A USP Field Theory Interpretation of the Cosmic Microwave Background: Delta-f Decay, Resonance Floors, and the Deterministic Photon Fate (msf:48000 v1.1.1)

This note proposes that the Cosmic Microwave Background (CMB) can be interpreted within USP Field Theory as a universal resonance floor reached through deterministic decay of Δf, the mismatch parameter that defines photon coherence and geometric identity. Using the measured CMB temperature T = 2.725 K, the resonance floor is mapped to a minimum sustainable frequency: Δf_min ≈ 56 GHz. This value is interpreted as a stability boundary of the cosmic field after long-distance coherence loss. The observed blackbody spectral peak near 160 GHz is treated as a statistical occupancy maximum above the floor — not as the floor itself. The document: • Formalizes Δf decay geometry and long-path survival filtering • Distinguishes resonance floor vs. spectral peak • Introduces a line-of-sight weighted decay mechanism for low-ℓ alignment • Defines falsifiable amplitude targets for anisotropy (microkelvin scale) • Establishes spectral guardrails consistent with COBE/FIRAS blackbody precision • Proposes cross-correlation tests with large-scale structure tracers This work remains compatible with observational constraints including: • Near-perfect blackbody spectrum • Time-dilation consistency • COBE / WMAP / Planck temperature stability The cosmological interpretation is presented as optional and layered, allowing compatibility with standard ΛCDM while offering an alternative geometric resonance framework. Harmonized references within the USP series include: msf:47650 — Δf corridor decay geometry msf:47300 — survival filtering and Δf_crit cutoff msf:45740 — time as Δf rhythm (CMB as slow-clock baseline) This document defines a physically constrained and observationally testable formulation of the “resonance floor” interpretation of the CMB.