arXiv subjects

Romain Gervalle

Publications and source records attributed to Romain Gervalle.

9 recordsLinked to original sources

arXiv2026

Boson Star Factory: Past, Present, and Future

Boson stars are self-gravitating solitons of the Einstein-Klein-Gordon equations for a massive complex scalar field, and arguably the simplest horizonless compact objects arising in General Relativity. Beyond spherical symmetry, their construction requires solving a system of nonlinear elliptic partial differential equations, a difficult task per se and that each group has traditionally addressed with its own numerical implementation. A systematic comparison of different methods applied to the same physical model has so far been lacking. In this work, we carry out such a comparison for the two simplest non-spherical configurations: rotating boson stars and static dipolar boson stars. We employ three independent codes, based respectively on finite differences (FIDISOL/CADSOL), finite elements (FreeFem), and spectral methods (Kadath), and assess their accuracy through different diagnostics. The three solvers agree on the global observables to typically eight significant digits. We further compare the runtime of each code on identical hardware and provide precise benchmark values of the relevant physical quantities, intended to serve as reference data for future numerical studies of boson stars. As a further proof of concept, we also present the use of physics-informed neural networks to construct a rotating Q-ball solution.

gr-qc↗

arXiv2026

Bosonic stars with dark electroweak fields

We construct bosonic stars in the Einstein-Weinberg-Salam theory, namely the bosonic sector of the electroweak Standard Model minimally coupled to Einstein's theory of gravity. These configurations are everywhere regular, spherically symmetric and asymptotically flat, consisting of a static condensate of the massive $W$ and $Z$ bosons, whose masses arise from the Higgs mechanism. Although the corresponding fields exist in nature, gravitational effects are negligible at the physical electroweak scale and the configurations would be microscopic. Keeping Newton's constant and the electroweak mass ratios at their physical values, we therefore interpret the model as the bosonic sector of a \textit{dark} electroweak theory with a radically different energy scale. Choosing the ultralight vector boson mass $m_{\mathrm{W}}c^{2}=8.7\times10^{-13}\,$eV, as suggested by the Proca star interpretation of GW190521, our bosonic stars can attain masses in the intermediate-mass black hole range.

gr-qc↗

arXiv2025

Black holes with electroweak hair -- the detailed derivation

We present a very detailed derivation of solutions describing hairy black holes within the gravity-coupled Weinberg-Salam theory, which were previously reported in \href{https://doi.org/10.1103/PhysRevLett.133.171402}{Phys.Rev.Lett. 133 (2024) 171402}. These black holes support a strong magnetic field that polarizes the electroweak vacuum and creates a condensate of massive fields carrying superconducting currents along the black hole horizon. The currents, in turn, generate a ``corona'' of magnetic vortex segments attached to the horizon at both ends. The condensate and corona together constitute the black hole hair. The extremal solutions approach, in the far field, the magnetic Reissner-Nordström configuration, with a total mass that is {\it lower} than the total charge, $M<|Q|$, due to the negative Zeeman energy of the condensate. This makes the removal of the hair energetically unfavorable. The maximally hairy black holes exhibit masses comparable to terrestrial values, with approximately 11\% of their total mass stored in the hair. Given that these solutions arise within a well-tested theoretical framework, they are likely to have physical relevance.

hep-th↗

arXiv2024

Black holes with electroweak hair

We construct static and axially symmetric magnetically charged hairy black holes in the gravity-coupled Weinberg-Salam theory. Large black holes merge with the Reissner-Nordström (RN) family, while the small ones are extremal and support a hair in the form of a ring-shaped electroweak condensate carrying superconducting W-currents and up to $22\%$ of the total magnetic charge. The extremal solutions are asymptotically RN with a mass {\it below} the total charge, $M<|Q|$, due to the negative Zeeman energy of the condensate interacting with the black hole magnetic field. Therefore, they cannot decay into RN black holes. As their charge increases, they show a phase transition when the horizon symmetry changes from spherical to oblate. At this point they have the mass typical for planetary size black holes of which $\approx 11\%$ is stored in the hair. Being obtained within a well-tested theory, our solutions are expected to be physically relevant.

hep-th↗

arXiv2024

Hairy black holes and other compact objects in theories of gravity

In the realm of spacetimes governed by Einstein's general relativity and containing only Maxwell's electromagnetic field, stationary black holes are fully characterized by their mass, electric or magnetic charge, and angular momentum -- a property encapsulated in a version of the no-hair theorem. However, the validity of this theorem is contingent on certain assumptions, and when these are relaxed, new solutions describing hairy black holes arise. To date, astronomical observations have not provided concrete evidence of any type of black hole hair. Nevertheless, the development of increasingly precise gravitational wave detectors has sparked renewed interest in hairy black holes. In this thesis, we delve into two approaches to circumvent the no-hair theorem. The first option consists in describing the spacetime metric by an alternative theory of gravitation. We investigate the dynamical stability of hairy black holes in a vacuum spacetime described by the theory of massive bigravity. We show that hairy black holes in bigravity can describe both stellar black holes and supermassive black holes. The second approach is to keep Einstein's equations but to consider a different material content than Maxwell's electromagnetic field. For this we choose the fields of the electroweak theory. In the absence of gravitation, this theory describes magnetic monopoles with infinite mass. General relativity allows for their regularization by concealing their Coulombian singularity within an event horizon. After providing a detailed analysis of the internal structure of monopoles in flat space, we investigate how their properties generalize to the black hole case. Lastly, we study a particular example of soliton -- boson stars -- arising when a complex scalar field is coupled to general relativity. We construct chains of boson stars by solving the elliptic field equations using the finite element method.

