Scalar–Tensor Cosmology with Controlled Deviations from ΛCDM Expansion

We present a scalar–tensor cosmological model in which a dynamical scalar field Ψ modifies the effective gravitational coupling on cosmological scales. The model is formulated in the Jordan frame and analyzed numerically using a stable parametrization in terms of the logarithmic scale factor. The resulting cosmological evolution is found to be stable and free of divergences. Comparison with the standard ΛCDM model shows a moderate enhancement of the Hubble parameter at higher redshifts, while converging to ΛCDM at late times. These results suggest that scalar–tensor modifications of gravity can produce controlled and potentially observable deviations from standard cosmology, while remaining consistent with late-time evolution.