LAUSR

We develop a new phenomenological model that addresses current tensions between observations of the early and late Universe. Our scenario features: (i) a decaying dark energy fluid, which undergoes a transition at z ∼ 5, 000, to raise today’s value of the Hubble parameter – addressing the “H0 tension,” and (ii) an ultra-light axion, which starts oscillating at z ∼ 16, 000, to suppress the matter power spectrum – addressing the “S8 tension.” Our Markov Chain Monte Carlo analyses show that such a Dark Sector model fits a combination of early time datasets slightly better than the ΛCDM model, while reducing both the H0 and S8 tensions to . 3σ level. Combined with measurements from cosmic shear surveys, we find that the discrepancy on S8 is reduced to the 1.4σ level, and the value of H0 is further raised. Adding local supernovae measurements, we find that the H0 and S8 tensions are reduced to the 1.5σ and 1.1σ level respectively, with a significant improvement ∆χ ' −17 compared to the ΛCDM model. We discuss a possible particle physics realization of this model, with a dark confining gauge sector and its associated axion, although embedding the full details within microphysics remains an urgent open question. Our scenario will be decisively probed with future CMB surveys.