Possible Hints of Sterile Neutrinos in Recent Measurements of the Hubble Parameter

Abstract

Local Universe observations find a value of the Hubble constant $H_0$ that is larger than the value inferred from the Cosmic Microwave Background and other early Universe measurements, assuming known physics and the $Λ$CDM cosmological model. We show that additional radiation in active neutrinos produced just before Big Bang Nucleosynthesis by an unstable sterile neutrino with mass $m_s=$ O(10) MeV can alleviate this discrepancy. The necessary masses and couplings of the sterile neutrino, assuming it mixes primarily with $ν_τ$ and/or $ν_μ$ neutrinos, are within reach of Super-Kamiokande as well as upcoming laboratory experiments such as NA62 and DUNE.

Figures (1)

• Figure 1: Narrow (blue/red) bands in ($m_s, \sin^2\theta)$ space where decaying sterile neutrinos can alleviate the Hubble constant tension, assuming coupling exclusively to either electron neutrinos [left], or muon neutrinos [center], or tau neutrinos [right]. Experimental limits from TRIUMF PIENU:2011aaBritton:1992Britton:1992prlAguilar-Arevalo:2019owf, KEK Yamazaki:1984sj, CHARM Bergsma:1985is, SIN Abela:1981nf, PS191 Bernardi:1985nyBernardi:1987ek, Michel spectrum deGouvea:2015euy, lepton universality deGouvea:2015euy, NA62 NA62:2020mcv and a re-analysis by Bryman and Shrock Bryman:2019ssiBryman:2019bjg, Blue/red bands indicate regions where $\Delta N_{\rm eff} = 0.2-1$ is allowed according to Ref. Dolgov:2000jw. In the red portion of each band