From Scratch: A Direct Test of the Sound-Horizon Assumption

From Scratch: A Direct Test of the Sound-Horizon Assumption | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article From Scratch: A Direct Test of the Sound-Horizon Assumption S. A. Cooper This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8320112/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract We evaluate the sound horizon at photon decoupling using only late-time observations and the directly measured acoustic angle from the CMB. The goal is to determine how much of the sound horizon can be recovered from data alone, without assuming any specific early-universe model. The analysis uses published cosmic-chronometer measurements of the expansion rate, the TRGB determination of the local Hubble constant, and the observed acoustic angle. A non-parametric reconstruction of the expansion history gives the distance to last scattering based solely on late-time information. This makes it possible to infer the physical sound horizon independently of the standard cosmological framework. The result is a broad estimate of the sound horizon that agrees with the commonly quoted value but carries a much larger uncertainty. Almost all of this uncertainty comes from the unobserved region between the highest redshifts reached by chronometer data and the epoch of recombination. Because no direct expansion-rate measurements exist in this range, the sound horizon cannot be sharply determined from observations alone. This evaluation shows a basic limitation in how late-time data can be used to test early-universe predictions. It also clarifies the structure behind the Hubble tension. The local determination of the Hubble constant is a direct measurement, while the CMB-derived value depends on adopting a theoretical sound horizon. The tension therefore reflects a comparison between observation and inference rather than between two independent measurements. Cosmology Hubble constant Sound horizon Baryon acoustic oscillations Gaussian Processes Expansion history Full Text Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8320112","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":570912798,"identity":"ef750a93-337e-441a-8212-f8d2fc8f51b5","order_by":0,"name":"S. A. 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