The Gago Model Gravitational Wave Amplification by Ultra-Light Scalar Field

This thesis presents the Gago Model, an elegant and self-contained resolution to the Hubble Tension through a novel mechanism of late-time Gravitational Wave (GW) amplification. The proposed mechanism is rigorously shown to be both achromatic (dispersion-free at first order) and fully compatible with the stringent propagation speed constraints derived from the multi-messenger event GW170817, overcoming major hurdles faced by many scalar-tensor theories. A critical breakthrough in this work is the rigorous asymptotic correction of the scalar field self-energy integral (Sigma_1). This correction reveals that the necessary coupling is naturally small (gamma ~ 10^-8), dramatically strengthening the model's robustness by expanding the viable parameter space from an initial 2.1% to effectively 100%, thereby eliminating fine-tuning issues. Anchored by a variational derivation of the critical dynamical point H=1 and incorporating a chameleon screening mechanism, the model is theoretically solid and yields definitive, falsifiable predictions for next-generation detectors (LIGO O5/O6), most notably a specific redshift-dependent bias in the GW-inferred Hubble constant (dH0_GW/dz > 0).