arXiv subjects

Mauricio Richartz

Publications and source records attributed to Mauricio Richartz.

9 recordsLinked to original sources

arXiv2025

Wind-Finslerian structure of black holes

Recently, there has been an increasing interest in the Finslerian interpretation of null geodesics in the exterior regions of stationary black holes, particularly through the Zermelo navigation problem and the Randers metric. In this work, we show that recent mathematical advancements in wind-Finslerian structures, which involve the critical and strong Zermelo navigation problems and their connections to Kropina and Lorentz-Finsler metrics, enable the extension of the Finslerian framework to encompass horizons and their interior regions of black holes. The Finslerian indicatrix, a key element of this framework, serves as an effective tool for identifying frame-dragging effects and the location of horizons and ergosurfaces. We illustrate our results with explicit physical examples, focusing on spherically symmetric black holes, Kerr black holes, and analog models of gravity. Our findings provide new insights into the ``river model'' of black holes, offering enhanced visual representations of null geodesics on the ergosurfaces, horizons, and within their interior regions.

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arXiv2019

Follow-up signals from superradiant instabilities of black hole merger remnants

Superradiant instabilities can trigger the formation of bosonic clouds around rotating black holes. If the bosonic field growth is sufficiently fast, these clouds could form shortly after a binary black hole merger. Such clouds are continuous sources of gravitational waves whose detection (or lack thereof) can probe the existence of ultralight bosons (such as axion-like particles) and their properties. Motivated by the binary black hole mergers seen by Advanced LIGO so far, we investigate in detail the parameter space that can be probed with continuous gravitational wave signals from ultralight scalar field clouds around black hole merger remnants with particular focus on future ground-based detectors (A+, Voyager and Cosmic Explorer). We also study the impact that the confusion noise from a putative stochastic gravitational-wave background from unresolved sources would have on such searches and we estimate, under different astrophysical priors, the number of binary black-hole merger events that could lead to an observable post-merger signal. Under our most optimistic assumptions, Cosmic Explorer could detect dozens of post-merger signals.

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arXiv2018

Black hole binaries: ergoregions, photon surfaces, wave scattering, and quasinormal modes

Closed photon orbits around isolated black holes are related to important aspects of black hole physics, such as strong lensing, absorption cross section of null particles and the way that black holes relax through quasinormal ringing. When two black holes are present -- such as during the inspiral and merger events of interest for gravitational-wave detectors -- the concept of closed photon orbits still exists, but its properties are basically unknown. With these applications in mind, we study here the closed photon orbits of two different static black hole binaries. The first one is the Majumdar-Papapetrou geometry describing two extremal, charged black holes in equilibrium, while the second one is the double sink solution of fluid dynamics, which describes (in a curved-spacetime language) two "dumb" holes. For the latter solution, we also characterize its dynamical response to external perturbations, and study how it relates to the photon orbits. In addition, we compute the ergoregion of such spacetime and show that it does not coincide with the event horizon.

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arXiv2018

Black hole quasibound states from a draining bathtub vortex flow

Quasinormal modes are a set of damped resonances that describe how an excited open system is driven back to equilibrium. In gravitational physics these modes characterise the ringdown of a perturbed black hole, e.g. following a binary black hole merger. A careful analysis of the ringdown spectrum reveals the properties of the black hole, such as its angular momentum and mass. In more complex gravitational systems the spectrum might depend on more parameters, and hence allows us to search for new physics. In this letter we present a hydrodynamic analogue of a rotating black hole, that illustrates how the presence of extra structure affects the quasinormal mode spectrum. The analogy is obtained by considering wave scattering on a draining bathtub vortex flow. We show that due to vorticity of the background flow, the resulting field theory corresponds to a scalar field on an effective curved spacetime which acquires a local mass in the vortex core. The obtained quasinormal mode spectrum exhibits long-lived trapped modes, commonly known as quasibound states. Our findings can be tested in future experiments, building up on recent successful implementations of analogue rotating black holes.

