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), which undergoes a transition at $z \sim
5,000$, to raise today's value of the Hubble parameter -- addressing the $H_0$
tension, and (ii) an ultra-light axion (ULA), which starts oscillating at
$z\gtrsim 10^4$, to suppress the matter power spectrum -- addressing the $S_8$
tension. 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
$\Lambda$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, and the value of $H_0$ is further raised.
Adding local supernovae measurements, we find that the $H_0$ and $S_8$ tensions
are reduced to the $1.4\sigma$ and $1.2\sigma$ level respectively, with a
significant improvement $\Delta\chi^2\simeq -18$ compared to the $\Lambda$CDM
model. With this complete dataset, the DDE and ULA are detected at $\simeq
4\sigma$ and $\simeq2\sigma$, 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.
