Macroscopic Cosmological Dynamics from a Nested Delayed-Release Horizon Sector

I propose a conjectural macroscopic cos-mological framework motivated by the possibility thatcosmological dynamics may retain coarse-grained scaleinformation beyond a single homogeneous Friedmann–Robertson–Walker scale factor. Rather than claiming amicroscopic derivation, we implement this idea at theeffective-theory level through a bounded horizon-sectoractivation variable. The defining structural feature ofthe construction is nestedness: flat ΛCDM is recoveredas an exact submanifold, while the additional contri-bution enters only through a reference-normalized rel-ative activation. We formulate the model as a reducedadiabatic system on the expanding branch, establish lo-cal well-posedness, derive boundedness and asymptoticproperties, and recover a finite transition to accelerationunder natural conditions. We then test the frameworkusing Pantheon+ supernovae with the full covariancematrix, a standard baryon acoustic oscillation compi-lation, and a first-pass growth-rate compilation in fσ8treated with a diagonal covariance approximation. Thedirect joint fit yields only a very small improvementin total χ2 relative to flat ΛCDM, while informationcriteria continue to favor the simpler baseline model.A profile-likelihood analysis of the present-day growthmodification is minimized at the exact ΛCDM limit andgives one-sided upper bounds consistent with a weakdelayed-release sector. Current background plus first-pass growth data therefore constrain the framework toremain close to its exact ΛCDM limit.