UltraLight Dark Matter Dynamics in the Language of Eigenstates*

Self-gravitating quantum matter may exist in a wide range of cosmological and astrophysical settings: from the very early universe through to present-day boson stars. Such quantum matter arises in UltraLight Dark Matter (ULDM): an exciting axion-like particle candidate which keeps the successes of CDM on large scales but alleviates tensions on small scales. This small-scale behavior is due to characteristic cores in ULDM called solitons, which also correspond to the ground state of the self-gravitating quantum system governing ULDM. We calculate the full spectrum of eigenstates and decompose simulations of ULDM into these states, allowing us to precisely track the evolution of the tell-tale soliton cores and the surrounding halo "skirt". Using this formalism, we investigate formation of halos through binary soliton collisions and the dependence of the final halo product on initial parameters, which allows us to comment on the core-halo mass relation of ULDM. We also link characteristic ULDM halo behavior—such as the soliton "breathing mode" and random walk of the center of mass—to the presence of certain modes. Finally, we comment on the relationship between eigenenergies and oscillatory timescales present in the system, as well as future directions for understanding ULDM through the language of its eigenstates.