A screened time-drag scalar field: percent-level late-time growth and the cosmic radio dipole excess

We present a minimal, ghost-free, gradient-stable scalar–tensor theory in which a single scalar field τ with a density-dependent kinetic coefficient Z(ρ_m) produces ~1% modifications to the structure-growth rate at z ≲ 2 while preserving the exact ΛCDM background expansion. The model exhibits strong kinetic screening, suppressing fifth forces by over 14 orders of magnitude locally and by more than 10²⁴ at recombination. Linear perturbations yield a modified effective Newton constant G_eff(a) = G[1 + α_eff(a)], with α ≈ 0.02 producing the correct percent-level enhancement in fσ₈.</p>\n\n<p>Large-scale gradients in the background time derivative τ̇ generate a non-kinematic contribution to cosmic number-count dipoles of order 0.01–0.02, automatically aligned with the CMB dipole and peaking at z ~ 1. This naturally explains the long-standing excess in radio and mid-infrared dipole measurements. The theory satisfies all constraints from BBN, CMB anisotropies, Solar System tests, gravitational-wave propagation, and PPN bounds.</p>\n\n<p>The model makes sharp, falsifiable predictions testable at percent-level precision by DESI, Euclid, LSST, and SKA over the next decade. This work provides a unified explanation for two independent late-time cosmological anomalies—suppressed growth and the radio dipole excess—using a single screened scalar degree of freedom without altering the ΛCDM expansion history.