<link>https://gerrfarr.github.io/</link></image><item><title>The Carousel Lens II: Cosmological Constraints with GIGA-Lens</h1> <article> <h1>The Carousel Lens II: Cosmological Constraints with GIGA-Lens</h1> <p>Sun, 01 Feb 2026 00:00:00 +0000</p> </article> <article> <h1>The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes</h1> <p>Thu, 01 Jan 2026 00:00:00 +0000</p> </article> <article> <h1>Unified and Consistent Structure Growth Measurements from Joint ACT, SPT, and Planck CMB Lensing</h1> <p>Thu, 01 Jan 2026 00:00:00 +0000</p> </article> <article> <h1>Multi-probe hints for beyond-ΛCDM physics — Untangling evidence from DESI, CMB, and more</h1> <p>Tue, 05 Aug 2025 00:00:00 +0000</p> </article> <article> <h1>Joint view of SZ and X-ray observations from ACT and eROSITA around DESI LRGs</h1> <p>Tue, 29 Jul 2025 00:00:00 +0000</p> </article> <article> <h1>Missing baryons recovered with kSZ + CMB lensing</h1> <p>Fri, 18 Jul 2025 00:00:00 +0000</p> <p>The paper — first-authored by Boryana Hadzhiyska, with Simone Ferraro, Noah Sailer, Rongpu Zhou and me — is now in Phys. Rev. D. It sheds new light on the distribution and abundance of baryons around DESI galaxy groups.</p> <p>More context .</p> </article> <article> <h1>Addressing Tensions in ΛCDM Cosmology by an Increase in the Optical Depth to Reionization</h1> <p>Wed, 23 Apr 2025 00:00:00 +0000</p> <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> </article> <article> <h1>PRL: Resolving cosmological tensions with reionization</h1> <p>Wed, 23 Apr 2025 00:00:00 +0000</p> <p> 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.</p> <p>More context on the paper page itself: .</p> </article> <article> <h1>The New York Times: An astronomer hunting dark energy with DESI</h1> <p>Wed, 19 Mar 2025 00:00:00 +0000</p> </article> <article> <h1>ACT DR6 main release</h1> <p>Tue, 18 Mar 2025 00:00:00 +0000</p> <p>The ACT collaboration has released the DR6 maps, power spectra, likelihoods, and the corresponding cosmological constraints — a milestone the collaboration has been building toward for years. The main papers:</p> <ul> <li> (Louis et al.)</li> <li> (Calabrese et al.)</li> <li> (Naess et al.)</li> </ul> </article> <article> <h1>Multi-probe cosmology with ACT CMB lensing and galaxy cross-correlations</h1> <p>Sun, 12 Jan 2025 00:00:00 +0000</p> <p>Invited talk reviewing the multi-probe cosmology program built around ACT DR6 CMB lensing and its cross-correlations with unWISE galaxies, with implications for $\Lambda$CDM and emerging tensions.</p> </article> <article> <h1>An iterative CMB lensing estimator minimizing instrumental noise bias</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Atacama Cosmology Telescope DR6 and DESI: Structure growth measurements from the cross-correlation of DESI legacy imaging galaxies and CMB lensing from ACT DR6 and Planck PR4</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Atacama Cosmology Telescope: DR6 gravitational lensing and SDSS BOSS cross-correlation measurement and constraints on gravity with the EG statistic</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>CLArXivR</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> <p>In some fields the arXiv now sees hundreds of new preprints every day. No working researcher can read all of them, and most existing tools stop at recommendation — surfacing a ranked list of papers that look superficially related to what you’ve read before. That helps with discovery, but it doesn’t solve the harder problem: actually reading enough of each paper to know whether and how it matters for <em>your</em> work.</p> <p><strong>CLArXivR</strong> adds a comprehension layer on top of recommendation. For each new preprint, the system reads the paper in the context of the researcher’s own work and produces a contextual summary that answers questions an expert actually cares about — <em>does this overlap with what I’m doing? does it build on it, contradict it, or extend it in a direction worth following up?</em> The goal is to skip past generic abstracts and deliver the kind of read you’d want from a thoughtful colleague who knows your work.</p> <p>Once you commit to summarising at that level of detail, the load-bearing properties shift. <strong>Faithfulness</strong> (the system doesn’t say things the paper didn’t) and <strong>relevance</strong> (the summary highlights what <em>this</em> researcher needs, not generic findings) become the things to design and engineer around — and most of the technical effort goes into the evaluation frameworks needed to measure both, and the iteration loops that let them improve together rather than at each other’s expense.</p> <p><strong>Coming soon</strong> — currently in private alpha. If you work in a field with a high preprint volume and would like early access, .