Angular momentum evolution can be predicted from cosmological initial conditions

The angular momentum of dark matter haloes controls their spin magnitude and orientation, which in turn influences the galaxies therein. However, the process by which dark matter haloes acquire angular momentum is not fully understood. We present an extension to the genetic modification technique which allows us to control the angular momentum of any region in the initial conditions. Using a suite of simulations, we investigate whether changes to the angular momentum of a specified region in the evolved Universe can be accurately predicted from changes in the initial conditions. We find that the angular momentum in regions with modified initial conditions can be predicted significantly more accurately than expected from applying tidal torque theory. This result is masked when analysing the angular momentum of haloes, because particles in the outskirts of haloes dominate the angular momentum budget. We conclude that the angular momentum of Lagrangian patches is highly predictable from the initial conditions, with apparent stochastic or chaotic behaviour being driven by halo membership criteria.