Paper 5 of the Interior Observer (IO) cosmological framework, which proposes that the observable universe exists inside a Schwarzschild black hole with the same physics inside the horizon as outside. Paper 4 located the framework's one quantified tension, the acoustic scale, in the radiation-dominated epoch that the dust cycloid alone does not describe. Paper 5 builds the early-time interior that epoch requires, through three results. First, the framework is fixed to the non-rotating limit. The interior temperature follows the horizon surface gravity, which falls monotonically with Kerr spin, so any rotation lowers the predicted temperature below the Schwarzschild value of 2. 663 K and only widens the gap with observation. The Schwarzschild limit is physically reinforced, not merely assumed, and cosmic rotation is bounded to a small perturbative role (a/M < 0. 096 costs only a quarter of a percent in temperature). Second, the discrete Vaidya null-dust metric proposed in the early papers for the radiation phase is shown, by direct computation of its Einstein tensor, to describe an anisotropic radial energy stream (a single nonvanishing component Gᵥv = 2λ/r², density ρ = λ/ (4πr²) ) rather than the isotropic thermal bath that nucleosynthesis and the cosmic microwave background acoustic oscillations require. It cannot serve as the cosmological radiation fluid and is retired. Third, it is replaced by a single continuous mixed-fluid closed Friedmann interior carrying radiation, matter, curvature, and the vacuum term together, H² (u) = H₀² (Ωᵣ/u⁴ + Ωₘ/u³ + Ωₖ/u² + Ω_Λ), which transitions smoothly from the radiation era to the present with no internal junction or shell. On the active projection branch (H₀ = 67. 58 km/s/Mpc, Ωₘ = 0. 349, Ωₖ = −0. 046, Ω_Λ = 0. 697) the expansion rate at nucleosynthesis is standard to within a tenth of a percent (HIO/H_ΛCDM = 1. 0009 at z = 10⁹), so the light-element abundances of Papers 22 and 24, all within one observational sigma with the lithium problem conditionally resolved, come from cross-section rate-dressing on a near-standard background, not from a modified expansion rate. The dead-branch acoustic-peak deficit that the earlier version treated as fundamental does not survive: the first acoustic peak matches Planck (ℓ₁ = 220), and the residual acoustic tension is the finer scale θ* = 0. 5994° against Planck 0. 5970° ± 0. 00026°, a 9. 2σ residual characterized in Paper 20. The one genuine open problem is the exterior matching of the pressurized radiation-era interior to the Schwarzschild vacuum. Reproducibility bundle paper5-v2. 0 (SHA256 af70b536cd3576e98e7a98fb4bce0831c4652085fbfd9516058ad6e6f89574b1, validator 17/17 PASS) at https: //github. com/dfife/io-framework-public/releases/tag/paper5-v2. 0. https: //dfife. github. io/index. html v2. 0 (May 2026): Storytelling rebuild onto the active projection branch (Papers 10 and 29: H₀ = 67. 58 km/s/Mpc, Ωₘ = 0. 349, Ωₖ = −0. 046, Ω_Λ = 0. 697, Ωᵣ = 9. 1575 × 10⁻⁵, zero continuously fitted cosmological parameters). The three durable results are retained: the Kerr-spin temperature bound, the proof that the Vaidya null-dust radiation phase cannot supply an isotropic thermal bath, and its replacement by a continuous mixed-fluid closed Friedmann interior. The dead-branch acoustic-peak section is superseded and removed as a standing result: the v1. 4 claim that the first acoustic peak is structurally locked near 181 was computed on the earlier branch (Ωₘ = 0. 197, Ωₖ = −0. 130, H₀ = 58. 2) ; on the active branch the first peak matches Planck (Paper 12), and the one quantified acoustic tension is the finer acoustic scale θ* (Paper 20). The early-universe expansion-rate comparison is recomputed on the active branch and reconciled with the rate-dressing treatment of Papers 22 and 24. The discrete Vaidya phase, the dead-branch density parameters, and the ledgered ℓ₁ = 181 tension are removed. Title updated to match content. Code and data availability and an Active Theorem and Definition Ledger added. Premises-led abstract; hyphens only. Reproducibility bundle: paper5-v2. 0 published (release tag paper5-v2. 0, SHA256 af70b536cd3576e98e7a98fb4bce0831c4652085fbfd9516058ad6e6f89574b1, commit e45582b, validator 17/17 PASS) at https: //github. com/dfife/io-framework-public/releases/tag/paper5-v2. 0. See https: //github. com/dfife/io-framework-public/tree/main for claim-naming convention. v1. 4 (March 2026): Title page reformatted to series standard. No content changes required: Paper 5's results (Kerr spin bounds, Vaidya incompatibility proof, mixed-fluid FRW interior, CMB first peak analysis, BBN expansion rate) are all convention-independent. The cycloid parameterization correction (Papers 1-4) does not affect any result in this paper. See Paper 21 v1. 1 for the full audit. v1. 3 (Paper 12, DOI: 10. 5281/zenodo. 18936508): Baryon sector annotations per Paper 12 (Baryon Dictionary Principle). Paper 12 derived fb = 2γ/x = 0. 313, giving ωb, geom = 0. 021 (~5% below ΛCDM, vs. 24% at fb = 0. 25). The BBN tensions flagged in §6. 4 and §7 as "requiring dedicated computation" are resolved: D/H = 2. 49 × 10⁻⁵ (−1. 2σ). The Kerr spin bound, Vaidya incompatibility, mixed-fluid peak ℓ₁ = 181, and HIO/H_ΛCDM = 0. 955 are all unaffected. v1. 2: Added bibliography, no other changes. v1. 1: Added note clarifying that the exact inclusion of Ωᵣ in the mixed-fluid boundary condition shifts H₀ from 58. 41 km/s/Mpc (Papers 1, 3, 4; dust-only) to 58. 2 km/s/Mpc. Self-consistency correction, not a parameter change. Companion references updated. Companion to Paper 1 (DOI: 10. 5281/zenodo. 18854813), Paper 2 (DOI: 10. 5281/zenodo. 18868612), Paper 3 (DOI: 10. 5281/zenodo. 18876346), Paper 4 (DOI: 10. 5281/zenodo. 18883069), and Paper 6 (DOI: 10. 5281/zenodo. 18891475).
David Fife (Mon,) studied this question.