The discovery of supermassive black holes (SMBHs) at redshifts $z\gtrsim 10$ by JWST challenges the standard Eddington-limited growth models. We propose a novel mechanism for the rapid assembly of primordial black hole (PBH) seeds through scalar-induced critical accretion during early universe phase transitions. By solving the generalized relativistic Euler equations with a non-minimal coupling between dark matter and a background scalar field $\varphi$, we demonstrate that a “scalar fifth force” significantly shifts the sonic point outward. This mechanism yields an accretion enhancement factor $\mathrm{\Upsilon }\approx (1+\beta {)}^2$, enabling a $10M_{\odot }$ seed to reach ${10}^6{M}_{\odot }$ by $z\approx 15$, far exceeding standard General Relativistic predictions. Furthermore, this rapid mass growth predicts a distinctive spectral redshift in the stochastic gravitational wave background (SGWB), shifting the peak from the LIGO/Virgo band toward the Einstein Telescope sensitivity window (1–10 Hz).

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• Quantum cosmology
• Relativistic aspects of cosmology
• Primordial black holes