Alleviating the H0 and σ8 anomalies with a decaying dark matter model

Abstract

The Hubble tension between the ΛCDM-model-dependent prediction of the current expansion rate H0 using Planck data and direct, model-independent measurements in the local universe from the SH0ES collaboration disagree at >3.5σ. Moreover, there exists a milder ~ 2σ tension between similar predictions for the amplitude S8 of matter fluctuations and its measurement in the local universe. As explanations relying on unresolved systematics have not been found, theorists have been exploring explanations for these anomalies that modify the cosmological model, altering early-universe-based predictions for these parameters. However, new cosmological models that attempt to resolve one tension often worsen the other. In this paper, we investigate a decaying dark matter (DDM) model as a solution to both tensions simultaneously. Here, a fraction of dark matter density decays into dark radiation. The decay rate Γ is proportional to the Hubble rate H through the constant αdr, the only additional parameter of this model. Then, this model deviates most from ΛCDM in the early universe, with αdr being positively correlated with H0 and negatively with S8. Hence, increasing αdr (and allowing dark matter to decay in this way) can then diminish both tensions simultaneously. When only considering Planck CMB data and the local SH0ES prior on H0, ~ 1% dark matter decays, decreasing the S8 tension to 0.3σ and increasing the best-fit H0 by 1.6 km/s/Mpc. However, the addition of intermediate-redshift data (the JLA supernova dataset and baryon acoustic oscillation data) weakens the effectiveness of this model. Only ~ 0.5% of the dark matter decays bringing the S8 tension back up to ~ 1.5 σ and the increase in the best-fit H0 down to 0.4 km/s/Mpc.