arXiv subjects

Miguel Zilhão

Publications and source records attributed to Miguel Zilhão.

At least 19 recordsLinked to original sources

arXiv2026

Turbulent gravitational wakes

We construct turbulent gravitational wakes in asymptotically AdS$_4$ spacetimes by solving the full non-linear Einstein equations in the characteristic formulation. Specifically, we show that a localized boundary deformation acting on a boosted black brane generates a wake whose boundary dynamics exhibit the formation of coherent vortical structures that are advected downstream by the background flow. Analysing a wake propagating along the x direction, we show that it displays the characteristic self-similar scaling of classical turbulent wakes. We then identify the geometric imprint of the wake on the horizon area density, the traceless horizon extrinsic curvature, and bulk metric perturbations, finding coherent structures that extend from the asymptotic boundary to the horizon. These results provide the first study of turbulent gravitational wakes in four-dimensional asymptotically AdS spacetimes.

hep-th↗

arXiv2026

Spontaneous spherical symmetry breaking of black holes with resonant hair

Black holes with resonant hair are static, spherical, electrically charged solutions of the Einstein-Maxwell-(gauged-)scalar system. Scalar self-interactions are mandatory for their existence. Initial dynamical studies restricted to spherical symmetry suggested stability; more recently, fully non-spherical dynamical studies revealed instabilities, at least for a particular class of self-interactions. Here, we provide a more detailed study of this instability together with a different decay channel, depending on the chosen solutions. Moreover, considering a second model, we provide evidence that the instabilities may be generic for different classes of self-interactions. We conclude these solutions are dynamically unstable and split into a bosonic lump and a bald black hole (via fission) or implode to the latter (via absorption). In both cases, the non-spherical dynamics seems to be key.

gr-qc↗

arXiv2026

Electromagnetic duality degeneracy in dynamical black hole mergers

Electromagnetic duality is a symmetry of the source-free Einstein-Maxwell equations that rotates electric and magnetic fields while leaving the stress-energy tensor invariant. We present the first fully nonlinear realization of this symmetry in dynamical strong-gravity regimes by performing numerical relativity simulations of charged black hole mergers across a continuous duality family. Starting from electrically charged binaries, we generate dyonic and magnetically charged configurations via duality rotations and evolve them within a common numerical framework. We find that all dual configurations exhibit identical spacetime dynamics, while the emitted electromagnetic radiation is related by a rotation of its polarization equal to the duality angle. Our results demonstrate a degeneracy of gravitational observables under electromagnetic duality and provide a concrete mapping between dual configurations at the level of radiation, establishing electromagnetic duality as an organizing principle for dynamical Einstein-Maxwell solutions.

gr-qc↗

arXiv2026

Splitting the Gravitational Atom: Instabilities of Black Holes with Synchronized or Resonant Hair

Black holes (BHs) with synchronized bosonic hair challenge the Kerr paradigm, linking superradiance from ultralight fields -- creating gravitational atoms -- to bosonic stars across parameter space. In the ''very hairy'' regime, where a small horizon lies inside a bosonic star containing most of the energy, they deviate sharply from Kerr, but their dynamics remain unexplored. We show that for such solutions the horizon gets naturally ejected from the center of its scalar environment, and observe a similar dynamics in a cousin model of BHs with resonant scalar hair, albeit with a different fate. This dynamical splitting is likely to be generic for sufficiently hairy BHs in the broader class of models with synchronized or resonant hair, but possible exceptions may exist.

gr-qc↗

arXiv2026

Dancing in the dark: probing Dark Matter through the dynamics of eccentric binary pulsars

We investigate the dynamics of eccentric binary pulsars embedded in dark matter environments. While previous studies have primarily focused on circular orbits in collisionless dark matter halos, we extend this framework to eccentric systems and explore their interaction with ultralight scalar fields. Adopting a perturbative approach, we compute the modifications to the orbital period induced by dark matter-driven dynamical friction. Our results show that orbital eccentricity amplifies the imprints of non-vacuum environments on binary dynamics, underscoring the potential of such systems as sensitive probes for dark matter signatures.

