Galactic Plane Emission Model Fail

Modeling and removing the diffuse emission from the Galactic plane is one of the most challenging aspects of CMB and intensity mapping data analysis. The plane emits synchrotron radiation, free-free emission, thermal dust emission, AME, and molecular line emission across a wide range of frequencies, and all of these foregrounds must be accurately characterized to isolate the cosmological signal. Standard foreground models — including those used by Planck, WMAP, and now CMB-S4 foreground studies — fail to perfectly reproduce the observed multi-frequency emission in the Galactic plane, leaving residuals that contaminate CMB polarization measurements, particularly at low multipoles where the residuals are most significant relative to the cosmological signal. These model failures introduce systematic uncertainties in the primordial B-mode amplitude constraint, the optical depth τ, and the reionization history — all of which depend on accurate foreground separation at large angular scales.

Successive Collision Theory identifies a physical origin for Galactic plane foreground modeling failures through the angular momentum inheritance mechanism applied to the large-scale organization of the Milky Way's interstellar medium. In SCT, the ISM is not a random superposition of independently evolving emission regions but a structured medium whose gas, dust, and magnetic field distributions are organized by the inherited angular momentum field of the collision debris. Standard foreground models are constructed from parametric templates that assume smooth, separable spectral and spatial dependencies — an assumption that fails when the true emission has large-scale coherent angular structure from angular momentum organization. The residuals between observed and modeled plane emission reflect the difference between the true angular-momentum-organized emission structure and the smooth parametric templates, producing systematic large-scale residuals aligned with the collision axis and the Galactic angular momentum direction.

The pre-existing stellar populations from the colliding pockets contributed chemical species and dust grain types not captured in standard Galactic emission templates calibrated on purely post-collision stellar evolution. Pre-existing carbonaceous grains, polycyclic aromatic hydrocarbons, and molecular species from prior stellar generations are mixed into the Galactic ISM at levels that vary spatially with the debris field's angular momentum structure. The resulting emission spectrum differs from the pure post-collision stellar template in subtle but systematic ways — particularly in the AME-to-thermal dust ratio and in molecular line emission intensities — producing frequency-dependent residuals that cannot be absorbed into standard template coefficients. SCT therefore predicts that improved Galactic plane foreground models will require physically motivated templates based on the angular momentum structure of the local ISM rather than purely empirical polynomial spectral fitting.