The Cosmological Implications of Self-Interacting Dark Matter

Abstract

In this thesis I study how dark matter particles that interact through forces other than just gravity would affect the formation of structure in the Universe. This begins with a theoretical calculation of the location and rate at which these interactions take place throughout cosmic history. Giant galaxy clusters are expected to have the highest rates of dark matter interactions, at least for the simplest dark matter particle models.

Predicting the formation of structure with non-standard dark matter requires the use of N-body simulations. I therefore introduce and test a set of modifications to the GADGET code that allow it to simulate a class of dark matter models known as self-interacting dark matter (SIDM). I focus particular attention on rarely discussed aspects of simulating SIDM; including how to handle particles scattering multiple times within a single time-step and how to implement scattering across processors. I also discuss how best to choose numerical parameters associated with the SIDM implementation and the range of numerical parameters that produce converged results.

Because galaxy clusters should have particularly high rates of dark matter interactions, I use this code to perform simulations of a pair of merging galaxy clusters known as the 'Bullet Cluster'. At first these employ simple SIDM particle physics models for the dark matter. I demonstrate the importance of analysing simulations in an observationally motivated manner, finding that the way in which simulation outputs are compared with observations can have a significant impact on the derived constraints upon dark matter’s properties. I then look at what happens to these constraints for more complicated particle physics models of SIDM. In isolated systems, the effects of a complicated scattering cross-sections can be modelled using an appropriately-matched simple cross-section, while in systems like the Bullet Cluster, complicated cross-sections lead to phenomenology not seen with simpler particle models. Overall I find that SIDM remains a viable class of dark matter models, consistent with current observations.