We present simplified models for the galactic center $\gamma$-ray excess where Dirac dark matter annihilates into pairs or triplets of on-shell bosonic mediators to the standard model. These annihilation modes allow the dark matter mass to be heavier than those of conventional effective theories for the $\gamma$-ray excess. Because the annihilation rate is set by the dark matter–mediator coupling, the standard model coupling can be made parametrically small to “hide” the dark sector by suppressing direct detection and collider signals. We explore the viability of these models as a thermal relic and on the role of the mediators for controlling the $\gamma$-ray spectral shape. We comment on ultraviolet completions for these simplified models and novel options for standard model final states.

We analyze infrared divergences arising in calculations involving light and massless fields in de~Sitter space. We show that these arise from an incorrect treatment of the constant mode of the field and that a correct quantization leads to a well-defined and calculable perturbation expansion. We illustrate this by computing the first nontrivial loop correction in a theory of a massless scalar field with a quartic interaction.

We study the leading quantum effects in the recently introduced matrix big bang model. This amounts to a study of supersymmetric Yang-Mills theory compactified on the Milne orbifold. We find a one-loop potential that is attractive near the big bang. Surprisingly, the potential decays very rapidly at late times where it appears to be generated by $D$-brane effects. Usually, general covariance constrains the form of any effective action generated by renormalization group flow. However, the form of our one-loop potential seems to violate these constraints in a manner that suggests a connection between the cosmological singularity and long wavelength, late time physics.

Complex scalar fields charged under a global $U(1)$ symmetry can admit nontopological soliton configurations called $Q$-balls, which are stable against decay into individual particles or smaller $Q$-balls. These $Q$-balls are interesting objects within quantum field theory, but are also of phenomenological interest in several cosmological and astrophysical contexts. The $Q$-ball profiles are determined by a nonlinear differential equation, and so they generally require solution by numerical methods. In this work, we derive analytical approximations for the $Q$-ball profile in a polynomial potential and obtain simple expressions for the important $Q$-ball properties of charge, energy, and radius. These results improve significantly on the often-used thin-wall approximation and make it possible to describe $Q$-balls to excellent precision without having to solve the underlying differential equation.