Dynamics of a Scalar-Tensor Vacuum Phase Transition at Cosmic Noon: Early Dark Energy and Electromagnetic Constraints

Context: The ΛCDM model faces challenges regarding the Hubble constant (H₀) and the detailed history of structure formation. Early Dark Energy (EDE) models offer a solution to the H₀ tension but often lack a natural trigger mechanism. Proposition: We propose a phenomenological framework where a scalar field φ undergoes a "freezing" phase transition at a critical redshift zc ≈ 3. Mechanism: Prior to the transition (z > zc), the field contributes a non-negligible energy density component (ρ_φ), acting as EDE. This injection increases the pre-recombination expansion rate, reducing the sound horizon rₛ and alleviating the Hubble tension. Constraints: We explore the coupling of this field to the electromagnetic sector. We find that while a large variation in the fine-structure constant α is excluded by CMB data, a perturbative shift of |Δ α/α| 0.3% remains consistent with Planck constraints. Such a shift provides a secondary channel to alleviate the Cosmological Lithium Problem and modify high-redshift cooling rates.