Axi-Higgs cosmology

If the electroweak Higgs vacuum expectation value v in early universe is ∼1% higher than its present value v₀=246 GeV, the ⁷Li puzzle in BBN and the CMB / ΛCDM tension with late-universe measurements on Hubble parameter are mitigated. We propose a model of an axion coupled to the Higgs field, named “axi-Higgs”, with its mass m_a ∼ 10^-30 - 10^-29 eV and decay constant f_a ∼ 10¹⁷ - 10¹⁸ GeV, to achieve this goal. The axion initial value a_ini yields an initial Δ v_ini/v₀ ∼ 0.01 throughout the BBN-recombination epoch and a percent level contribution to the total matter density today. Because of its very large de Broglie wavelength, this axion matter density ω_a suppresses the matter power spectrum, alleviating the CMB/ΛCDM S₈/σ₈ tension with the weak-lensing data. It also explains the recently reported isotropic cosmic birefringence by its coupling with photons. Adding the axion (m ∼ 10^-22 eV) in the fuzzy dark matter model to the axi-Higgs model allows bigger Δv_rec and ω_a to address the Hubble and S₈ / σ₈ tensions simultaneously. The model predicts that Δv may be detected by the spectral measurements of quasars, while its oscillation may be observed in the atomic clock measurements. © 2021 IOP Publishing Ltd and Sissa Medialab