UCLA

Pseudo-Nambu Goldstone bosons (pNGBs), such as axion-like particles (ALPs), are a compellingcandidate for cold dark matter. In the following thesis, I will discuss two separate ALP models of cold dark matter. In both models, a spontaneously broken approximate global U(1) symmetry results in the formation of an interlocking network of 1- and 2-dimensional topological defects that is stable unless a bias term is present in the Lagrangian. The annihilation of this system results in the production of ALPs, gravitational waves, and possibly primordial black holes (PBHs). We dub this process “catastrogenesis”, from the Greek word κατ αoτ ρoϕ�η, meaning “annihilation.” The first model I will discuss assumes that the ALPs are stable and therefore constitute some or all of the dark matter. In this case, if produced at temperatures below 100 eV gravitational waves could be detected by future cosmological probes for ALP masses anywhere between 10−16 and 106 eV. The second possibility I discuss is that ALPs could decay into Standard Model products that then thermalize, in which case ALPs do not make up any of the dark matter. However, if PBHs are produced during the last stages of string-wall annihilation they could themselves constitute up to 100% of the dark matter. In addition, produced gravitational waves could be detectable by future interferometers.