EUT - Temperature-Dependent Ultralight Scalar Cosmology Early Stiff Phase and kpc-Scale Dwarf Galaxy Cores with Mild Self-Interactions

We explore a minimal, inflation-free cosmology driven by an ultralight axion-like particle (ALP) whose mass is strongly temperature-dependent. Above ∼100 GeV the field is effectively massless, producing a prolonged kinetic-dominated stiff-matter phase (w = +1) lasting ∼64 e-folds that substantially ameliorates the horizon and flatness problems. At late times the zero-temperature mass is mₐ ≈ 3 × 10⁻¹⁹ eV, yielding solitonic cores of radius 1. 1–1. 8 kpc in 10⁸–10⁹ M⊙ dwarf galaxies — in excellent agreement with observed core sizes. An extremely mild, technically natural sextic self-interaction λ₆ ≈ 10⁻⁹⁴ restores full compatibility with the most recent black-hole superradiance constraints (November 2025) without affecting any other prediction. The model predicts clear, falsifiable deviations from ΛCDM on 0. 5–5 kpc scales testable with upcoming Euclid, LSST, and 30-m class telescope data. Have a look also at my other project: Eternal Universe Theory – Mirror Edition v0. 2. 5 E (u₁) = –E (u₂) A Two-Sheeted Energy-Antisymmetric Cosmology and the Natural Origin of the Cosmological Constant https: //zenodo. org/records/17683904