Astrophysics

    

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

Authors: S. A. Cooper

This paper evaluates whether the comoving sound horizon can be determined directly from late-time observations. Using only empirically measured expansion rates up to redshift two, together with BAO angle measurements, the analysis shows that the majority of the sound-horizon integral lies in an unmeasured high-redshift domain. Monte Carlo sampling demonstrates that different high-redshift continuations of the expansion history produce consistent late-time distances yet yield widely different sound-horizon values. As a result, the sound horizon is not an empirically recoverable quantity with current data. All late-time determinations of r_s depend on assumptions about the early-Universe expansion history rather than measurements.

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[v1] 2025-12-12 00:43:22

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