Diffuse Gamma-Ray Background

The isotropic diffuse gamma-ray background (IGRB) detected by the Fermi Large Area Telescope is the residual gamma-ray flux after removing resolved sources and the Galactic diffuse emission. It encodes contributions from unresolved blazars, star-forming galaxies, radio galaxies, and possibly more exotic sources. The measured spectral shape and amplitude of the IGRB are broadly consistent with the known unresolved source populations in ΛCDM, but residuals persist at both low and high energies: there is an excess at GeV energies that has attracted attention as a possible dark matter annihilation signal, and the overall normalization of the IGRB requires careful accounting of the contribution from star-forming galaxies that itself depends on the assumed cosmic star formation history. The angular anisotropy of the IGRB — its fluctuation spectrum across the sky — encodes the clustering of unresolved sources and deviates from pure Poisson noise at large angular scales.

Successive Collision Theory addresses the IGRB through the enhanced star formation history and pre-existing compact object populations. The higher effective star formation rate density at high redshift implied by SCT's pre-existing stellar populations produces more star-forming galaxies contributing to the IGRB at all energies, naturally adjusting the normalization without invoking dark matter annihilation. The pre-existing compact objects — including neutron stars and black hole systems with non-standard spin and accretion histories — contribute to the unresolved gamma-ray source population through accretion-powered emission and magnetospheric processes at rates set by their pre-collision configurations rather than by standard stellar population synthesis models. This shifts the predicted source count distribution and energy spectrum of the IGRB in ways that reduce the residuals without requiring new particles.

The large-angular-scale anisotropy excess in the IGRB naturally follows from the coherent organization of gamma-ray source populations along the angular momentum strata and collision geometry of the debris field. In SCT, unresolved blazars and star-forming galaxies are not distributed with purely Poisson angular statistics — they trace the large-scale structure seeded by the collision's angular momentum field, which has coherence on scales up to the collision axis alignment scale of hundreds of megaparsecs. This introduces non-Poissonian angular correlations in the IGRB at large angular scales aligned with the CMB quadrupole/octupole axis — the same collision axis direction — producing the observed large-scale IGRB anisotropy as a predictable signature of the angular momentum inheritance mechanism.