arXiv subjects

Marco Astorino

Publications and source records attributed to Marco Astorino.

At least 19 recordsLinked to original sources

arXiv2026

Reissner-Nordstrom in the Bertotti-Robinson-Bonnor-Melvin electromagnetic field

The magnetic Reissner-Nordstrom black hole embedded into the Bertotti-Robinson magnetic field is presented. Then it is used, as a seed, to generate, through the Harrison transformation of the Ersnt's equations, the Reissner-Nordstrom metric in the Bertotti-Robinson-Bonnor-Melvin magnetic field. Special cases generalising, the vacuum hairy type I Schwarzschild solution is discussed. The local equivalence between various forms of the Schwarzschild-Bertotti-Robinson is shown by exhibiting the explicit diffeomorphism.

gr-qc↗

arXiv2026

Black holes in rotating, electromagnetic backgrounds and topological Kerr-Newman-NUT spacetimes

We observe that a large class of well behaved stationary and axisymmetric black hole solutions in general relativity and in the Einstein-Maxwell theory can be classified according to the properties of their background. Indeed all these backgrounds belong to a unique family which includes simultaneously all the known axisymmetric and regular backgrounds: the swirling, the Bertotti-Robinson, the Bonnor-Melvin universe, Witten's expanding bubble and also other novel, regular, rotating gravitational or electromagnetic environments. All these can be, fundamentally, traced back to the double Wick rotation of the topological generalisation of (accelerating) Kerr-Newman-NUT metric. We present a black hole embedded in an unexplored sector of the general background: Schwarzschild inside a generalised rotating (and possibly electromagnetic) universe. These results indicate that basically all the known analytical and exact single black hole solutions in the four-dimensional Einstein-Maxwell theory belong to the (accelerating) Kerr-Newman-NUT family embedded into backgrounds that are a subcase of the conjugated Kerr-Newman-NUT space-time with an angular manifold of arbitrary topology.

gr-qc↗

arXiv2026

Ultrarelativistic boost of a black hole in the magnetic universe of Levi-Civita--Bertotti--Robinson

We consider an exact Einstein-Maxwell solution constructed by Alekseev and Garcia which describes a Schwarzschild black hole immersed in the magnetic universe of Levi-Civita, Bertotti and Robinson (LCBR). After reviewing the basic properties of this spacetime, we study the ultrarelativistic limit in which the black hole is boosted to the speed of light, while sending its mass to zero. This results in a non-expanding impulsive wave traveling in the LCBR universe. The wave front is a 2-sphere carrying two null point particles at its poles -- a remnant of the structure of the original static spacetime. It is also shown that the obtained line-element belongs to the Kundt class of spacetimes, and the relation with a known family of exact gravitational waves of finite duration propagating in the LCBR background is clarified. In the limit of a vanishing electromagnetic field, one point particle is pushed away to infinity and the single-particle Aichelburg-Sexl pp-wave propagating in Minkowski space is recovered.

gr-qc↗

arXiv2026

Static hairy black hole in 4D General Relativity

In four-dimensional vacuum general relativity the only known static, exact and analytical black hole solution is given by the Schwarzschild spacetime. In this paper this renowned metric is generalised by adding another integrating constant, a hair that switches the metric from the Petrov type D to the type I. This new parameter represents the intensity of an external gravitational field, which can be considered the hyperbolic generalisation of the Witten's bubble of nothing. No curvature or conical singularities are present outside the event horizon. The no hair arguments are circumvented because the metric is not asymptotically flat, and neither the black hole is spherical. The gravitational hair continuously deforms the Schwarzschild geometry: the horizon becomes oblate, while its area is reduced. Conserved charges and thermodynamic properties of the black hole are studied.

gr-qc↗

arXiv2025

Kerr Black Holes in an Expanding Bubble

An exact and analytical solution, in four-dimensional general relativity, describing a collinear array of an arbitrary number of Kerr black holes inside an expanding bubble of nothing is built, thanks to the inverse scattering technique. Physical properties and thermodynamics of the single Kerr in the bubble are studied. No cosmic strings or struts are present. The binary black hole system displays equilibrium configurations, because the expanding bubble surrounding the black holes balances the mutual gravitational attraction of the two constituents.

gr-qc↗

arXiv2025

Black holes in the external Bertotti-Robinson-Bonnor-Melvin electromagnetic field

An exact and analytical solution, in four-dimensional general relativity coupled with Maxwell electromagnetism, is built by means of a Lie point symmetry of the Ernst equations, the Harrison transformation. The new spacetime describes a Schwarzschild-like black hole embedded into a general external back-reacting electromagnetic field, which is the superposition of the Levi-Civita-Bertotti-Robinson and the Bonnor-Melvin ones. The relation between the two homogeneous electromagnetic fields is clarified. Conserved charges and the first law of thermodynamics are analysed. Swirling generalisations are also considered. Limits to the known metrics such as Schwarzschild-Bertotti-Robinson, Schwarzschild-Bonnor-Melvin and Bertotti-Robinson-Bonnor-Melvin are discussed.

gr-qc↗

arXiv2025

Rotating and swirling binary black hole system balanced by its gravitational spin-spin interaction

