Angular momentum evolution can be predicted from cosmological initial conditions

TL;DR

This paper demonstrates that the angular momentum of dark matter regions can be accurately predicted from initial cosmological conditions, revealing less stochasticity than previously thought.

Contribution

It introduces a genetic modification technique to control initial angular momentum and shows improved predictability over tidal torque theory.

Findings

Angular momentum of regions can be predicted 2-4 times more accurately than tidal torque theory suggests.

Stochasticity in halo angular momentum is due to boundary definitions, not initial conditions.

Lagrangian patch angular momentum is highly predictable from initial conditions.

Abstract

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; in particular, it is unclear whether angular momentum growth is stochastic. To address this question, we extend the genetic modification technique to allow control over the angular momentum of any region in the initial conditions. Using this technique to produce a sequence of modified simulations, we can then 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 alone. We find that the angular momentum in regions with modified initial conditions can be predicted between 2 and 4 times 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 chaotic behaviour being driven by stochastic changes to the arbitrary boundary defining the halo.