gr-qc↗

arXiv2022

Electroweak multi-monopoles

We construct the multi-charge generalizations for the electroweak magnetic monopole solution of Cho and Maison within a wide range of values of the magnetic charge. We use the same ansatz for the axially symmetric fields as the one previously employed to construct the electroweak sphalerons and compare the internal structure of monopoles with that of sphalerons. The monopoles have zero dipole moment but a finite quadrupole momentum that rapidly increases with growing magnetic charge. For large charges, the monopole configurations are strongly squashed and show inside a bubble of symmetric phase filled with a U(1) hypercharge field produced by a pointlike magnetic charge at the origin, strong enough to suppress all other fields and restore the full gauge symmetry. The bubble is surrounded by a large belt of broken phase containing a magnetically charged ring filled with a nonlinear W-condensate, squeezed between two superconducting rings of opposite electric currents. In the far field region there remains only the magnetic field supported by the total magnetic charge contained at the origin and in the magnetic ring. The axially symmetric monopoles are probably just a special case of more general monopole solutions not possessing any continuous symmetries. The Cho-Maison monopole is stable but the stability of its multi-charge generalizations is not yet confirmed. All electroweak monopoles have infinite energy due to the pointlike U(1) charge at the origin, but the energy is expected to become finite after taking gravity into account, which should provide a cutoff via creating an event horizon to shield the U(1) charge.

hep-th↗

arXiv2022

Chains of rotating boson stars

Boson Stars are stationary, axially symmetric solutions of a complex scalar field theory coupled to gravity. Recently, multi-solitonic configurations interpreted as static chains of multiple Boson Stars bound by gravity and carrying no angular momentum were reported. We propose to generalize those solutions to the stationary case by constructing chains of rotating Boson Stars and analyze their properties. The non-linear elliptic field equations are solved using the finite element method. We find that chains with an even number of constituents exhibit the same spiral-like frequency dependence of their mass, angular momentum and Noether charge as single Boson Stars. In contrast, sequences of chains with an odd number of constituents show nontrivial loops starting and ending at the flat vacuum. As a consequence, such solutions cannot be uniquely parametrized by a single parameter. We conjecture that all rotating chains correspond to excitations of single Boson Stars or pairs of them. We also analyze their flat space limit and find that they reduce to chains of $Q$-balls.

gr-qc↗

arXiv2022

Electroweak monopoles and their stability

We apply a generalized field ansatz to describe the spherically symmetric sector of classical solutions of the electroweak theory. This sector contains Abelian magnetic monopoles labeled by their magnetic charge $n=\pm 1,\pm 2,\ldots$, the non-Abelian monopole for $n=\pm 2$ found previously by Cho and Maison (CM), and also the electric oscillating solutions. All magnetic monopoles have infinite energy. We analyze their perturbative stability and use the method of complex spacetime tetrad to separate variables and reduce the perturbation equations to multi-channel Schroedinger-type eigenvalue problems. The spectra of perturbations around the CM monopole do not contain negative modes hence this solution is stable. The $n=\pm 1$ Abelian monopole is also stable, but all monopoles with $|n|\geq 2$ are unstable with respect to perturbations with angular momentum $j=|n|/2-1$. The Abelian $|n|=2$ monopole is unstable only within the $j=0$ sector whereas the CM monopole also has $|n|=2$ and belongs to the same sector, hence it may be viewed as a stable remnant of the decay of the Abelian monopole. One may similarly conjecture that stable remnants exist also for monopoles with $|n|>2$, hence the CM monopole may be just the first member of a sequence of non-Abelian monopoles with higher magnetic charges. Only the CM monopole is spherically symmetric while all non-Abelian monopoles with $|n|>2$ are not rotationally invariant.

hep-th↗

arXiv2020

Asymptotically flat hairy black holes in massive bigravity

We study asymptotically flat black holes with massive graviton hair within the ghost-free bigravity theory. There have been contradictory statements in the literature about their existence -- such solutions were reported some time ago, but later a different group claimed the Schwarzschild solution to be the only asymptotically flat black hole in the theory. As a result, the controversy emerged. We have analyzed the issue ourselves and have been able to construct such solutions within a carefully designed numerical scheme. We find that for given parameter values there can be one or two asymptotically flat hairy black holes in addition to the Schwarzschild solution. We analyze their perturbative stability and find that they can be stable or unstable, depending on the parameter values. The masses of stable hairy black holes that would be physically relevant range form stellar values up to values typical for supermassive black holes. One of their two metrics is extremely close to Schwarzschild, while all their "hair" is hidden in the second metric that is not coupled to matter and not directly seen. If the massive bigravity theory indeed describes physics, the hair of such black holes should manifest themselves in violent processes like black hole collisions and should be visible in the structure of the signals detected by LIGO/VIRGO.

hep-th↗