Axionic instabilities and new black hole solutions

The coupling between scalar and vector fields has a long and interesting history. Axions are one key possibility to solve the strong CP problem and axion-like particles could be one solution to the dark matter puzzle. Given the nature of the coupling, and the universality of free fall, nontrivial important effects are expected in regions where gravity is strong. Here, we show that i. A background EM field induces an axionic instability in flat space, for large enough electric fields. Conversely, a homogeneous harmonic axion field induces an instability in the Maxwell sector. When carried over to curved spacetime, this phenomena translates into generic instabilities of charged black holes (BHs). ii. In the presence of charge, BH uniqueness results are lost. We find solutions which are small deformations of the Kerr-Newman geometry and hairy stationary solutions without angular momentum, which are `dragged' by the axion. Axion fields must exist around spinning BHs if these are immersed in external magnetic fields. The axion profile can be obtained perturbatively from the electro-vacuum solution derived by Wald. iii. Ultralight axions trigger superradiant instabilities of spinning BHs and form an axionic cloud in the exterior geometry. The superradiant growth can be interrupted or suppressed through axionic or scalar couplings to EM. These couplings lead to periodic bursts of light, which occur throughout the history of energy extraction from the BH. We provide numerical and simple analytical estimates for the rates of these processes. iv. Finally, we discuss how plasma effects can affect the evolution of superradiant instabilities.Comment: 28 pages, RevTeX4. v2: overall improvements, typos corrected; version to appear in Physical Review

Towards numerical relativity in scalar Gauss-Bonnet gravity: 3+1 decomposition beyond the small-coupling limit

Scalar Gauss-Bonnet gravity is the only theory with quadratic curvature corrections to general relativity whose field equations are of second differential order. This theory allows for nonperturbative dynamical corrections and is therefore one of the most compelling case studies for beyond-general relativity effects in the strong-curvature regime. However, having second-order field equations is not a guarantee for a healthy time evolution in generic configurations. As a first step towards evolving black-hole binaries in this theory, we here derive the 3+1 decomposition of the field equations for any (not necessarily small) coupling constant and we discuss potential challenges of its implementation.Comment: 8 page

Evolution of black hole shadows from superradiance

Black holes have turned into cosmic laboratories to search for ultra-light scalars by virtue of the superradiant instability. In this paper we present a detailed study of the impact of the superradiant evolution on the black hole shadow and investigate the exciting possibility to explore it with future observations of Very Long Baseline Interferometry. We simulated the superradiant evolution numerically, in the adiabatic regime, and derived analytic approximations modelling the process. Driven by superradiance, we evolve the black hole shadow diameter and (i) find that it can change by a few $\mu$as, just below the current resolution of the Event Horizon Telescope, albeit on timescales that are longer than realistic observation times; (ii) show that the shadow diameter can either shrink or grow; and (iii) explore in detail how the shadow's end state is determined by the initial parameters and coupling.Comment: 22 pages, 16 figures. Updated to match published versio

Impact of multiple modes on the black-hole superradiant instability

Ultralight bosonic fields in the mass range $\sim (10^{-20}-10^{-11})\,{\rm eV}$ can trigger a superradiant instability that extracts energy and angular momentum from an astrophysical black hole with mass $M\sim(5,10^{10})M_\odot$, forming a nonspherical, rotating condensate around it. So far, most studies of the evolution and end-state of the instability have been limited to initial data containing only the fastest growing superradiant mode. By studying the evolution of multimode data in a quasi-adiabatic approximation, we show that the dynamics is much richer and depend strongly on the energy of the seed, on the relative amplitude between modes, and on the gravitational coupling. If the seed energy is a few percent of the black-hole mass, a black hole surrounded by a mixture of superradiant and nonsuperradiant modes with comparable amplitudes might not undergo a superradiant unstable phase, depending on the value of the boson mass. If the seed energy is smaller, as in the case of an instability triggered by quantum fluctuations, the effect of nonsuperradiant modes is negligible. We discuss the implications of these findings for current constraints on ultralight fields with electromagnetic and gravitational-wave observations.Comment: 21 pages, 12 figures; matches version accepted in PR

Black holes in a box: towards the numerical evolution of black holes in AdS

The evolution of black holes in "confining boxes" is interesting for a number of reasons, particularly because it mimics the global structure of Anti-de Sitter geometries. These are non-globally hyperbolic space-times and the Cauchy problem may only be well defined if the initial data is supplemented by boundary conditions at the time-like conformal boundary. Here, we explore the active role that boundary conditions play in the evolution of a bulk black hole system, by imprisoning a black hole binary in a box with mirror-like boundary conditions. We are able to follow the post-merger dynamics for up to two reflections off the boundary of the gravitational radiation produced in the merger. We estimate that about 15% of the radiation energy is absorbed by the black hole per interaction, whereas transfer of angular momentum from the radiation to the black hole is only observed in the first interaction. We discuss the possible role of superradiant scattering for this result. Unlike the studies with outgoing boundary conditions, both the Newman-Penrose scalars \Psi_4 and \Psi_0 are non-trivial in our setup, and we show that the numerical data verifies the expected relations between them.Comment: REvTex4, 17 pages, 12 Figs. v2: Minor improvements. Published version. Animation of a black hole binary in a box can be found at http://blackholes.ist.utl.pt