[Paper Review] Self-interacting neutrinos, the Hubble parameter tension, and the Cosmic Microwave Background

Thejs Brinckmann, Jae Hyeok Chang+1 more|arXiv (Cornell University)|Dec 21, 2020

Astrophysics and Cosmic PhenomenaPhysics and Astronomy158 references97 citations

TL;DR

This paper investigates whether self-interacting neutrinos can alleviate the Hubble parameter tension using Planck 2018 CMB and BAO data. It examines multiple scenarios—full self-interaction, two free-streaming neutrinos, variable interacting fractions, and varying effective neutrino species—finding no compelling evidence for new neutrino interactions; instead, all models fail to reduce the Hubble tension more than a variable Neff of free-streaming particles.

ABSTRACT

We perform a comprehensive study of cosmological constraints on non-standard neutrino self-interactions using cosmic microwave background (CMB) and baryon acoustic oscillation data. We consider different scenarios for neutrino self-interactions distinguished by the fraction of neutrino states allowed to participate in self-interactions and how the relativistic energy density, N$_{ extrm{eff}}$, is allowed to vary. Specifically, we study cases in which: all neutrino states self-interact and N$_{ extrm{eff}}$ varies; two species free-stream, which we show alleviates tension with laboratory constraints, while the energy in the additional interacting states varies; and a variable fraction of neutrinos self-interact with either the total N$_{ extrm{eff}}$ fixed to the Standard Model value or allowed to vary. In no case do we find compelling evidence for new neutrino interactions or non-standard values of N$_{ extrm{eff}}$. In several cases we find additional modes with neutrino decoupling occurring at lower redshifts $z_{ extrm{dec}} \sim 10^{3-4}$. We do a careful analysis to examine whether new neutrino self-interactions solve or alleviate the so-called $H_0$ tension and find that, when all Planck 2018 CMB temperature and polarization data is included, none of these examples ease the tension more than allowing a variable N$_{ extrm{eff}}$ comprised of free-streaming particles. Although we focus on neutrino interactions, these constraints are applicable to any light relic particle.

Motivation & Objective

• To assess whether self-interacting neutrinos can resolve the Hubble parameter tension.
• To constrain non-standard neutrino self-interactions using CMB and BAO data.
• To explore scenarios with varying numbers of self-interacting neutrino states and effective relativistic degrees of freedom (Neff).
• To evaluate whether late-decoupling neutrino interactions impact CMB anisotropies.
• To determine if such models worsen or alleviate the S8 tension.

Proposed method

• Uses Planck 2018 temperature and polarization CMB data as primary observational input.
• Applies cosmological perturbation theory to model neutrino self-interactions and their impact on CMB power spectra.
• Considers four distinct scenarios: full self-interaction with variable Neff, two free-streaming neutrinos with variable interacting states, fixed Neff with variable self-interaction fraction, and combined variable Neff and fraction.
• Employs Markov Chain Monte Carlo (MCMC) sampling to constrain model parameters.
• Compares model predictions to BAO data from BOSS and DES to improve constraints.
• Analyzes decoupling redshifts (zdec ~ 10^3–10^4) for additional neutrino interaction modes.

Experimental results

Research questions

• RQ1 Can self-interacting neutrinos reduce the Hubble parameter tension compared to standard models?
• RQ2 Do scenarios with two free-streaming neutrinos alleviate tensions with laboratory constraints on new interactions?
• RQ3 What constraints arise when Neff is fixed at the Standard Model value (3.046) but the fraction of self-interacting neutrinos varies?
• RQ4 How do models with both variable Neff and variable self-interaction fraction compare to free-streaming Neff in easing the Hubble tension?
• RQ5 Do these self-interaction models worsen the S8 tension in late-time cosmological probes?

Key findings

• No compelling evidence is found for new neutrino self-interactions across all four scenarios studied.
• In several cases, additional neutrino interaction modes are inferred with decoupling redshifts zdec ≈ 10^3–10^4.
• When all Planck 2018 CMB data is included, none of the self-interaction models alleviate the Hubble tension more than a variable Neff of free-streaming particles.
• The scenario with two free-streaming neutrino states reduces tension with laboratory constraints on new interactions.
• Models with variable Neff and self-interaction fraction do not outperform standard free-streaming Neff in resolving the Hubble tension.
• Constraints on light relic particles apply universally, as the effects are purely gravitational and depend only on energy density and decoupling redshift.