We present the first exact and analytical solution in General Relativity describing an equilibrium configuration for two stationary black holes. The metric models two collinear extremal Kerr black holes immersed in an external and back-reacting rotating tidal drag. The gravitational attraction is balanced by the repulsive gravitational spin-spin interaction generated by the interplay between black holes angular momenta and the rotational background. The new solution is built by embedding the double Kerr metric into a swirling universe by means of the Ehlers transformation. The geometry is completely regular outside the event horizons. Thermodynamic properties of the binary black hole system are studied, the Smarr law, the first law and the Christodoulou-Ruffini formulas are verified. Microscopic degrees of freedom of the entropy are computed from the dual CFT living on the boundary of the near horizon geometries.

gr-qc↗

arXiv2025

Revisiting black holes of algebraic type D with a cosmological constant

As an extension of our previous work [1] (arXiv:2409.02308), we study a complete family of type D black holes with Kerr-like rotation, NUT twist, acceleration, electric and magnetic charges, and any value of the cosmological constant $Λ$. We relate various metric forms of these spacetimes, namely those found by Plebanski-Demianski (PD), Griffiths-Podolsky (GP), and most recently Astorino (A). By explicit coordinate transformations and proper identification of the physical parameters we show that these representations are locally equivalent, and cover the entire class of type D solutions of the Einstein-Maxwell-$Λ$ equations, such that the (non-null) electromagnetic field is aligned with both the (double-degenerate) principal null directions of the Weyl tensor. In particular, we concentrate on the subclass which describes accelerating NUT black holes without the Kerr-like rotation.

gr-qc↗

arXiv2024

Black holes of type D revisited: relating their various metric forms

We investigate a complete family of spacetimes which represent black holes with rotation, NUT twist, acceleration, electric and magnetic charges. These are exact solutions of the Einstein-Maxwell equations with any cosmological constant, such that the (non-null) electromagnetic field is aligned with both the double-degenerate principal null directions of the Weyl tensor. In particular, we explicitly relate various coordinates and the corresponding physical parameters of such solutions, namely the original Plebanski-Demianski (PD) form, the convenient Astorino (A) form which was found recently, and formally improved here (A+), the Griffiths-Podolsky (GP), and Podolsky-Vratny (PV) form of the metric. It is demonstrated that, if properly mapped and physically interpreted, all these representations cover the complete class of type D black holes. Using the new A-parameters, the two main PD quartic metric functions are factorized into the product of quadratic expressions, enabling thus an explicit analysis. Moreover, we clarify the role of the twist parameter $ω$, related to both the Kerr-like rotation and the NUT parameters $a$ and $l$, respectively. Special attention is payed to the elusive subclass of accelerating NUT black holes with $a=0$.

gr-qc↗

arXiv2024

Equivalence principle and generalised accelerating black holes from binary systems

The Einstein equivalence principle in general relativity allows us to interpret accelerating black holes as a black hole immersed into the gravitational field of a larger companion black hole. Indeed it is demonstrated that C-metrics can be obtained as a limit of a binary system where one of the black holes grows indefinitely large, becoming a Rindler horizon. When the bigger black hole, before the limiting process, is of Schwarzschild type we recover usual accelerating black holes belonging to the Plebanski-Demianski class, thus type D. Whether the greater black hole carries some extra features, such as electric charges or rotations, we get generalised accelerating black holes which belong to a more general class, the type I. In that case the background has a richer structure, reminiscent of the physical features of the inflated companion, with respect to the standard Rindler spacetime. This insight allows us to build a general type D metric, describing an accelerating Kerr-NUT black hole. It has well defined limits to all the type-D black holes of general relativity, including the elusive (type-D) accelerating Taub-NUT spacetime. Extension to the presence of the cosmological constant is also provided.

gr-qc↗

arXiv2024

Most general Type-D Black Hole and the Accelerating Reissner-Nordstrom-NUT-(A)dS solution

The Plebanski-Demianski metric is commonly thought to describe the most general Type D black hole in general relativity (possibly coupled with Maxwell electromagnetism), but in the form we know at the moment, it fails to include the elusive accelerating spacetimes with gravitomagnetic mass without angular momentum. We propose a very general type D metric, stemming from a binary black hole system, which represents a novel branch of the type D spacetimes: accelerating Kerr-Newman-NUT black hole with cosmological constant. It has straightforward limits to all the known type D black holes in the Einstein-Maxwell theory and in particular to the accelerating Reissner-Nordstrom-NUT solution, yet unknown in the literature.

gr-qc↗

arXiv2024

Black holes in a swirling universe

We present a new solution in Einstein's General Relativity representing a Schwarzschild black hole immersed in a rotating universe. Such a solution is constructed analytically by means of the last unexplored Lie point symmetry of the Ernst equations for stationary and axisymmetric spacetimes. This kind of the Ehlers transformation is able to embed any given solution into a rotating background, which is not of NUT type. We analyse the physical properties, ergoregions and geodesics of the new metric, which is regular outside the event horizon and has a well defined thermodynamics. We finally consider the Kerr generalisation.

