Some Properties of Multi-Component Axion Dark Matter

We introduce a mechanism for multi-component dark matter (DM) that originates from axion mixing and present some of its defining properties. In this context, multi-component DM implies that the cold DM is composed of the QCD axion and many ultra-light axion-like particles (ALPs). This framework can be realized in the type IIB string axiverse with hierarchical axion masses and decay constants. Our investigation reveals that in the light QCD axion scenario, the energy density of the lightest ALP often dominates after mixing. On the other hand, in the heavy QCD axion scenario, both the QCD axion and non-lightest ALPs may dominate, depending on the ALP decay constants. Under certain conditions, the QCD axion can dominate the DM budget. Finally, we briefly discuss a theoretical framework featuring $\sim\mathcal{O}(100)$ axions, with hierarchical axion masses and decay constants.