2018/03/06 by William J. Sutherland, Will Sutherland
Physics and Astronomy · #Astrophysics #Astrophysics and Cosmic Phenomena #Cosmic background radiation #Cosmic microwave background #Cosmological constant #Cosmology #Cosmology and Gravitation Theories #Dark energy #Degeneracy (biology) #Degenerate energy levels #Dimensionless quantity #Galaxy #Hubble's law #Mathematical physics #Neutrino #Particle physics #Physics #Planck #Quantum mechanics #Radio Astronomy Observations and Technology #Redshift #astro-ph.CO #hep-ph
paper · pdf · doi:10.1093/mnras/sty687
Latex, 9 pages, 2 figures. Accepted by MNRAS
arxiv created 2018/03/06 · openalex publication_date 2018/03/15 · arxiv updated 2018/03/28 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
It is well known that estimating cosmological parameters from cosmic microwave background (CMB) data alone results in a significant degeneracy between the total neutrino mass and several other cosmological parameters, especially the Hubble constant H0 and the matter density parameter Ωm. Adding low-redshift measurements such as baryon acoustic oscillations (BAOs) breaks this degeneracy and greatly improves the constraints on neutrino mass. The sensitivity is surprisingly high, for example, adding the ∼1 percent measurement of the BAO ratio rs/DV from the BOSS survey leads to a limit Σ mν < 0.19 eV, equivalent to Ων < 0.0045 at 95 per cent confidence. For the case of Σ mν < 0.6 eV, the CMB degeneracy with neutrino mass almost follows a track of constant sound horizon angle (Howlett et al. 2012). For a ΛCDM + mν model, we use simple but quite accurate analytic approximations to derive the slope of this track, giving dimensionless multipliers between the neutrino to matter ratio (xν ≡ ων/ωcb) and the shifts in other cosmological parameters. The resulting multipliers are substantially larger than 1: conserving the CMB sound horizon angle requires parameter shifts δln H0 ≈ −2 δxν, δln Ωm ≈ +5 δxν, δln ωΛ ≈ −6.2 δxν, and most notably δωΛ ≈ −14 δων. These multipliers give an intuitive derivation of the degeneracy direction, which agrees well with the numerical likelihood results from the Planck team.