Abstract
We study the cosmology of the dark sector consisting of (ultra)light scalars. Since the scalar mass is radiatively unstable, a special explanation is required to make the mass much smaller than the UV scale. There are two well-known mechanisms for the origin of scalar mass. The scalar can be identified as a pseudo-Goldstone boson, whose shift symmetry is explicitly broken by nonperturbative corrections, like the axion. Alternatively, it can be identified as a composite particle like the glueball, whose mass is limited by the confinement scale of the theory because no scalar degree of freedom exists at high scales. In both cases, the scalar can be naturally light, but interaction behavior is quite different. The lighter the axion (glueball), the weaker (stronger) its interaction. As the simplest nontrivial example, we consider the dark axion whose shift symmetry is anomalously broken by the hidden non-Abelian gauge symmetry. After the confinement of the gauge group, the dark axion and the dark glueball get masses and both form multicomponent dark matter. We carefully consider the effects of energy flow from the dark gluons to the dark axions and derive the full equations of motion for the background and the perturbed variables. The effect of the dark axion--dark gluon coupling on the evolution of the entropy and the isocurvature perturbations is also clarified. Finally, we discuss the gravothermal collapse of the glueball subcomponent dark matter after the halos form, in order to explore the potential to contribute to the formation of seeds for the supermassive black holes observed at high redshifts. With the simplified assumptions, the glueball subcomponent dark matter with the mass of 0.01--0.1 MeV and the axion main dark matter component with the decay constant ${f}_{a}=\mathcal{O}({10}^{15}--{10}^{16})\text{ }\text{ }\mathrm{GeV}$ and the mass of $\mathcal{O}({10}^{\ensuremath{-}14}--{10}^{\ensuremath{-}18})\text{ }\text{ }\mathrm{eV}$ can provide a hint on the origin of the supermassive black holes at high redshifts.
Highlights
The discovery of the Higgs boson completed the Standard Model, which explains most of the phenomena in the Universe including nuclei, atoms, and their interactions
We have studied the cosmological evolution of dark light scalars, whose masses and interactions originate from the approximate global symmetry and the nonperturbative dynamics of the hidden gauge symmetry
We explore the possibility that the subcomponent glueball dark matter contributes to the formation of the supermassive black hole at redshift z ∼ 7
Summary
The discovery of the Higgs boson completed the Standard Model, which explains most of the phenomena in the Universe including nuclei, atoms, and their interactions. The numberchanging interactions are active, so the occupation number is always limited by its temperature during its cosmological evolution In this case, the relic density is determined by the freeze-out mechanism. The relic density is determined by the freeze-out mechanism Because of this property, the light dark glueball becomes a good candidate for self-interacting dark matter (SIDM) [8,9], which is one of the ways to make the cored density profile around the centers of galaxies [10]. As a subcomponent hot dark matter, it can play the role to suppress small-scale perturbations [11] These two mechanisms to obtain a light scalar dark matter provide a completely different microscopic nature of dark matter and yield different predictions for small-scale evolution.
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