups using the kinematic Sunyaev–Zel'dovich effect is out in Phys. Rev. D.
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PRL: Resolving cosmological tensions with reionization
Our joint first-author paper on the optical depth to reionization is out in Physical Review Letters.
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The New York Times: An astronomer hunting dark energy with DESI
The New York Times on DESI's hint of evolving dark energy and the people behind the survey.
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