科研之友
微信
新浪微博
Facebook
分享链接
摘要
We study a testable dark-matter (DM) model outside of the standard weakly interacting massive particle paradigm in which the observed ratio Omega(dark) similar or equal to Omega(visible) for visible and dark-matter densities finds its natural explanation as a result of their common QCD origin when both types of matter (DM and visible) are formed at the QCD phase transition and both are proportional to Lambda(QCD). Instead of the conventional "baryogenesis" mechanism, we advocate a paradigm when the "baryogenesis" is actually a charge separation process which always occurs in the presence of the CP odd axion field a(x). In this scenario, the global baryon number of the Universe remains zero, while the unobserved antibaryon charge is hidden in the form of heavy nuggets, similar to Witten's strangelets and compromise the DM of the Universe. In the present work, we study in great detail a possible formation mechanism of such macroscopically large heavy objects. We argue that the nuggets will be inevitably produced during the QCD phase transition as a result of Kibble-Zurek mechanism on formation of the topological defects during a phase transition. Relevant topological defects in our scenario are the closed bubbles made of the N-DW = 1 axion domain walls. These bubbles, in general, accrete the baryon (or antibaryon) charge, which eventually results in the formation of the nuggets and antinuggets carrying a huge baryon (antibaryon) charge. A typical size and the baryon charge of these macroscopically large objects are mainly determined by the axion mass m(a). However, the main consequence of the model, Omega(dark) similar or equal to Omega(visible), is insensitive to the axion mass which may assume any value within the observationally allowed window 10(-6) eV less than or similar to m(a) less than or similar to 10(-3) eV. We also estimate the baryon-to-entropy ratio eta equivalent to n(B)/n(gamma) similar to 10-10 within this scenario. Finally, we comment on implications of these results to the axion search experiments, including the microwave cavity and the Orpheus experiments.
- 出版日期2016-10-3
