arXiv subjects

Keanu Müller

Publications and source records attributed to Keanu Müller.

6 recordsLinked to original sources

arXiv2026

From Bertotti--Robinson to Vacuum: New Exact Solutions in General Relativity via Harrison and Inversion Symmetries

We construct new vacuum solutions of the Einstein equations generated from electrovacuum configurations embedded in external electromagnetic backgrounds. Starting from accelerating Bertotti--Robinson black holes, we exploit two independent symmetries of the electrovacuum: a Melvin--Bonnor-type magnetization and a magnetic Inversion. In both constructions, the external electromagnetic field can be removed while still leaving a non-trivial gravitational backreaction in the metric, yielding new accelerating vacuum spacetimes of Petrov type I. In the static, non-accelerating limit, the magnetized Bertotti--Robinson--Schwarzschild case reproduces known results, while the Inversion symmetry produces a genuinely new vacuum configuration, a two-parameter extension of the Schwarzschild--Levi-Civita geometry. These constructions provide a systematic method for generating algebraically general vacuum geometries and illustrate how electromagnetic embeddings can induce non-trivial vacuum metrics in General Relativity. The main geometrical properties of these spacetimes are analyzed. Additionally, we present two further results: a stationary generalization of these vacuum geometries and two new static vacuum configurations obtained by applying the same symmetries to the Alekseev--Garc\'ia black hole seed.

gr-qc↗

arXiv2026

Melvin--Bonnor and Bertotti--Robinson spacetimes with Baryonic charge

In this work, we exploit a dictionary that maps solutions of the Einstein--Scalar--Maxwell theory to those of gauged Skyrme--Maxwell--Einstein models in $(3+1)$ dimensions, allowing gravitating scalar-field configurations in external electromagnetic backgrounds to be interpreted in terms of baryonic quantities. Using the definition of Baryonic charge as the integral of a topological density, we derive novel mass formulas that relate the spacetime mass parameter to the Baryonic charge, the external magnetic field, and other solution parameters. As a result, the mass and Baryonic charge are not independent. These closed analytic expressions encode physical information that would otherwise be difficult to extract. In particular, for the class of solutions considered here, the relation between mass and Baryonic charge is linear at large mass, while significant nonlinear deviations appear at intermediate values. This framework provides a useful tool for both extracting new physical insights from known solutions and constructing new baryonic configurations using the solution-generating techniques available for Einstein--Scalar--Maxwell theory.

gr-qc↗

arXiv2026

Magnetized dynamical black holes

We construct a novel exact solution of the Einstein-scalar-Maxwell equations describing a dynamical black hole immersed in an external, time-dependent electromagnetic field. Motivated by the need for more realistic analytical black hole models, our construction incorporates two key ingredients often neglected in exact solutions: a fully dynamical cosmological background and the non-perturbative backreaction of external electromagnetic fields. The compact object is obtained by dressing a Schwarzschild black hole with a radially and temporally dependent scalar field, yielding a time-dependent generalization of the Fisher-Janis-Newman-Winicour solution within the Fonarev framework. The external electromagnetic field is generated via a Lie point symmetry of the Einstein-scalar-Maxwell system, which exports the effect of a Harrison transformation to dynamical settings provided a spacelike Killing vector is present. The resulting spacetime combines a spherically symmetric dynamical horizon with an axisymmetric electromagnetic field and exhibits a rich asymptotic structure mixing Friedmann-Lema\^itre-Robertson-Walker and Levi-Civita geometries. We show that the time dependence of the configuration plays a crucial role in potentially cloaking curvature singularities, which would otherwise be generically naked in the stationary limit. We analyze the geometric and physical properties of the solution, including its asymptotic behavior, algebraic classification, and the structure of trapped surfaces defining the dynamical horizon. Possible implications for primordial black holes and some astrophysical applications, as well as extensions to higher dimensions, are also discussed.

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arXiv2025

A new exact rotating spacetime in vacuum: The Kerr--Levi-Civita Spacetime

We construct a new rotating solution of Einstein's theory in vacuum by exploiting the Lie point symmetries of the field equations in the complex potential formalism of Ernst. In particular, we perform a discrete symmetry transformation, known as inversion, of the gravitational potential associated with the Kerr metric. The resulting metric describes a rotating generalization of the Schwarzschild--Levi-Civita spacetime, and we refer to it as the Kerr--Levi-Civita metric. We study the key geometric features of this novel spacetime, which turns out to be free of curvature singularities, topological defects, and closed timelike curves. These attractive properties are also common to the extremal black hole and the super-spinning case. The solution is algebraically general (Petrov-type I), and its horizons lie at the horizon radii of the Kerr black hole. The ergoregions, however, are strongly influenced by the Levi-Civita-like asymptotic structure, producing an effect akin to the magnetized Kerr--Newman and swirling solutions. Interestingly, while its static counterpart permits a Kerr--Schild representation, the Kerr--Levi-Civita metric does not admit such a formulation.

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arXiv2024

Mixing "Magnetic'' and "Electric'' Ehlers--Harrison transformations: The Electromagnetic Swirling Spacetime and Novel Type I Backgrounds

In this paper, we obtain a complete list of stationary and axisymmetric spacetimes, generated from a Minkowski spacetime using the Ernst technique. We do so by operating on the associated seed potentials with a composition of Ehlers and Harrison transformations. In particular, assigning an additional ``electric'' or ``magnetic'' tag to the transformations, we investigate the new spacetimes obtained either via a composition of magnetic Ehlers and Harrison transformations (first part) or via a magnetic-electric combination (second part). In the first part, the resulting type D spacetime, dubbed electromagnetic swirling universe, features key properties, separately found in swirling and (Bonnor--)Melvin spacetimes, the latter recovered in appropriate limits. A detailed analysis of the geometry is included, and subtle issues are addressed. A detailed proof that the spacetime belongs to the Kundt family, is included, and a notable relation to the planar-Reissner-Nordstr\"om-NUT black hole is also meticulously worked out. This relation is further exploited to reverse-engineer the form of the solution in the presence of a nontrivial cosmological constant. A Schwarzschild black hole embedded into the new background is also discussed. In the second part, we present four novel stationary and axisymmetric asymptotically nonflat type I spacetimes, which are naively expected to be extensions of the Melvin or swirling solution including a NUT parameter or electromagnetic charges. We actually find that they are, under conditions, free of curvature and topological singularities, with the physical meaning of the electric transformation parameters in these backgrounds requiring further investigation.

gr-qc↗

arXiv2023

Exact rotating wormholes via Ehlers transformations

In this paper, we construct exact rotating wormholes using Ehlers solution-generating technique. This is based on the Ernst description of four-dimensional, stationary, and axially symmetric solutions of the Einstein-Maxwell theory. We adopt the static Barceló-Visser wormhole derived from the Einstein-Maxwell-conformal-scalar theory as a seed, and demonstrate, through the Ernst approach, how to construct two novel geometries of rotating wormholes. These geometries correspond to the Barceló-Visser wormhole embedded within a rotating and a magnetic background, respectively. In the first case, the rotation is a result of a dragging force (due to the rotating background) acting on the initial static wormhole, while in the second case it is caused by the electromagnetic interaction between the electric charge of the static wormhole and the external magnetic field. We conduct a comprehensive analysis of the geometric properties of these configurations, and examine the new features introduced by rotation, such as the emergence of ergoregions. Recent evidence suggests that incorporating slow rotation can stabilise wormholes, rendering these exact, fully rotating solutions particularly appealing.

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