Axiverse and Black Hole
Hideo Kodama, Hirotaka Yoshino
TL;DR
This work surveys the axiverse, a landscape of ultralight axions predicted by string/M-theory, and highlights black-hole superradiance as a robust, testable cosmophysical phenomenon. By analyzing the massive-scalar Klein–Gordon equation in Kerr backgrounds, it derives instability conditions and growth rates across large- and small-mass regimes, and demonstrates how axion clouds around astrophysical black holes can emit gravitational waves and undergo nonlinear phenomena such as bosenova. The paper identifies an instability strip in the $\mu$–$M$ plane and describes the gravitational atom (G-atom) picture, where axion bound states around black holes form quantum-like systems with observable GW signatures. These insights provide a potential route to probe string compactifications and the ultimate theory through upcoming gravitational-wave observations and black-hole spin studies. The results emphasize the interplay between high-energy theory, BH physics, and cosmophysical data in testing fundamental physics.
Abstract
String theory/M-theory generally predicts that axionic fields with a broad mass spectrum extending below 10^{-10}eV are produced after compactification to four dimensions. These axions/fields provoke a rich variety of cosmophysical phenomena on different scales depending on their masses and provide us new windows to probe the ultimate theory. In this article, after overviewing this axiverse idea, I take up the black hole instability as the most fascinating one among such axionic phenomena and explain its physical mechanism and astrophysical predictions.