Explanation for the Hubble constant tension

Determine the explanation for the more-than-4σ tension between direct measurements of the Hubble constant from Type-Ia supernovae (approximately 72.3 ± 1.4 (stat) ± 1.4 (syst) km s^{-1} Mpc^{-1}) and values inferred from Cosmic Microwave Background anisotropy measurements (approximately 67.4 ± 0.5 km s^{-1} Mpc^{-1}) under the standard ΛCDM cosmological framework.

Background

The paper highlights a major discrepancy—commonly referred to as the Hubble tension—between local determinations of the Hubble constant using Type-Ia supernovae and the value inferred from CMB observations within the ΛCDM model. The authors note that this disagreement exceeds 4σ and explicitly state that its explanation is currently unknown.

Motivated by this unresolved issue, the paper explores a phenomenological extension to ΛCDM via a thermal y-type distortion of the cosmic neutrino background. This distortion effectively increases the relativistic energy density, reducing the sound horizon and permitting higher H0 values, thereby potentially mitigating the tension. Nonetheless, the underlying cause of the tension remains unidentified in the broader cosmological context.

References

The most significant of these is the disagreement between direct measurement of the Hubble Constant $H_0$ using standard candles such as Type-Ia supernovae resulting in a value of $72.3 \pm 1.4\ \mathrm{(stat)}\ \pm 1.4\ \mathrm{(syst)}\ \mathrm{km\ s{-1}\ Mpc{-1}$ , and measurement of $H_0$ obtained from measurements of the anisotropy of the Cosmic Microwave Background, which give a value of $67.4 \pm 0.5 \ \mathrm{km\ s{-1}\ Mpc{-1}$ , resulting in a tension in excess of $4 \sigma$. The explanation for this tension is currently unknown.

— A bound on thermal y-distortion of the cosmic neutrino background (2407.18102 - Barenboim et al., 2024) in Section 1 (Introduction)