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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.
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It has recently been suggested that the presence of a plenitude of light axions, an Axiverse, is evidence for the extra dimensions of string theory. We discuss the observational consequences of these axions on astrophysical black holes through the Penrose superradiance process. When an axion Compton wavelength is comparable to the size of a black hole, the axion binds to the black hole nucleus forming a gravitational atom in the sky. The occupation number of superradiant atomic levels, fed by the energy and angular momentum of the black hole, grows exponentially. The black hole spins down and an axion Bose-Einstein condensate cloud forms around it. When the attractive axion self-interactions become stronger than the gravitational binding energy, the axion cloud collapses, a phenomenon known in condensed matter physics as Bosenova. The existence of axions is first diagnosed by gaps in the mass vs spin plot of astrophysical black holes. For young black holes the allowed values of spin are quantized, giving rise to Regge trajectories inside the gap region. The axion cloud can also be observed directly either through precision mapping of the near horizon geometry or through gravitational waves coming from the Bosenova explosion, as well as axion transitions and annihilations in the gravitational atom. Our estimates suggest that these signals are detectable in upcoming experiments, such as Advanced LIGO, AGIS, and LISA. Current black hole spin measurements imply an upper bound on the QCD axion decay constant of 2 x 10^17 GeV, while Advanced LIGO can detect signals from a QCD axion cloud with a decay constant as low as the GUT scale. We finally discuss the possibility of observing the gamma-rays associated with the Bosenova explosion and, perhaps, the radio waves from axion-to-photon conversion for the QCD axion.
The process of superradiance can extract angular momentum and energy from astrophysical black holes (BHs) to populate gravitationally-bound states with an exponentially large number of light bosons. We analytically calculate superradiant growth rates for vectors around rotating BHs in the regime where the vector Compton wavelength is much larger than the BH size. Spin-1 bound states have superradiance times as short as a second around stellar BHs, growing up to a thou- sand times faster than their spin-0 counterparts. The fast rates allow us to use measurements of rapidly spinning BHs in X-ray binaries to exclude a wide range of masses for weakly-coupled spin-1 particles, $5 times 10^{-14} - 2 times 10^{-11}$ eV; lighter masses in the range $6 times 10^{-20} - 2 times 10^{-17}$ eV start to be constrained by supermassive BH spin measurements at a lower level of confidence. We also explore routes to detection of new vector particles possible with the advent of gravitational wave (GW) astronomy. The LIGO-Virgo collaboration could discover hints of a new light vector particle in statistical analyses of masses and spins of merging BHs. Vector annihilations source continuous monochromatic gravitational radiation which could be observed by current GW observatories. At design sensitivity, Advanced LIGO may measure up to thousands of annihilation signals from within the Milky Way, while hundreds of BHs born in binary mergers across the observable universe may superradiate vector bound states and become new beacons of monochromatic gravitational waves.
Black hole superradiance is a powerful probe of light, weakly-coupled hidden sector particles. Many candidate particles, such as axions, generically have self-interactions that can influence the evolution of the superradiant instability. As pointed out in arXiv:1604.06422 in the context of a toy model, much of the existing literature on spin-0 superradiance does not take into account the most important self-interaction-induced processes. These processes lead to energy exchange between quasi-bound levels and particle emission to infinity; for large self-couplings, superradiant growth is saturated at a quasi-equilibrium configuration of reduced level occupation numbers. In this paper, we perform a detailed analysis of the rich dynamics of spin-0 superradiance with self-interactions, and the resulting observational signatures. We focus on quartic self-interactions, which dominate the evolution for most models of interest. We explore multiple distinct regimes of parameter space introduced by a non-zero self-interaction, including the simultaneous population of two or more bound levels; at large coupling, we confirm the basic picture of quasi-equilibrium saturation and provide evidence that the bosenova collapse does not occur in most of the astrophysical parameter space. Compared to gravitational superradiance, we find that gravitational wave annihilation signals and black hole spin-down are parametrically suppressed with increasing interactions, while new gravitational wave transition signals can take place for moderate interactions. The novel phenomenon of scalar wave emission is less suppressed at large couplings, and if the particle has Standard Model interactions, then coherent, monochromatic axion wave signals from black hole superradiance may be detectable in proposed axion dark matter experiments.
We remind that the ring down features observed in the LIGO GWs resulted from trembling of photon spheres (Rp=3M) of newly formed compact objects and not from the trembling of their event horizons (R=2M). Further, the tentative evidences for late time echoes in GWs might be signatures of horizonless compact objects rather than vacuum black holes (BHs). Similarly, even for an ideal BH, the radius of its shadow is R_shad = sqrt{3}Rp is actually the gravitationally lensed shadow of its photon sphere. Accordingly any compact object having R geq R = 3M would generate similar shadow. Thus, no observation has ever detected any event horizon or any exact BH. Also note that the magnetic field embedded in the accreting plasma close to the compact object is expected to have a radial pattern of B sim 1/r while the stronger BHM dipole magnetic field should fall off as B sim 1/r3. Accordingly it has been suggested that one may try to infer the true nature of the so-called astrophysical BHs by studying the radial pattern of the magnetic field in their vicinity. But here we highlight that close to the surface of BHMs, the magnetic field pattern differs significantly from the same for non-relativistic dipoles. In particular, we point out that for ultra-compact BHMs, the polar field is weaker than the equatorial field by an extremely large factor of sim z_s/lnz_s, where z_s>>1 is the surface gravitational redshift. We suggest that by studying the of radial variation as well as significant angular asymmetry of magnetic field structure near the compact object, future observations might differentiate a theoretical black hole from a astrophysical BH mimicker. This study also shows that even if some BHMs would be hypothesized to possess magnetic fields even stronger than that of magnetars, in certain cases, they may effectively behave as atoll type neutron stars possessing extremely low magnetic fields.
It is widely believed that axions are ubiquitous in string theory and could be the dark matter. The peculiar features of the axion dark matter are coherent oscillations and a coupling to the electromagnetic field through the Chern-Simons term. In this paper, we study consequences of these two features of the axion with the mass in a range from $10^{-13},{rm eV}$ to $10^{3},{rm eV}$. First, we study the parametric resonance of electromagnetic waves induced by the coherent oscillation of the axion. As a result of the resonance, the amplitude of the electromagnetic waves is enhanced and the circularly polarized monochromatic waves will be generated. Second, we study the velocity of light in the background of the axion dark matter. In the presence of the Chern-Simons term, the dispersion relation is modified and the speed of light will oscillate in time. It turns out that the change of speed of light would be difficult to observe. We argue that the future radio wave observations of the resonance can give rise to a stronger constraint on the coupling constant and/or the density of the axion dark matter.
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