Dark sector to restore cosmological concordance

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 (DDE) with a transition at z similar to 5; 000, to raise today's value of the Hubble parameter, and (ii) an ultralight axion (ULA), which starts oscillating at z greater than or similar to 104, to suppress the matter power spectrum. Our Markov chain Monte Carlo analyses show that such a dark sector model fits a combination of cosmic microwave background (CMB), baryon acoustic oscillations, and Large Scale Structure (LSS) data slightly better than the ACDM model, while importantly reducing both the H0 and S8 tensions with late universe probes (less than or similar to 3u). Combined with measurements from cosmic shear surveys, we find that the discrepancy on S8 is reduced to the 1.4u level. Adding local supernovae measurements, we find that the H0 and S8 tensions are reduced to the 1.4u and 1.2u levels respectively, with a significant improvement Delta chi 2 similar or equal to -18 compared to the ACDM model. With this complete dataset, the DDE and ULA are detected at similar or equal to 4u and similar or equal to 2u, respectively. 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 and LSS surveys.