gr-qc↗

arXiv2026

Stability and collisions of excited spherical boson stars: glimpses of chains and rings

Scalar, spherically symmetric, radially excited boson stars were previously shown to be stabilized, against spherical dynamics, by sufficiently strong self-interactions. Here, we further test their stability now in a full 3+1D evolution. We show that the stable stars in the former case become afflicted by a non-spherical instability. Then, we perform head-on collisions of both (stable) fundamental and (sufficiently long-lived) excited boson stars. Depending on the stars chosen, either a black hole or a bosonic remnant are possible. In particular, collisions of excited stars result in a bosonic bound state which resembles a dynamical superposition of chains and rings, akin to the ones found as equilibrium solutions in Liang:2025myf. These evolutions emphasize a key difference concerning the dynamical robustness of fundamental vs. excited spherical boson stars, when generic (beyond spherical) dynamics is considered.

gr-qc↗

arXiv2025

Spinodal Gravitational Waves

We uncover a new gravitational-wave production mechanism in cosmological, first-order, thermal phase transitions. These are usually assumed to proceed via the nucleation of bubbles of the stable phase inside the metastable phase. However, if the nucleation rate is sufficiently suppressed, then the Universe may supercool all the way down the metastable branch and enter the spinodal region. In this case the transition proceeds via the exponential growth of unstable modes and the subsequent formation, merging and relaxation of phase domains. We use holography to follow the real-time evolution of this process in a strongly coupled, four-dimensional gauge theory. The resulting gravitational wave spectrum differs qualitatively from that in transitions mediated by bubble nucleation. We discuss the possibility that the spinodal dynamics may be preceded by a period of thermal inflation.

hep-th↗

arXiv2025

Fluctuations and Correlations in Causal Set Theory

We study the statistical fluctuations (such as the variance) of causal set quantities, with particular focus on the causal set action. To facilitate calculating such fluctuations, we develop tools to account for correlations between causal intervals with different cardinalities. We present a convenient decomposition of the fluctuations of the causal set action into contributions that depend on different kinds of correlations. This decomposition can be used in causal sets approximated by any spacetime manifold $\mathcal M$. Our work paves the way for investigating a number of interesting discreteness effects, such as certain aspects of the Everpresent $Λ$ cosmological model.

gr-qc↗

arXiv2024

Bubble dynamics in a QCD-like phase diagram

A line of first-order phase transitions is conjectured in the phase diagram of Quantum Chromodynamics at non-zero baryon density. If this is the case, numerical simulations of neutron star mergers suggest that various regions of the stars may cross this line multiple times. This results in the nucleation of bubbles of the preferred phase, which subsequently expand and collide. The resulting gravitational wave spectrum is highly sensitively to the velocity of the bubble walls. We use holography to perform the first microscopic simulation of bubble dynamics in a theory that qualitatively mirrors the expected phase diagram of Quantum Chromodynamics. We determine the wall velocity in the metastable regions and we compare it to theoretical estimates. We discuss implications for gravitational wave production.

hep-th↗

arXiv2024

Full 3D nonlinear dynamics of charged and magnetized boson stars

Gauged boson stars are exotic compact objects that can potentially mimic black holes or magnetized neutron stars in both their gravitational and electromagnetic signatures, offering a compelling new description or even an alternative explanation for various multimessenger phenomena. As a crucial step toward establishing boson stars as viable multimessenger sources, we perform 3D numerical simulations of the fully nonlinear Einstein-Maxwell-Klein-Gordon system, focusing on both spherical and axisymmetric boson star configurations that vary in their electromagnetic coupling between the neutral case up to values close to the critical case, and so their magnetic field content. For spherical configurations, we consistently find stable solutions. In contrast, for axially symmetric, electrically neutral, magnetized configurations, the dynamics are highly sensitive to the electromagnetic coupling. Configurations with stronger coupling develop a one-armed mode instability, which leads to collapse into black holes. Configurations with weaker coupling undergo a two-stage process: an initial bar-mode instability that triggers a one-armed spiral deformation. This eventually also results in black hole formation, accompanied by emissions of both gravitational and electromagnetic radiation. A similar instability and two-stage pattern is observed in all charged rotating boson stars analyzed. However, all of these configurations become stable when self-interactions are introduced.