</p> </article> <article> <h1>Constraints on primordial non-Gaussianity from the cross-correlation of DESI luminous red galaxies and Planck CMB lensing</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Cosmological constraints from the cross-correlation of DESI Luminous Red Galaxies with CMB lensing from Planck PR4 and ACT DR6</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Cosmological limits on the neutrino mass sum for beyond-ensuremathŁambdaCDM models</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Cosmology from Planck CMB lensing and DESI DR1 quasar tomography</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Evidence for large baryonic feedback at low and intermediate redshifts from kinematic Sunyaev-Zeltextquoterightdovich observations with ACT and DESI photometric galaxies</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Missing baryons recovered: A measurement of the gas fraction in galaxies and groups with the kinematic Sunyaev-Zeltextquoterightdovich effect and CMB lensing</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: DR6 constraints on extended cosmological models</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: DR6 maps</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: DR6 power spectra, likelihoods and ensuremathŁambdaCDM parameters</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: high-redshift measurement of structure growth from the cross-correlation of Quaia quasars and CMB lensing from ACT DR6 and Planck PR4</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>The Simons Observatory: science goals and forecasts for the enhanced Large Aperture Telescope</h1> <p>Wed, 01 Jan 2025 00:00:00 +0000</p> </article> <article> <h1>Joining LBNL as Owen Chamberlain Postdoctoral Fellow</h1> <p>Sun, 22 Sep 2024 00:00:00 +0000</p> <p>I’ve started my Owen Chamberlain Postdoctoral Fellowship in the Physics Division at Lawrence Berkeley National Laboratory, after completing my PhD at Cambridge in July. Looking forward to leading new CMB lensing × galaxy survey analyses with the DESI and Simons Observatory collaborations here at LBNL.</p> </article> <article> <h1>New paper: multi-probe cosmology with unWISE × ACT DR6 CMB lensing</h1> <p>Tue, 03 Sep 2024 00:00:00 +0000</p> <p>Our paper is now on the arXiv ( ) and submitted to Phys. Rev. D. It builds directly on our 2023 ApJ paper, jointly fitting galaxy auto-correlation, galaxy–lensing cross-correlation, and the CMB lensing auto-spectrum to deliver tighter constraints on structure growth.</p> </article> <article> <h1>The Atacama Cosmology Telescope: Multi-probe cosmology with unWISE galaxies and ACT DR6 CMB lensing</h1> <p>Tue, 03 Sep 2024 00:00:00 +0000</p> <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> </article> <article> <h1>unWISE × ACT DR6 CMB lensing</h1> <p>Tue, 03 Sep 2024 00:00:00 +0000</p> <p>Cross-correlations between CMB lensing and galaxy surveys are a key piece of my research. In this work we correlate the <strong>unWISE</strong> infrared galaxy catalog (~500 million galaxies, between about $z \sim 0.2$ and $z \sim 1.6$) with <strong>ACT DR6 CMB lensing</strong>, extracting information on the physics of cosmic expansion and structure growth.</p> <p>For this analysis I build a measurement pipeline that carefully treats and verifies a long list of potential systematics — from foregrounds (both galactic and extragalactic) to redshift and modelling systematics. Ultimately, we delivered percent-level precision with a new level of systematic rigor for this type of analysis.</p> <h2 id="what-weve-shown">What we’ve shown</h2> <p>The core of this work was presented in two papers:</p> <ul> <li> (arXiv:2309.05659) established the cross-correlation pipeline and measured the amplitude of structure, $S_8$, at intermediate redshift, a regime complementary to galaxy weak lensing and CMB lensing auto-spectra.</li> <li> (arXiv:2409.02109) extends this to a full joint fit of the galaxy auto-correlation, the cross-correlation, and the CMB lensing auto-spectrum — tightening constraints across multiple redshift slices.</li> </ul> <p>Going beyond these two-point function analyses, we used the unWISE samples for the <strong>first detection</strong> of the three-point cross-correlation between CMB lensing and galaxies , opening a new non-Gaussian probe of large-scale structure.