gr-qc↗

arXiv2023

Accelerating and Charged Type I Black Holes

A new, exact and analytical class of accelerating and charged black holes is built, in the Einstein-Maxwell theory, thanks to the Harrison transformation. The diagonal metric does not belong to the Petrov type D classification, therefore it is not part of the Plebanski-Demianski spacetimes. The simplest subcase of this family recovers the Reissner-Nordstrom black hole in the vanishing acceleration limit and the standard C-metric in the limit of null electric charge. More general cases can have two independent electric charges, which can be tuned as desired, even to remain with an uncharged black hole, such as Petrov Type I Schwarzschild, embedded in an accelerating charged Rindler background. These accelerating black holes can be considered as a limit of charged binary systems. Conical singularities can be possibly removed in extremal configurations. The entropy of the conformal field theory model dual to the extreme black hole is obtained from near horizon analysis. Magnetic, dyonic, NUTty and Kerr-like extensions are also discussed.

gr-qc↗

arXiv2023

Plebanski-Demianski goes NUTs (to remove the Misner string)

We present a general procedure, based on the Ehlers transformation of the Ernst equations, to add the gravitomagnetic mass to the whole Plebanski-Demianski family of solutions. We can efficiently generate a large class of accelerating black holes, such as Reissner-Nordstrom or Kerr-Newman, endowed with the NUT parameter. The full rotating version carries a couple of independent NUT charges, one associated to the black hole and the other to the accelerating Rindler background. The two NUT parameters can be coupled to remove the axial irregularity which causes the Misner string, still remaining with a Lorentzian spacetime, without the need to impose periodic time. All the metrics we build are not of D-type according to the Petrov classification, but type-I, so they belong to a more general category with respect to C-metrics and the Plebanski-Demianski seed. A convenient form of the most general type D black hole solution in general relativity, coupled with Maxwell electromagnetism, is obtained when switching off one of the two NUT parameters.

gr-qc↗

arXiv2023

Removal of conical singularities from rotating C-metrics and dual CFT entropy

We show how to remove from the rotating C-metric spacetime, which describes accelerating Kerr black holes, both conical singularities. This can be done by embedding the metric into a swirling gravitational universe, through a proper Ehlers transformation. The spin-spin interaction between the external rotational background and the black hole provides the source of the acceleration without the need of rods or strings. The physical properties and entropy of the new solution are studied using near horizon and dual conformal techniques of the Kerr/CFT correspondence. The charged case is also analysed: Accelerating Reissner-Nordstrom and Accelerating Kerr-Newman space-times embedded in a swirling universe are also generated.

gr-qc↗

arXiv2022

Bubbles of nothing in binary black holes and black rings, and viceversa

We argue that expanding bubbles of nothing are a widespread feature of systems of black holes with multiple or non-spherical horizons, appearing as a limit of regions that are narrowly enclosed by the horizons. The bubble is a minimal cycle that links the Einstein-Rosen bridges in the system, and its expansion occurs through the familiar stretching of space in black hole interiors. We demonstrate this idea (which does not involve any Wick rotations) with explicit constructions in four and five dimensions. The geometries of expanding bubbles in these dimensions arise as a limit of, respectively, static black hole binaries and black rings. The limit is such that the separation between the two black holes, or the inner hole of the black ring, becomes very small, and the horizons of the black holes correspond to acceleration horizons of the bubbles. We also explain how a five-dimensional black hole binary gives rise to a different type of expanding bubble. We then show that bubble spacetimes can host black hole binaries and black rings in static equilibrium, with their gravitational attraction being balanced against the background spacetime expansion. Similar constructions are expected in six or more dimensions, but most of these solutions can be obtained only numerically. Finally, we argue that the Nariai solution can be regarded as containing an expanding circular bubble of nothing.

hep-th↗

arXiv2021

Many accelerating distorted black holes

An analytical metric of four-dimensional General Relativity, representing an array of collinear and accelerating black holes, is constructed with the inverse scattering method. The solution can be completely regularised from any conical singularity, thanks to the presence of an external gravitational field. Therefore the multi-black hole configuration can be maintained at equilibrium without the need of string or struts. Some notable subcases such as the accelerating distorted Schwarzschild black hole and the double distorted C-metric are explicitly presented. The Smarr law and the thermodynamics of these systems is studied. The Bonnor-Swaminarayan and the Bičák-Hoenselaers-Schmidt particle metrics are recovered, through appropriate limits, from the multi-black holes solutions.

gr-qc↗

arXiv2021

Charged and rotating multi-black holes in an external gravitational field

We construct analytical and regular solutions in four-dimensional General Relativity which represent multi-black hole systems immersed in external gravitational field configurations. The external field background is composed by an infinite multipolar expansion, which allows to regularise the conical singularities of an array of collinear static black holes. A stationary rotating generalisation is achieved by adding independent angular momenta and NUT parameters to each source of the binary configuration. Moreover, a charged extension of the binary black hole system at equilibrium is generated. Finally, we show that the binary Majumdar-Papapetrou solution is consistently recovered in the vanishing external field limit. All of these solutions reach an equilibrium state due to the external gravitational field only, avoiding in this way the presence of any string or strut defect.

gr-qc↗