gr-qc↗

arXiv2024

The pseudospectra of black holes in AdS

We study the stability of quasinormal modes (QNMs) in electrically charged black brane spacetimes that asymptote to AdS by means of the pseudospectrum. Methodologically, we adopt ingoing Eddington-Finkelstein coordinates to cast QNMs in terms of a generalised eigenvalue problem involving a non-selfadjoint operator; this simplifies the computation significantly in comparison with previous results in the literature. Our analysis reveals spectral instability for (neutral) scalar as well as gravitoelectric perturbations. This indicates that the equilibration process of perturbed black branes is sensitive to external perturbations. Particular attention is given on the hydrodynamic modes, which are found to be the least unstable. In contrast with computations in hyperboloidal coordinates, we find that the pseudospectral contour lines cross to the upper half plane. This indicates the existence of pseudo-resonances as well as the possibility of transient instabilities. We also investigate the asymptotic structure of pseudospectral contour levels and we find remarkable universality across all sectors, persistent in the extremal limit.

gr-qc↗

arXiv2024

Self-interactions can (also) destabilize bosonic stars

We study the dynamical stability of Proca-Higgs stars, in spherical symmetry. These are solutions of the Einstein-Proca-Higgs model, which features a Higgs-like field coupled to a Proca field, both of which minimally coupled to the gravitational field. The corresponding stars can be regarded as Proca stars with self-interactions, while avoiding the hyperbolicity issues of self-interacting Einstein-Proca models. We report that these configurations are stable near the Proca limit in the candidate stable branches, but exhibit instabilities in certain parts of the parameter space, even in the candidate stable branches, regaining their stability for very strong self-interactions. This shows that for these models, unlike various examples of scalar boson stars, self-interactions can deteriorate, rather than improve, the dynamical robustness of bosonic stars.

gr-qc↗

arXiv2023

Numerical convergence of model Cauchy-characteristic extraction and matching

Gravitational waves provide a powerful enhancement to our understanding of fundamental physics. To make the most of their detection we need to accurately model the entire process of their emission and propagation toward interferometers. Cauchy-characteristic extraction and matching are methods to compute gravitational waves at null infinity, a mathematical idealization of detector location, from numerical relativity simulations. Both methods can in principle contribute to modeling by providing highly accurate gravitational waveforms. An underappreciated subtlety in realizing this potential is posed by the (mere) weak hyperbolicity of the particular PDE systems solved in the characteristic formulation of the Einstein field equations. This shortcoming results from the popular choice of Bondi-like coordinates. So motivated, we construct toy models that capture that PDE structure and study Cauchy-characteristic extraction and matching with them. Where possible we provide energy estimates for their solutions and perform careful numerical norm convergence tests to demonstrate the effect of weak hyperbolicity on Cauchy-characteristic extraction and matching. Our findings strongly indicate that, as currently formulated, Cauchy-characteristic matching for the Einstein field equations would provide solutions that are, at best, convergent at an order lower than expected for the numerical method, and may be unstable. In contrast, under certain conditions, the extraction method can provide properly convergent solutions. Establishing however that these conditions hold for the aforementioned characteristic formulations is still an open problem.

gr-qc↗

arXiv2023

Piercing of a solitonic boson star by a black hole

Recently, the piercing of a mini boson star by a black hole was studied, with tidal capture and the discovery of a "gravitational atom" being reported ( arXiv:2206.00021 [gr-qc] ). Building on this research, we extend the study by including a hexic solitonic potential and explore the piercing of a solitonic boson star by a black hole. Notably, the solitonic boson star can reach higher compactness, which one might expect could alter the dynamics in this context. Our findings suggest that even when the black hole's size approaches the test particle limit, the solitonic boson star is easily captured by the black hole due to an extreme tidal capture process. Regardless of the black hole initial mass and velocity, our results indicate that over 85% of the boson star material is accreted. Thus, the self-interaction does not alter the qualitative behavior of the system.