</p> <h2 id="foundational-act-dr6-lensing-release">Foundational ACT DR6 lensing release</h2> <p>This work sits on top of the ACT DR6 lensing data release, which I contributed to:</p> <ul> <li>Madhavacheril et al. ( ) — DR6 gravitational lensing map and cosmological parameters</li> <li>Qu et al. ( ) — DR6 CMB lensing power spectrum</li> <li>MacCrann et al. ( ) — mitigating extragalactic foregrounds</li> </ul> <p>And, more recently, the joint structure-growth analysis combining CMB lensing from ACT, SPT, and Planck:</p> <ul> <li>Qu et al. ( , PRL) — unified structure growth measurements</li> </ul> <h2 id="other-cross-correlationas-building-on-this-work">Other cross-correlationas building on this work</h2> <p>The methodology developed here has been carried over to other galaxy samples I’ve worked on:</p> <ul> <li><strong>DESI Legacy Survey galaxies × ACT DR6 + Planck PR4 lensing</strong> <ul> <li>Sailer et al. ( , JCAP 2024) — cosmological constraints from the cross-correlation</li> <li>Kim et al. ( , JCAP 2024) — structure formation over cosmic time</li> <li>Qu et al. ( , Phys. Rev. D 2025) — structure growth measurements</li> </ul> </li> <li><strong>Quaia quasars × ACT DR6 + Planck PR4 lensing</strong> <ul> <li>Embil Villagra et al. ( , JCAP 2025) — high-redshift structure growth</li> </ul> </li> </ul> </article> <article> <h1>Probing LSS with ACT DR6 CMB lensing and unWISE</h1> <p>Mon, 22 Jul 2024 00:00:00 +0000</p> </article> <article> <h1>Tracing the gas around DESI galaxies with SZ and X-ray</h1> <p>Tue, 09 Jul 2024 00:00:00 +0000</p> <p>On large scales the cosmic matter distribution is relatively well understood, from observations and simulations - it is largely governed by the laws of gravity. On smaller scales - inside clusters and groups of galaxies - a rich interplay of gravity and feedback processes driven by black holes and stars significantly complicate the picture. These processes are crucial for our understanding of galaxy formation, but they also bias weak-lensing measurements that don’t account for the redistribution of matter.</p> <h2 id="what-we-have-shown-so-far">What we have shown so far</h2> <p>One way to probe the distribution of matter inside these objects is through the Sunyaev–Zel’dovich (SZ) effect - a tiny CMB temperature shift produced when CMB photons scatter off electrons in galaxy clusters and groups. In a paper led by Boryana Hadzhiyska together with the ACT and DESI teams (incl. myself; , Phys. Rev. D 2024) we used ACT CMB data to observe the <strong>kinematic Sunyaev–Zel’dovich (kSZ) effect</strong> - scattering of electrons with bulk velocities - around galaxies identified by DESI. We showed that the feedback processes appear to be stronger than expected from numerical simulations.</p> <p>This analysis had one critical caveat; we didn’t know the mass of the objects we were looking at. We calibrated group masses by looking at how they gravitationally lense the CMB. In keeping with my focus on ensuring this type of analysis is robust, I contributed important systematics checks, demonstrating that CMB lensing measurements could be used to constrain the mass of the galaxies we had mapped without bias (Hadzhiyska et al. , Phys. Rev. D 2025).</p> <h2 id="what-we-are-working-on-now">What we are working on now</h2> <p>The kSZ (and the related thermal SZ - scattering off thermal electrons) are not the only way to constrain the gas distribution. Another popular method is observations of the X-ray emission from these objects.</p> <p>We are currently mapping the X-ray emission around the same objects and plan to analyse it jointly with kSZ and tSZ data, unveiling the new details about the formation of cosmic structure on the smallest scales.</p> </article> <article> <h1>DESI × CMB lensing</h1> <p>Fri, 05 Jul 2024 00:00:00 +0000</p> <p> — the Dark Energy Spectroscopic Instrument — is mapping tens of millions of galaxies and quasars to produce the most detailed redshift survey of the universe to date. Cross-correlating that map with CMB lensing measurements gives us a clean view of how cosmic structure has grown across cosmic time.</p> <p>I lead the development and deployment of the <strong>measurement and analysis pipeline</strong> for the DESI DR2 CMB lensing cross-correlation analyses. This is data engineering at scale: galaxy catalogs spanning tens of millions of objects across six distinct samples, CMB lensing maps covering tens of thousands of square degrees, distributed computing pipelines that thread careful systematics control from raw data through to cosmological constraints.</p> <h2 id="whats-next">What’s next</h2> <p>The same machinery extends naturally to DESI’s full 5-year dataset and future CMB lensing maps from the <strong>Simons Observatory</strong>. The pipeline is built to be reusable across galaxy samples, lensing experiments, and parameter spaces.