Dark Matter: from the Universe to the Lattice

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Dark matter governs the structure of the cosmos, from the smallest galaxies to the observable Universe. Through its gravitational pull on visible stars and gas, we can infer that dark matter makes up 85% of the total matter in the Universe. However, the microphysical properties of dark matter remain unknown despite intense scientific inquiry spanning high and low energy experiments and astronomical observations. While it is firmly established that dark matter exists through its gravity, the key to uncovering its nature lies in detecting its interactions beyond gravity. This proposal in theoretical physics focuses on the particle physics and astrophysics of dark matter and dark sectors. Under this hypothesis, dark matter particles have interactions and forces among themselves, analogous to the rich physics of visible matter and its strong, weak, and electromagnetic interactions. Under this broad theme, I will pursue two parallel research objectives. First, I will investigate tidal streams in the Milky Way as a probe of dark matter substructure. Tidal streams are the tidally-disrupted remnants of globular clusters, which, being dynamically fragile, are easily disrupted by dark matter clumps in the Milky Way. Therefore, tidal streams provide a method for detecting invisible substructures predicted to exist for dark matter that are otherwise completely invisible since they are too small to bind stars and gas on their own. Depending on the underlying particle model for dark matter, clumps are predicted to be concentrated or diffuse or absent altogether. This physics may be related to the long-standing core-cusp problem for rotation curves in galaxies. These observations are consistent with the dark sector model for cored dark matter profiles that more diffuse compared to the cuspy profiles predicted theoretically. However, the jury is still out due to the confounding influence of gas and stars. I will look to tidal streams to settle this controversy once and for all. Free from the confounding effect of stars and gas, tidal streams have the potential to measure the density profiles for substructures that purely dark matter. Second, I will explore the physics of strongly-interacting dark sectors. Dark matter models along these lines are appealing theoretically, but remain under-explored due to the breakdown of the usual Feynman-diagram-based approaches. I will use lattice field theory to study the interactions of dark matter particles within a minimal strongly-interacting dark sector. I will investigate the formation of dark elements through fusion between dark matter particles. I will also calculate the scattering cross section for dark matter particles with one another. These effects can influence phenomenology and astrophysics of dark matter, potentially providing new signatures for discovering its properties.