Systematic CMB Constraints on Early-Universe Modifications: Parameter Degeneracies and Null Results

AbstractWe perform a systematic test of Energy-Flow Cosmology (EFC) inspired modifications to the Cosmic Microwave Background (CMB) using Planck 2018 TT data and the CLASS Boltzmann solver. Two distinct mechanisms are investigated: (1) a microphysical modification of the photon–baryon coupling rate (κ̇), and (2) a macroscopic background modification via a localized, radiation-like energy component affecting H(z) near recombination. Initial one-parameter scans suggested possible improvements for the H(z) channel; however, full two-parameter grid analyses demonstrate that these effects are fully degenerate with standard cosmological parameters, particularly H₀. When parameter degeneracies are correctly accounted for, both tested mechanisms yield minima at zero amplitude, indicating no preference over standard ΛCDM. The results show that the CMB regime (L0–L1) strongly constrains modifications to background dynamics while being comparatively tolerant of small microphysical changes. Within the EFC-R (regime-dependent) framework, this outcome is consistent with expectations: the CMB epoch represents a near-linear radiation-dominated regime where EFC effects are not expected to dominate. The study establishes a reproducible methodology for testing EFC-type models against CMB data and demonstrates how regime-based reasoning can guide the interpretation of null results.