Aspects of black holes in alternative theories of gravity

Black holes are among the simplest objects in the Universe, yet they are also the perfect probes of the strong-field regime of gravity, where alternative theories of gravity are expected to yield new predictions for the physics of compact astrophysical objects. In the era of gravitational wave astronomy, some of the predictions can finally be put to a test. We start by investigating the Horndeski class of scalar-tensor theories. It incorporates many alternative theories of gravity that feature a scalar degree of freedom in addition to the metric, either explicitly or through a specific representation. Such scalars have been employed to explain the puzzles of cosmology and they often arise in the effective field theories of more fundamental physics. With the additional field comes the possibility of black-hole solutions that differ from those of general relativity. We investigate the dynamics of a theory that admits only solutions of this kind and find evidence suggesting that such black holes form through gravitational collapse. Moreover, we study the causal structure of any solution with a non-trivial scalar, as the equations permit superluminal propagation for scalar perturbations. We find that the notion of a horizon that bounds all field excitations persists. This is not the case in theories with a preferred frame where black holes have multiple nested horizons. We address the claim that superluminal modes, the hallmark of Lorentz-violating theories, allow for processes that violate the generalized second law of black hole thermodynamics. We derive the necessary conditions for such a violation and find those unlikely to arise from a theory where gravity is attractive. Finally, we sketch a path to advance the research programme featured in this thesis in view of the recent progress in gravitational wave astronomy

Dynamical scalar hair formation around a Schwarzschild black hole

Scalar fields coupled to the Gauss-Bonnet invariant evade the known no-hair theorems and have nontrivial configurations around black holes. We focus on a scalar field that couples linearly to the Gauss-Bonnet invariant and hence exhibits shift symmetry. We study its dynamical evolution and the formation of scalar hair in a Schwarzschild background. We show that the evolution eventually settles to the known static hairy solutions in the appropriate limit

Dynamical obstruction to perpetual motion from Lorentz-violating black holes

Black holes in Lorentz-violating theories have been claimed to violate the second law of thermodynamics by perpetual motion energy extraction. We revisit this question for a Penrose splitting process in a spherically symmetric setting with two species of particles that move on radial geodesics that extend to infinity. We show that energy extraction by this process cannot happen in any theory in which gravity is attractive, in the sense of a geometric inequality that we describe. This inequality is satisfied by all known Einstein-æther and Hořava black hole solutions

New horizons for fundamental physics with LISA

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of gravitational waves can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas

Causal structure of black holes in shift-symmetric Horndeski theories

In theories with derivative (self-)interactions, the propagation of perturbations on nontrivial field configurations is determined by effective metrics. Generalized scalar-tensor theories belong in this class and this implies that the matter fields and gravitational perturbations do not necessarily experience the same causal structure. Motivated by this, we explore the causal structure of black holes as perceived by scalar fields in the Horndeski class. We consider linearized perturbations on a fixed background metric that describes a generic black hole. The effective metric that determines the propagation of these perturbations does not generally coincide with the background metric (to which matter fields couple minimally). Assuming that the metric and the scalar respect stationarity and that the surface gravity of the horizon is constant, we prove that Killing horizons of the background metric are always Killing horizons of the effective metric as well. Hence, scalar perturbations cannot escape the region that matter fields perceive as the interior of the black hole. This result does not depend on asymptotics but only on local considerations and does not make any reference to no-hair theorems. We then demonstrate that, when one relaxes the stationarity assumption for the scalar, solutions where the horizons of the effective and the background metrics do not match can be found in the decoupling limit

Black hole hair formation in shift-symmetric generalised scalar-tensor gravity

A linear coupling between a scalar field and the Gauss–Bonnet invariant is the only known interaction term between a scalar and the metric that: respects shift symmetry; does not lead to higher order equations; inevitably introduces black hole hair in asymptotically flat, 4-dimensional spacetimes. Here we focus on the simplest theory that includes such a term and we explore the dynamical formation of scalar hair. In particular, we work in the decoupling limit that neglects the backreaction of the scalar onto the metric and evolve the scalar configuration numerically in the background of a Schwarzschild black hole and a collapsing dust star described by the Oppenheimer–Snyder solution. For all types of initial data that we consider, the scalar relaxes at late times to the known, static, analytic configuration that is associated with a hairy, spherically symmetric black hole. This suggests that the corresponding black hole solutions are indeed endpoints of collapse

Restaurant outbreak of Legionnaires' disease associated with a decorative fountain: an environmental and case-control study

BACKGROUND: From June to November 2005, 18 cases of community-acquired Legionnaires' disease (LD) were reported in Rapid City South Dakota. We conducted epidemiologic and environmental investigations to identify the source of the outbreak. METHODS: We conducted a case-control study that included the first 13 cases and 52 controls randomly selected from emergency department records and matched on underlying illness. We collected information about activities of case-patients and controls during the 14 days before symptom onset. Environmental samples (n = 291) were cultured for Legionella. Clinical and environmental isolates were compared using monoclonal antibody subtyping and sequence based typing (SBT). RESULTS: Case-patients were significantly more likely than controls to have passed through several city areas that contained or were adjacent to areas with cooling towers positive for Legionella. Six of 11 case-patients (matched odds ratio (mOR) 32.7, 95% CI 4.7-infinity) reported eating in Restaurant A versus 0 controls. Legionella pneumophila serogroup 1 was isolated from four clinical specimens: 3 were Benidorm type strains and 1 was a Denver type strain. Legionella were identified from several environmental sites including 24 (56%) of 43 cooling towers tested, but only one site, a small decorative fountain in Restaurant A, contained Benidorm, the outbreak strain. Clinical and environmental Benidorm isolates had identical SBT patterns. CONCLUSION: This is the first time that small fountain without obvious aerosol-generating capability has been implicated as the source of a LD outbreak. Removal of the fountain halted transmission

Black holes, gravitational waves and fundamental physics: a roadmap

The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'

Aspects of black holes in alternative theories of gravity

Black holes are among the simplest objects in the Universe, yet they are also the perfect probes of the strong-field regime of gravity, where alternative theories of gravity are expected to yield new predictions for the physics of compact astrophysical objects. In the era of gravitational wave astronomy, some of the predictions can finally be put to a test. We start by investigating the Horndeski class of scalar-tensor theories. It incorporates many alternative theories of gravity that feature a scalar degree of freedom in addition to the metric, either explicitly or through a specific representation. Such scalars have been employed to explain the puzzles of cosmology and they often arise in the effective field theories of more fundamental physics. With the additional field comes the possibility of black-hole solutions that differ from those of general relativity. We investigate the dynamics of a theory that admits only solutions of this kind and find evidence suggesting that such black holes form through gravitational collapse. Moreover, we study the causal structure of any solution with a non-trivial scalar, as the equations permit superluminal propagation for scalar perturbations. We find that the notion of a horizon that bounds all field excitations persists. This is not the case in theories with a preferred frame where black holes have multiple nested horizons. We address the claim that superluminal modes, the hallmark of Lorentz-violating theories, allow for processes that violate the generalized second law of black hole thermodynamics. We derive the necessary conditions for such a violation and find those unlikely to arise from a theory where gravity is attractive. Finally, we sketch a path to advance the research programme featured in this thesis in view of the recent progress in gravitational wave astronomy