• Physique/Astrophysique/Cosmologie et astrophysique extra-galactique
  
• Physique/Physique des Hautes Energies - Phénoménologie
  

Recent weak lensing surveys have revealed that the direct measurement of the parameter combination S8≡σ8(Ωm/0.3)0.5S8​≡σ8​(Ωm​/0.3)0.5-- where σ8σ8​ is a measure of the amplitude of matter fluctuations on 8 h−1h−1Mpc scales -- is ∼3σ∼3σ discrepant with the value reconstructed from cosmic microwave background (CMB) data assuming the ΛΛCDM model. In this article, we show that it is possible to resolve the tension if dark matter (DM) decays with a lifetime of Γ−1≃55 GyrsΓ−1≃55 Gyrs into one massless and one massive product, and transfers a fraction ε≃0.7 %ε≃0.7 % of its rest mass energy to the massless component. The velocity-kick received by the massive daughter leads to a suppression of gravitational clustering below its free-streaming length, thereby reducing the σ8σ8​ value as compared to that inferred from the standard ΛΛCDM model, in a similar fashion to massive neutrino and standard warm DM. Contrarily to the latter scenarios, the time-dependence of the power suppression and the free-streaming scale allows the 2-body decaying DM scenario to accommodate CMB, baryon acoustic oscillation, growth factor and un-calibrated supernova Ia data. We briefly discuss implications for DM model building, galactic small-scale structure problems and the recent Xenon-1T excess. Future experiments measuring the growth factor to high accuracy at 0≲z≲10≲z≲1 can further test this scenario.

We investigate constraints on early dark energy (EDE) using ACT, SPT-3G 2018, {\it Planck} polarization, and restricted {\it Planck} temperature data (at $\ell<650$), finding a $3.3\sigma$ preference ($\Delta\chi^2=-16.2$ for three additional degrees of freedom) for EDE over $\Lambda$CDM. The EDE contributes a maximum fractional energy density of $f_{\rm EDE}(z_c) = 0.163^{+0.047}_{-0.04}$ at a redshift $z_c = 3357\pm 200$ and leads to a CMB inferred value of the Hubble constant $H_0 = 74.2^{+1.9}_{-2.1}$ km/s/Mpc. We find that {\it Planck} and ACT data provide the majority of the improvement in $\chi^2$, and that the inclusion of SPT-3G pulls the posterior of $f_{\rm EDE}(z_c)$ away from $\Lambda$CDM. This is the first time that a moderate preference for EDE has been reported for these three combined CMB data sets. We find that including measurements of supernovae luminosity distances and the baryon acoustic oscillation standard ruler only minimally affects the preference ($3.0\sigma$), while measurements that probe the clustering of matter at late times -- the lensing potential power spectrum from {\it Planck} and $f \sigma_8$ from BOSS -- decrease the significance of the preference to 2.6$\sigma$. Conversely, adding a prior on the $H_0$ value as reported by the \SHOES{} collaboration increases the preference to the $4-5\sigma$ level. In the absence of this prior, the inclusion of \textit{Planck} TT data at $\ell>1300$ reduces the preference from $3.0\sigma$ to $2.3\sigma$ and the constraint on $f_{\rm EDE}(z_c)$ becomes compatible with \LCDM{} at $1\sigma$. We explore whether systematic errors in the \textit{Planck} polarization data may affect our conclusions and find that changing the TE polarization efficiencies significantly reduces the \textit{Planck} preference for EDE. More work will be necessary to establish whether these hints for EDE within CMB data alone are the sole results of systematic errors or an opening to new physics.