ITU and Cosmology: A Single-Axiom View of Big Bang, Inflation, Dark Matter, Dark Energy, Hubble/S₈ Tensions, CMB B-modes, Cosmic Horizons, and the 2026-2050 Observational Roadmap

We apply the Information-Theoretic Unification (ITU) framework (Terada 2026, concept DOI 10. 5281/zenodo. 20109209; current latest Tier 0 v3. 0 at 10. 5281/zenodo. 20200156) to cosmology. This is Tier 1 paper #19, the third paper of Block A (Physics/Math deepening) and the dual specialization of Tier 1 #17 Quantum Gravity (DOI 10. 5281/zenodo. 20230667) and Tier 1 #18 Black Holes (DOI 10. 5281/zenodo. 20233070), completing the HORIZON TRIAD with Kcosmic (observer INSIDE the de Sitter horizon). The ITU polytope reaches 19 vertices with Tier 1 #19 attaining degree 18 (new maximum). Pass-1 progress: 134 of 220 phases (60. 9 %). Phase 127 establishes the Big Bang ΛCDM framework. Friedmann equations integrate to age 13. 80 Gyr (Planck 2018: 13. 797 Gyr). Planck 2018 parameters: H₀ = 67. 4 km/s/Mpc, Ωₘ = 0. 315, Ω_Λ = 0. 685, σ₈ = 0. 811, nₛ = 0. 965. de Sitter horizon rdS = 5. 37 Gpc, SdS = 3. 31e+122 nats (Gibbons-Hawking). #18 BH and #19 Cosmic are symmetric specializations of Kgeom. Phase 128 covers inflation (Guth 1981, Linde 1982). Slow-roll for V = (1/2) m²φ²: nₛ = 0. 965 matched at φ ≈ 15. 11 MPl, r ≈ 0. 14 (EXCLUDED by BICEP/Keck 2021 r 6σ confirmed 2027 P=0. 80, DESI evolving DE 2027 P=0. 75, S₈ >4σ 2030 P=0. 70, CMB-S4 2035 P=0. 60, PBH DM 2038 P=0. 55, EDE+IDM 2035 P=0. 55, anthropic Λ 2040 P=0. 45, LiteBIRD τ 2034 P=0. 75, multiverse Level II 2050 P=0. 30. Grand Pₐvg = 0. 630. Strong/Medium/Weak = 6/3/1. Central thesis: ITU axiom delta S = delta specializes to Kcosmic at cosmological horizons, unifying Big Bang ΛCDM, inflation, DM, DE, Hubble+S₈ tensions, CMB B-modes, multiverse, and holographic information bound under one principle. Block A papers 1-3 (#17 QG + #18 BH + #19 Cosmology) form HORIZON TRIAD covering all spacetime-horizon physics. LiteBIRD (2034) and DESI Y3 (2027) provide strongest near-term tests. Pass-2 (Phase 223) will derive evolving K_Λ (t), KDM dynamical correction, and Compound EDE+IDM in ITU language. Honest framing: Pass-1 interpretive paper. Numerical results agree with established literature (Friedmann 1922, Hubble 1929, Penzias-Wilson 1965, Guth 1981, Linde 1982, Smoot 1992, Riess 1998, Perlmutter 1999, Planck 2018, KiDS-1000 2021, BICEP/Keck 2021, DESI 2024). No new physical predictions beyond cited works; contribution is unified ITU axiom narrative under Kcosmic and HORIZON TRIAD with #17 and #18. Includes 8 theory documents (theoryₚhase127-134. md), 8 Python numerical experiments, 8 figures (PNG), 8 JSON summaries. Total runtime ~15 seconds.