Superradiance: New Frontiers in Black Hole Physics

Preface
	References
Acknowledgments
Contents
About the Authors
Notation and Conventions
Acronyms
1 Milestones
	References
2 Superradiance in Flat Spacetime
	2.1 Klein Paradox: The First Example of Superradiance
		2.1.1 Bosonic Scattering
		2.1.2 Fermionic Scattering
	2.2 Superradiance and Pair Creation
	2.3 Superradiance and Spontaneous Emission by a Moving Object
		2.3.1 Cherenkov Emission and Superradiance
		2.3.2 Cherenkov Radiation by Neutral Particles
		2.3.3 Superradiance in Superfluids and Superconductors
	2.4 Sound Amplification by Shock Waves
		2.4.1 Sonic ``Booms''
		2.4.2 Superradiant Amplification at Discontinuities
	2.5 Rotational Superradiance
		2.5.1 Thermodynamics and Dissipation: Zel'dovich and Bekenstein's Argument
		2.5.2 EFT Approach
		2.5.3 Example 1. Scalar Waves
		2.5.4 Example 2. Amplification of Sound and Surface Waves at the Surface of a Spinning Cylinder
		2.5.5 Example 3. Tidal Heating and Acceleration
			Tidal Acceleration and Superradiance in the ``Newtonian'' Approximation
	References
3 Superradiance in Black-Hole Physics
	3.1 Action, Equations of Motion, and Black-Hole Spacetimes
		3.1.1 Static, Charged Backgrounds
		3.1.2 Spinning, Neutral Backgrounds
		3.1.3 Geodesics and Frame Dragging in the Kerr Geometry
		3.1.4 The Ergoregion
		3.1.5 Intermezzo: Stationary and Axisymmetric Black Holes Have an Ergoregion
	3.2 Area Theorem Implies Superradiance
	3.3 Energy Extraction from Black Holes: The Penrose Process
		3.3.1 The Original Penrose Process
		3.3.2 The Newtonian Carousel Analogy
		3.3.3 Penrose's Process: Energy Limits
		3.3.4 The Penrose Process in Generic Spacetimes
		3.3.5 The Collisional Penrose Process: Ultra-High-Energy Debris
	3.4 The ABC of Black-Hole Superradiance
	3.5 Horizons are Unrelated to Superradiance
	3.6 Superradiance from Charged Static Black Holes
		3.6.1 Linearized Analysis: Amplification Factors
		3.6.2 Backreaction on the Geometry: Mass and Charge Loss
	3.7 Superradiance from Rotating Black Holes
		3.7.1 Bosonic and Fermionic Fields in the Kerr Geometry
		3.7.2 Energy Fluxes of Bosonic Fields at Infinity and on the Horizon
		3.7.3 Amplification Factors
		3.7.4 Dirac Fields on the Kerr Geometry
		3.7.5 Linearized Analysis: Analytic vs Numerics
		3.7.6 Scattering of Plane Waves
			Scalar Waves
			Electromagnetic Waves
			Gravitational Waves
			Acoustic Geometries
		3.7.7 Nonlinear Superradiant Scattering from Spinning Black Holes
	3.8 Superradiance from Stars
	3.9 Superradiance in Analogue Black-Hole Geometries
	3.10 The Experimental Observation of Superradiance
	3.11 Superradiance in Higher-Dimensional Spacetimes
	3.12 Superradiance in Nonasymptotically Flat Spacetimes
	3.13 Superradiance Beyond General Relativity
		Superradiance of Black Holes Surrounded by Matter in Scalar–Tensor Theories
	3.14 Microscopic Description of Superradiance and the Kerr/CFT Duality
	3.15 Boosted Black Holes: Energy Extraction Without Ergoregions
	3.16 Boosted Black Strings: Ergoregions Without Superradiance
	3.17 Open Issues
	References
4 Black Holes and Superradiant Instabilities
	4.1 No Black Hole Fission Processes
	4.2 Spinning Black Holes in Confining Geometries Are Unstable
	4.3 Superradiant Instabilities: Time-Domain Evolutions Versus an Eigenvalue Search
	4.4 Black Holes Enclosed in a Mirror
		4.4.1 Rotating Black-Hole Bombs
		4.4.2 Charged Black-Hole Bombs
	4.5 Black Holes in AdS Backgrounds
