2024/07/25 by Gabriela Barenboim, Héctor Sanchis, Barenboim, Gabriela +5 · 1 citation
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Cosmology and Gravitation Theories #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #Particle physics theoretical and experimental studies
paper · pdf · doi:10.48550/arxiv.2407.18102
openalex publication_date 2024/07/25 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/01
We consider the possibility that the cosmic neutrino background might have a nonthermal spectrum, and investigate its effect on cosmological parameters relative to standard Λ-Cold Dark Matter (ΛCDM) cosmology. As a specific model, we consider a thermal y-distortion, which alters the distribution function of the neutrino background by depleting the population of low-energy neutrinos and enhancing the high-energy tail. We constrain the thermal y-parameter of the cosmic neutrino background using Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillation (BAO) measurements, and place a 95%-confidence upper bound of y ≤ 0.043. The y-parameter increases the number of effective relativistic degrees of freedom, reducing the sound horizon radius and increasing the best-fit value for the Hubble constant H0. We obtain an upper bound on the Hubble constant of H0 = 71.12 km/s/Mpc at 95% confidence, substantially reducing the tension between CMB/BAO constraints and direct measurement of the expansion rate from Type-Ia supernovae. Including a spectral distortion also allows for a higher value of the spectral index of scalar fluctuations, with a best-fit of nS = 0.9720 ± 0.0063, and a 95%-confidence upper bound of nS ≤ 0.9842.