Departmental Doctoral Oral Examination (Thesis Title: “Axion Quark Nugget Dark Matter Model: Developments in Model Building and Observations”)
Physics and Astronomy
The axion quark nugget (AQN) model was initially proposed with the motivation to explain the observed similarity between the visible and dark matter abundances in the Universe. AQNs are dense objects made of standard model quarks in color superconducting (CS) phase. AQNs can be made of matter as well as antimatter. Matter AQNs and antimatter AQNs together form the dark matter, while the disparity between them will lead to the observed matter-antimatter asymmetry. Thus, the similarity between visible and dark matter abundances can be naturally explained since they have the same origin in the AQN framework.
This thesis focuses on recent developments in model building and some potential observational evidence of AQNs. First, we show how the coherent nonzero axion field in the early Universe generates the disparity between matter and antimatter AQNs. Then, we calculate the real-time evolution of an AQN from its initial state as a closed axion domain wall with baryon charge trapped inside to its final CS state.
Next, we show that for the most part of axion parameter space, AQNs are the dominant part of dark matter compared to the contribution of the free axions from the misalignment mechanism. After that, we calculate the size distribution of AQNs based on percolation theory. We also demonstrate that after formation, the size distribution can survive the subsequent evolution in the early Universe. Finally, we study potential observational evidence of the AQN model, focusing on the following two phenomena: the impulsive radio events in quiet solar corona recorded by the Murchison Widefield Array and the seasonal variation of the near-Earth X-ray background observed by the XMM-Newton observatory.