DEPARTMENT OF PHYSICS DISSERTATION DEFENSE: Bryen Irving

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Title: Asymptotic Assembly Histories

Abstract: The cosmic web—the network of dark matter halos and filaments that hosts galaxies—has grown throughout most of cosmic history, but accelerated expansion is gradually bringing that growth to an end. What, precisely, does it mean for structure formation to end? To answer this question, we designed and created the Last Objects Simulation Suite (LOSS), a collection of collisionless cosmological N-body simulations that follows the matter field and halos to scale factors at least 100 times today's value. In this talk, I will present three connected results. First, LOSS shows that freeze-out appears differently in comoving and physical descriptions: the comoving field follows a scale-dependent ordering, the coarse-grained physical field approaches a later plateau, and a characteristic crossover mass unifies these behaviors within a two-regime picture. Second, evolving multiple halo definitions into the far future exposes their physical differences: conventional boundaries either continue changing as the background density falls or bridge neighboring structures, whereas splashback remains tied to orbital dynamics and most cleanly identifies the coherent orbiting halo. Finally, splashback-based histories reveal an ordered freeze-out: major mergers fade before typical mass growth and common abundances settle, while rare populations and internal structure evolve longer. Taken together, these results recast the end of structure formation not as a single epoch, but as an ordered, scale- and boundary-dependent progression toward effective stability across the cosmic web, halo populations, and internal structure.