		4.5.1 Instability of Small Kerr-AdS Black Holes and New Black-Hole Solutions
		4.5.2 Instabilities of Charged AdS Black Holes: Superradiance, Spontaneous Symmetry Breaking, and Holographic Superconductors
	4.6 Massive Bosonic Fields
		4.6.1 The Zoo of Light Bosonic Fields in Extensions of the Standard Model
		4.6.2 Massive Scalar Fields
		4.6.3 Massive Vector Fields
		4.6.4 Massive Tensor Fields
		4.6.5 A Unified Picture of the Linearized Superradiant Instability of Massive Bosonic Fields
		4.6.6 Superradiant Instabilities of Massive Bosonic Fields: Nonlinear Evolutions
		4.6.7 Anatomy of Scalar Clouds Around Spinning Black Holes
		4.6.8 Circumventing the No-Hair Theorem with Superradiance
	4.7 Tidal Effects Induced by a Companion Star or Black Hole
	4.8 Superradiant Instabilities and Other Absorption Channels
		4.8.1 Axionic Couplings and Bursts of Light
		4.8.2 Nonlinear Self-Interactions and ``bosenova'' Explosions
		4.8.3 Plasma-Triggered Superradiant Instabilities
	4.9 Superradiance in Scalar-Tensor Theories
		4.9.1 Spontaneous Superradiant Instabilities in Scalar-Tensor Theories
		4.9.2 Floating Orbits
	4.10 Superradiant Instability from Stars
	4.11 Black Holes Immersed in a Magnetic Field
	4.12 Superradiant Instability of Black Holes Surrounded by Conducting Rings
	4.13 Kaluza–Klein Mass: superradiant Instabilities in Higher Dimensions
	4.14 Ergoregion Instability
		4.14.1 Ergoregion Instability of Rotating Objects: A Consistent Approach
		4.14.2 Ergoregion Instability and Long-Lived Modes
		4.14.3 Ergoregion Instability in Fluids
		4.14.4 Ergoregion Instability and Hawking Radiation
	4.15 Black-Hole Lasers and Superluminal Corrections to Hawking Radiation
	4.16 Black Holes in Lorentz-Violating Theories: Nonlinear Instabilities
	4.17 Open Issues
	References
5 Black Hole Superradiance in Astrophysics
	5.1 Evolution of Superradiant Instabilities for Astrophysical BHs
		5.1.1 A Simplified Model Including Gas Accretion
		5.1.2 Superradiant Instabilities Imply No Highly Spinning Black Holes
		5.1.3 Summary of the Evolution of Superradiant Instabilities
	5.2 Astrophysical Black Holes as Particle Detectors
		5.2.1 Bounds on the Mass of Bosonic Fields from Gaps in the Regge Plane
		5.2.2 Gravitational-Wave Signatures
		5.2.3 Electromagnetic Signatures
		5.2.4 Signatures in Binary Systems
	5.3 Bounds on Ultralight Particles from Pulsar Timing
	5.4 Summary of the Bounds on Ultralight Bosons from Superradiant Instabilities
	5.5 Plasma Interactions
	5.6 Intrinsic Limits on Magnetic Fields
	5.7 Phenomenology of the Ergoregion Instability
		5.7.1 Ergoregion Instability of Ultracompact Stars
		5.7.2 Testing the Black-Hole Paradigm: Extreme Compact Objects
	5.8 Superradiance and Relativistic Jets
		5.8.1 Blandford–Znajek Process
		5.8.2 Blandford–Znajek Process and the Membrane Paradigm
	5.9 Superradiance, CFS Instability, and r-Modes of Spinning Stars
	5.10 Open Issues
	References
6 Conclusions and Outlook
A List of Publicly Available Codes
B Analytic Computation of the Amplification Coefficients
C Angular Momentum and Energy
	C.1 Energy and Angular Momentum Fluxes at the Horizon
D Electromagnetic Fluctuations Around a Rotating Black Hole Enclosed in a Mirror
E Hartle–Thorne Formalism for Slowly Rotating Spacetimes and Perturbations
	E.1 Background
	E.2 Perturbations of a Slowly Rotating Object
		E.2.1 Scalar Perturbations of a Slowly Rotating Star
F WKB Analysis of Long-Lived and Unstable Modes of Ultracompact Objects
	References