No abstract

Ultra light axion fields, motivated by the string theory, form a large condensate (axion cloud) around rotating black holes through superradiant instability. Several effects due to the axion cloud, such as the spin-down of black holes and the emission of monochromatic gravitational waves, open a new window to search for axions by astrophysical observations. When the axion self-interaction is considered, the evolution of cloud is altered significantly, and an explosive phenomenon called bosenova can happen. Thus, it is necessary to understand the precise evolution of self-interacting clouds for the detection of axions by astrophysical observations. In this paper, we propose a new method to track the whole process of the growth of self-interacting axion clouds employing the adiabatic approximation. We emphasize that our method relies neither on the non-relativistic approximation nor on perturbative treatment of the self-interaction, which is often used in literature. Our main finding is that the evolution of cloud in the strongly self-interacting regime depends on the strength of the gravitational coupling between the axion and the black hole. For a large coupling, the cloud evolves into a quasi-stationary state where the superradiant energy gain is balanced with the energy dissipation to infinity by the self-interaction. On the other hand, when one decreases the size of coupling, clouds become unstable at some energy, which would be interpreted as the onset of bosenova.

There are strong interests in considering the ultra-light scalar field (especially axion) around a rapidly rotating black hole because of the possibility of observing a gravitational waves from axion condensate (axion cloud) around black hole. Motivated by this consideration, we study dynamics of ultra-light scalar field with self-interaction around a rapidly rotating black hole by the Renormalization group method. We found that for the relativistic cloud, saturation of the superradiant instability by the scattering of the axion due to the self-interaction does not occur in the weakly non-linear regime. This means that for the relativistic axion cloud, explosive phenomena called the Bosenova might happen in the realistic situation.

Ultralight scalar fields such as axions can form clouds around rotating black holes (BHs) by the superradiant instability. It is important to consider the evolution of clouds associated with BH binaries for the detectability of the presence of clouds through gravitational wave signals and observations of the mass and spin parameters of BHs. The impact on the axion cloud due to the tidal perturbation from the companion in a binary system was first studied in D. Baumann et al., Phys. Rev. D, 101, 083019. Here, we re-examine this issue taking into account the following points. First, we study the influence of higher-multipole moments. Second, we consider the backreaction due to the angular momentum transfer between the cloud and the orbital motion. This angular momentum transfer further causes the backreaction to the hyperfine split through the change in geometry. Finally, we calculate the particle number flux to infinity induced by the tidal interaction. As a result, we find that the scalar field is not reabsorbed by the BH. Instead, the scalar particles are radiated away to evaporate during the inspiral, irrespective of the direction of the orbital motion, for almost equal mass binaries.

Double holography plays a crucial role in recent studies of Hawking radiation and information paradox by relating an intermediate picture, in which a dynamical gravity living on an end-of-the-world brane is coupled to a non-gravitational heat bath, to a much better-understood BCFT picture as well as a bulk picture. In this paper, causal structures in generic double holographic setups are studied. We find that the causal structure in the bulk picture is compatible with causality in the BCFT picture, thanks to a generalization of the Gao-Wald theorem. On the other hand, consistency with the bulk causal structure requires the effective theory in the intermediate picture to contain a special type of super-luminal and nonlocal effect which is significant at long range or IR. These are confirmed by both geometrical analysis and commutators of microscopic fields. Subregion correspondences in double holography are discussed with the knowledge of this nonlocality. Possible fundamental origins of this nonlocality and its difference with other types of nonlocality will also be discussed.