Understanding acoustic scale observations: the one-sided fight against Λ

2024/12/18 by Antony Lewis, Lewis, Antony, Ewan Chamberlain +1 · 16 citations
Earth and Planetary Sciences · Engineering · #Acoustic Wave Phenomena Research #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #Geophysical Methods and Applications #Underwater Acoustics Research

paper · pdf · doi:10.48550/arxiv.2412.13894

openalex publication_date 2024/12/18 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/30

Abstract

The cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) provide precise measurements of the cosmic expansion history through the comoving acoustic scale. The CMB angular scale measurement θ_* is particularly robust, constraining the ratio of the sound horizon to the angular diameter distance to last scattering independently of the late-time cosmological model. For models with standard early-universe physics, this measurement strongly constrains possible deviations from ΛCDM at late times. We show that the null energy condition imposes strict inequalities on the BAO observables D_H(z), D_M(z), D_V(z) and F_{\rm AP}(z) relative to ΛCDM predictions. These inequalities demonstrate that certain deviations from ΛCDM are impossible for any physical non-interacting dark energy model that respects the null energy condition within the context of FRW cosmological models. We also identify the regions of parameter space in the CPL parameterization w(a) = w₀ + w_a(1-a) that can give predictions consistent with both the null energy condition and the observed CMB scale. While current DESI DR2 BAO measurements exhibit some joint-constraint parameter tensions with ΛCDM, this tension arises primarily in directions that are inconsistent with the null-energy condition, so ΛCDM is favoured by current acoustic scale measurements unless the null-energy condition is violated.

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