</p> </article> <article> <h1>Probing the large-scale structure with cross-correlations between ACT DR6 CMB lensing and unWISE</h1> <p>Mon, 08 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>Atacama Cosmology Telescope: High-resolution component-separated maps across one third of the sky</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>Constraining gravity with a new precision EG estimator using Planck + SDSS BOSS data</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>Cosmological constraints from the tomography of DES-Y3 galaxies with CMB lensing from ACT DR4</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>Cosmology from cross-correlation of ACT-DR4 CMB lensing and DES-Y3 cosmic shear</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope DR6 and DESI: structure formation over cosmic time with a measurement of the cross-correlation of CMB lensing and luminous red galaxies</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: A Measurement of the DR6 CMB Lensing Power Spectrum and Its Implications for Structure Growth</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: DR6 Gravitational Lensing Map and Cosmological Parameters</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: Mitigating the Impact of Extragalactic Foregrounds for the DR6 Cosmic Microwave Background Lensing Analysis</h1> <p>Mon, 01 Jan 2024 00:00:00 +0000</p> </article> <article> <h1>Cosmology from DR6 CMB lensing cross-correlations with unWISE</h1> <p>Wed, 15 Nov 2023 00:00:00 +0000</p> </article> <article> <h1>Cosmology from DR6 CMB lensing and cross-correlations with unWISE</h1> <p>Wed, 08 Nov 2023 00:00:00 +0000</p> </article> <article> <h1>Detection of the CMB lensing – galaxy bispectrum</h1> <p>Tue, 07 Nov 2023 00:00:00 +0000</p> <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> </article> <article> <h1>Experience</h1> <p>Tue, 24 Oct 2023 00:00:00 +0000</p> </article> <article> <h1>DR6 lensing release and cross-correlations with unWISE</h1> <p>Sun, 15 Oct 2023 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: Extragalactic Point Sources in the Southern Surveys at 150, 220 and 280 GHz observed between 2008-2010</h1> <p>Sun, 01 Oct 2023 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: Cosmology from Cross-correlations of unWISE Galaxies and ACT DR6 CMB Lensing</h1> <p>Mon, 11 Sep 2023 00:00:00 +0000</p> <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> </article> <article> <h1>New results from unWISE × Planck lensing and towards unWISE × ACT DR6 lensing</h1> <p>Tue, 15 Aug 2023 00:00:00 +0000</p> </article> <article> <h1>Towards high-precision CMB lensing cross-correlations with unWISE galaxies</h1> <p>Tue, 01 Aug 2023 00:00:00 +0000</p> </article> <article> <h1>BBC News: Mysterious dark matter mapped in finest detail yet</h1> <p>Tue, 11 Apr 2023 00:00:00 +0000</p> </article> <article> <h1>Measuring H₀ using the equality scale with present and future galaxy surveys</h1> <p>Sat, 09 Apr 2022 00:00:00 +0000</p> </article> <article> <h1>H₀ from the matter–radiation equality scale</h1> <p>Wed, 06 Apr 2022 00:00:00 +0000</p> <p>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 <strong>sound horizon</strong> at recombination, which depends on early-universe physics. If the tension reflects something missing in our early-universe model, those measurements all move together.</p> <p>This project develops an <strong>independent route</strong> to $H_0$ from galaxy surveys that does not rely on the sound horizon at all. It uses the <strong>matter–radiation equality scale</strong> — a different feature imprinted in the matter power spectrum — as a standard ruler.</p> <h2 id="what-weve-shown">What we’ve shown</h2> <ul> <li>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.</li> <li> (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.</li> <li>Philcox, Farren, Sherwin, Baxter & Brout ( , Phys. Rev. D 2022) — combining galaxy surveys, CMB lensing, and Type-Ia supernovae to deliver a <strong>3.6% sound-horizon-independent constraint</strong> on $H_0$.</li> </ul> <p>The result is an alternative angle on the Hubble tension that future high-redshift spectroscopic data will sharpen further.</p> <h2 id="where-it-has-been-applied">Where it has been applied</h2> <p>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).</p> </article> <article> <h1>Determining the Hubble constant without the sound horizon: A 3.6% constraint on H0 from galaxy surveys, CMB lensing, and supernovae</h1> <p>Sat, 01 Jan 2022 00:00:00 +0000</p> </article> <article> <h1>Ultralight axions and the kinetic Sunyaev-Zeltextquoterightdovich effect</h1> <p>Sat, 01 Jan 2022 00:00:00 +0000</p> </article> <article> <h1>Determining the Hubble constant without the sound horizon: Perspectives with future galaxy surveys</h1> <p>Mon, 20 Dec 2021 00:00:00 +0000</p> <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> </article> <article> <h1>Confirming the Calibration of ALMA Using Planck Observations</h1> <p>Wed, 01 Sep 2021 00:00:00 +0000</p> </article> <article> <h1>Determining the Hubble Constant without the Sound Horizon: Measurements from Galaxy Surveys</h1> <p>Fri, 01 Jan 2021 00:00:00 +0000</p> </article> <article> <h1>The Atacama Cosmology Telescope: Two-season ACTPol Extragalactic Point Sources and their Polarization properties</h1> <p>Tue, 01 Jan 2019 00:00:00 +0000</p> </article> </main></body></html>