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arXiv2016

Detecting Rotational Superradiance in Fluid Laboratories

Rotational superradiance was predicted theoretically decades ago, and is chiefly responsible for a number of important effects and phenomenology in black-hole physics. However, rotational superradiance has never been observed experimentally. Here, with the aim of probing superradiance in the lab, we investigate the behavior of sound and surface waves in fluids resting in a circular basin at the center of which a rotating cylinder is placed. We show that with a suitable choice for the material of the cylinder, surface and sound waves are amplified. Two types of instabilities are studied: one sets in whenever superradiant modes are confined near the rotating cylinder and the other, which does not rely on confinement, corresponds to a local excitation of the cylinder. Our findings are experimentally testable in existing fluid laboratories and, hence, offer experimental exploration and comparison of dynamical instabilities arising from rapidly rotating boundary layers in astrophysical as well as in fluid dynamical systems.

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arXiv2016

Observation of superradiance in a vortex flow

Wave scattering phenomena are ubiquitous to almost all Sciences, from Biology to Physics. When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. Since the scatterer absorbs part of the incident energy, the reflected wave carries less energy than the incident one. However, if the obstacle is rotating, this process can be reversed and waves can be amplified, extracting energy from the scatterer. Even though this phenomenon, known as superradiance, has been thoroughly analysed in several theoretical scenarios (from eletromagnetic radiation scattering on a rotating cylinder to gravitational waves incident upon a rotating black hole), it has never been observed. Here we describe in detail the first laboratory detection of superradiance. We observed that plane waves propagating on the surface of water are amplified after being scattered by a draining vortex. The maximum amplification measured in the experiment was 20%, obtained for 3.70 Hz waves, in a 6.25 cm deep fluid. Our results are consistent with superradiant scattering caused by rapid rotation. In particular, a draining fluid can transfer part of its rotational energy to incident low-frequency waves. Our experimental findings will shed new light on Black Hole Physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole. We believe, especially in view of the recent observations of gravitational waves, that our results will motivate further research (both theoretical and experimental) on the observation of superradiance of gravitational waves.

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arXiv2015

Rotating black holes in a draining bathtub: superradiant scattering of gravity waves

In a draining rotating fluid flow background, surface perturbations behave as a scalar field on a rotating effective black hole spacetime. We propose a new model for the background flow which takes into account the varying depth of the water. Numerical integration of the associated Klein-Gordon equation using accessible experimental parameters shows that gravity waves in an appropriate frequency range are amplified through the mechanism of superradiance. Our numerical results suggest that the observation of this phenomenon in a common fluid mechanical system is within experimental reach. Unlike the case of wave scattering around Kerr black holes, which depends only on one dimensionless background parameter (the ratio $a/M$ between the specific angular momentum and the mass of the black hole), our system depends on two dimensionless background parameters, namely the normalized angular velocity and surface gravity at the effective black hole horizon.

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arXiv2009

Generalised superradiant scattering

We analyse the necessary and sufficient conditions for the occurrence of superradiance. Starting with a wave equation we examine the possibility of superradiance in terms of an effective potential and boundary conditions. In particular, we show that the existence of an ergoregion is not sufficient; an appropriate boundary condition, e.g. only ingoing group velocity waves at an event horizon, is also crucial. After applying our scheme to the standard examples of superradiance, we show that analogue models of gravity without an event horizon do not necessarily exhibit superradiance. Particularly, we show that the superradiant phenomenon is absent in purely rotating inviscid fluids with vorticity. We argue that there should be a catalogue of superradiant systems that can be found by focusing on the necessary and sufficient conditions outlined below.

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arXiv2008

Overspinning a nearly extreme black hole and the Weak Cosmic Censorship conjecture

We revisit here the recent proposal for overspinning a nearly extreme black hole by means of a quantum tunneling process. We show that electrically neutral massless fermions evade possible back reactions effects related to superradiance, confirming the view that it would be indeed possible to form a naked singularity due to quantum effects.

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