Solving the Hubble tension without spoiling big bang nucleosynthesis

Guo-Yuan Huang,Werner Rodejohann

doi:10.1103/physrevd.103.123007

Abstract

The Hubble parameter inferred from cosmic microwave background observations is consistently lower than that from local measurements, which could hint towards new physics. Solutions to the Hubble tension typically require a sizable amount of extra radiation $\Delta N^{}_{\rm eff}$ during recombination. However, the amount of $\Delta N^{}_{\rm eff}$ in the early Universe is unavoidably constrained by Big Bang Nucleosynthesis (BBN), which causes problems for such solutions. We present a possibility to evade this problem by introducing neutrino self-interactions via a simple Majoron-like coupling. The scalar is slightly heavier than $1~{\rm MeV}$ and allowed to be fully thermalized throughout the BBN era. The rise of neutrino temperature due to the entropy transfer via $\phi \to \nu\overline{\nu}$ reactions compensates the effect of a large $\Delta N^{}_{\rm eff}$ on BBN. Values of $\Delta N^{}_{\rm eff}$ as large as $0.7$ are in this case compatible with BBN. We perform a fit to the parameter space of the model.

Highlights

The Hubble parameter inferred from the Planck observations of the cosmic microwave background (CMB), H0 1⁄4 67.4 Æ 0.5 km=s=Mpc [1], is in tension with that of local measurements at low redshifts
The tension for the Hubble parameter could suggest the existence of new physics beyond the Standard Model or beyond the ΛCDM framework [4]
We have explored the role of big bang nucleosynthesis (BBN) for the Majoron-like scalar solution in light of the H0 tension

Summary

INTRODUCTION

The Hubble parameter inferred from the Planck observations of the cosmic microwave background (CMB), H0 1⁄4 67.4 Æ 0.5 km=s=Mpc [1], is in tension with that of local measurements at low redshifts. As long as the coupling in Eq (1) is strong enough, φ will be in thermal equilibrium with the neutrino plasma, contributing to extra radiation by ΔNeff 1⁄4 1=2 · 8=7 ≃ 0.57 for mφ ≪ Tν, where Tν is the plasma temperature. Incorporating the latest observations, BBN sets a strong constraint on the effective number of neutrino species [41], Neff 1⁄4 2.88 Æ 0.27: ð2Þ. This can be translated into a 2σ upper bound ΔNeff < 0.42, which severely limits the presence of extra radiation to solve the Hubble problem.

BBN as a function of the scalar mass mφ in the upper panel of

LARGE EXTRA RADIATION FOR BBN

PREFERRED PARAMETER SPACE

Findings

CONCLUDING REMARKS