gr-qc↗

arXiv2023

Self-interacting dipolar boson stars and their dynamics

We construct and dynamically evolve dipolar, self-interacting scalar boson stars in a model with sextic (+ quartic) self-interactions. The domain of existence of such dipolar $Q$-stars has a similar structure to that of the fundamental monopolar stars of the same model. For the latter it is structured in a Newtonian plus a relativistic branch, wherein perturbatively stable solutions exist, connected by a middle unstable branch. Our evolutions support similar dynamical properties of the dipolar $Q$-stars that: 1) in the Newtonian and relativistic branches are dynamically robust over time scales longer than those for which dipolar stars without self-interactions are seen to decay; 2) in the middle branch migrate to either the Newtonian or the relativistic branch; 3) beyond the relativistic branch decay to black holes. Overall, these results strengthen the observation, seen in other contexts, that self-interactions can mitigate dynamical instabilities of scalar boson star models.

gr-qc↗

arXiv2023

Stability and physical properties of spherical excited scalar boson stars

We study the time evolution of spherical, excited -- with $n$ radial nodes -- scalar boson stars in General Relativity minimally coupled to a complex massive scalar field with quartic self-interactions. We report that these stars, with up to $n=10$, can be made dynamically stable, up to timescales of $t\sim\frac{10^{4}}{cμ}$, where $μ$ is the inverse Compton wavelength of the scalar particle, for sufficiently large values of the self-interactions coupling constant $λ$, which depend on $n$. We observe that the compactness of these solutions is rather insensitive to $n$, for large $λ$ and fixed frequency. Generically, along the branches where stability was studied, these excited boson stars are not compact enough to allow for innermost stable circular orbits or light rings. Finally, we discuss the angular velocity of particles along timelike circular orbits, suggesting an application, for solutions in the Newtonian limit, to galactic rotation curves.

gr-qc↗

arXiv2023

Holographic Bubbles with Jecco: Expanding, Collapsing and Critical

Cosmological phase transitions can proceed via the nucleation of bubbles that subsequently expand and collide. The resulting gravitational wave spectrum depends crucially on the properties of these bubbles. We extend our previous holographic work on planar bubbles to circular bubbles in a strongly-coupled, non-Abelian, four-dimensional gauge theory. This extension brings about two new physical properties. First, the existence of a critical bubble, which we determine. Second, the bubble profile at late times exhibits a richer self-similar structure, which we verify. These results require a new 3+1 evolution code called Jecco that solves the Einstein equations in the characteristic formulation in asymptotically AdS spaces. Jecco is written in the Julia programming language and is freely available. We present an outline of the code and the tests performed to assess its robustness and performance.

hep-th↗

arXiv2022

Parasitic black holes: the swallowing of a fuzzy dark matter soliton

Fuzzy dark matter is an exciting alternative to the standard cold dark matter paradigm, reproducing its large scale predictions, while solving most of the existing tension with small scale observations. These models postulate that dark matter is constituted by light bosons and predict the condensation of a solitonic core -- also known as boson star, supported by wave pressure -- at the center of halos. However, solitons which host a \emph{parasitic} supermassive black hole are doomed to be swallowed by their guest. It is thus crucial to understand in detail the accretion process. In this work, we use numerical relativity to self-consistently solve the problem of accretion of a boson star by a central black hole, in spherical symmetry. We identify three stages in the process, a {\it boson-quake}, a {\it catastrophic stage} and a linear phase, as well as a general accurate expression for the lifetime of a boson star with an endoparasitic black hole. Lifetimes of these objects can be large enough to allow them to survive until the present time.

gr-qc↗