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 $z\sim 5,000$, to raise today’s value of the Hubble parameter, and (ii) an ultralight axion (ULA), which starts oscillating at $z\gtrsim {10}^4$, 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 $\mathrm{\Lambda }\mathrm{CDM}$ model, while importantly reducing both the $H_0$ and $S_8$ tensions with late universe probes ($\lesssim 3\sigma$). Combined with measurements from cosmic shear surveys, we find that the discrepancy on $S_8$ is reduced to the $1.4\sigma$ level. Adding local supernovae measurements, we find that the $H_0$ and $S_8$ tensions are reduced to the $1.4\sigma$ and $1.2\sigma$ levels respectively, with a significant improvement $\mathrm{\Delta }{\chi }^2\simeq -18$ compared to the $\mathrm{\Lambda }\mathrm{CDM}$ model. With this complete dataset, the DDE and ULA are detected at $\simeq 4\sigma$ and $\simeq 2\sigma$, 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.

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• Cosmic microwave background
• Cosmological parameters
• Cosmology
• Dark energy
• Evolution of the Universe
• Large scale structure of the Universe
• Particle dark matter
• Axions