The Sigma-8 Tension is a Drag

Measurements of weak gravitational lensing at low redshifts ($z\lesssim 0.5-1$), quantified by the parameter $S_8$, favor weaker matter clustering than that expected from the standard $\Lambda$CDM cosmological model with parameters determined by cosmic microwave background (CMB) measurements. However, the amplitude of matter clustering at higher redshifts, as probed by lensing of the CMB, is consistent with $\Lambda$CDM. This apparent paradox suggests a connection between the $S_8$ tension and the transition from matter to dark-energy domination. Here we show that the tension can be resolved by introducing a friction between dark matter and dark energy without altering the tightly constrained expansion history. The low-$S_8$ measurements favor (at $\gtrsim3\sigma$, in this one parameter model) a non-zero drag leading to a suppression of low-redshift power right around the transition from matter to dark-energy domination. We suggest ways to further probe the scenario.