Sigma-8 tension is a drag

Measurements of weak gravitational lensing at low redshifts (z≲0.5-1), quantified by the parameter S8, favor weaker matter clustering than that expected from the standard Λ cold dark matter (ΛCDM) cosmological model with the 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 ΛCDM. In the literature, it has been found that the tension can be resolved by introducing a friction between dark matter and dark energy without altering the tightly constrained expansion history. Here, we show that in order to get a low S8 value consistent with the findings of cosmic shear under ΛCDM, cosmological measurements favor (at ∼3σ, in this one parameter model) a nonzero drag leading to a suppression of low-redshift power right precisely around the transition from matter to dark-energy domination. Our results hint at a connection between the S8 tension and the long-standing "cosmic coincidence problem."We suggest ways to further probe the scenario.