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Sökning: WFRF:(Andreasson Håkan)

  • Resultat 1-10 av 93
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2.
  • Ames, Ellery, et al. (författare)
  • Cosmic string and black hole limits of toroidal Vlasov bodies in general relativity
  • 2019
  • Ingår i: Physical Review D. - : AMER PHYSICAL SOC. - 2470-0010 .- 2470-0029. ; 99:2
  • Tidskriftsartikel (refereegranskat)abstract
    • We numerically investigate limits of a two-parameter family of stationary solutions to the Einstein-Vlasov system. The solutions are toroidal and have nonvanishing angular momentum. As the parameters are tuned to more relativistic solutions (measured e.g., by an increasing redshift) we provide evidence for a sequence of solutions which approaches the extreme Kerr black hole family. Solutions with angular momentum larger than the square of the mass are also investigated, and in the relativistic limit the near-field geometry of such solutions is observed to become locally rotationally symmetric about the matter density. The existence of a deficit angle in these regions is investigated.
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3.
  • Ames, E., et al. (författare)
  • Dynamics of gravitational collapse in the axisymmetric Einstein-Vlasov system
  • 2021
  • Ingår i: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 38:10
  • Tidskriftsartikel (refereegranskat)abstract
    • We numerically investigate the dynamics near black hole formation of solutions to the Einstein-Vlasov system in axisymmetry. Our results are obtained using a particle-in-cell and finite difference code based on the (2 + 1) + 1 formulation of the Einstein field equations in axisymmetry. Solutions are launched from non-stationary initial data and exhibit type I critical behaviour. In particular, we find lifetime scaling in solutions containing black holes, and support that the critical solutions are stationary. Our results contain examples of solutions that form black holes, perform damped oscillations, and appear to disperse. We prove that complete dispersal of the solution implies that it has nonpositive binding energy.
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4.
  • Ames, Ellery, et al. (författare)
  • Hoop and weak cosmic censorship conjectures for the axisymmetric Einstein-Vlasov system
  • 2023
  • Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 108:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider gravitational collapse for the axially symmetric Einstein-Vlasov system. We investigate the weak cosmic censorship conjecture in the case of highly prolate initial data and we investigate the "only if"part of the Hoop conjecture. Shapiro and Teukolsky initiated a similar study in 1991 [Formation of Naked Singularities: The Violation of Cosmic Censorship, Phys. Rev. Lett. 66, 994 (1991)PRLTAO0031-900710.1103/PhysRevLett.66.994] where they found support that the weak cosmic censorship conjecture was violated for sufficiently prolate spheroidal initial data. More recently, independent studies of this problem have been carried out by Yoo et al. [3D simulation of spindle gravitational collapse of a collisionless particle system, Classical Quantum Gravity 34, 105010 (2017)CQGRDG0264-938110.1088/1361-6382/aa6ad5] and by East [Cosmic Censorship Upheld in Spheroidal Collapse of Collisionless Matter, Phys. Rev. Lett. 122, 231103 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.231103]. A common feature in these works is that the initial data are dustlike. Dust can be considered as a singular case of matter described by the Einstein-Vlasov system. The original motivation by Shapiro and Teukolsky to study this problem is based on the Lin-Mestel-Shu instability for gravitational collapse of uniform spheroids in the case of dust in Newtonian gravity. We argue that the Lin-Mestel-Shu solution is not relevant for studying the weak cosmic censorship conjecture of the Einstein-Vlasov system and we argue that dustlike initial data is also not relevant. To investigate collapse of highly prolate spheroidal configurations for the Einstein-Vlasov system is nevertheless interesting in view of the Hoop conjecture. By choosing highly prolate initial data the weak cosmic censorship conjecture is seriously tested. We carry out such a study for initial data which are not dustlike. We find formation of an apparent horizon in all cases we consider, which provides support for the weak cosmic censorship conjecture. In our tests of the Hoop conjecture we compute the polar circumference CH,p at the time when the apparent horizon forms and find that it is less than 12% above 4πM, where M is the irreducible mass of the apparent horizon, which agrees with the spirit of the Hoop conjecture.
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5.
  • Ames, Ellery, 1984, et al. (författare)
  • On axisymmetric and stationary solutions of the self-gravitating Vlasov system
  • 2016
  • Ingår i: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 33:15
  • Tidskriftsartikel (refereegranskat)abstract
    • Axisymmetric and stationary solutions are constructed to the Einstein-Vlasov and Vlasov-Poisson systems. These solutions are constructed numerically, using finite element methods and a fixed-point iteration in which the total mass is fixed at each step. A variety of axisymmetric stationary solutions are exhibited, including solutions with toroidal, disk-like, spindle-like, and composite spatial density configurations, as are solutions with non-vanishing net angular momentum. In the case of toroidal solutions, we show for the first time, solutions of the Einstein-Vlasov system which contain ergoregions.
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6.
  • Ames, Ellery, 1984, et al. (författare)
  • Stationary solutions of the axially symmetric Einstein-Vlasov system: present status and open problems
  • 2024
  • Ingår i: Classical and Quantum Gravity. - 0264-9381 .- 1361-6382. ; 41:7
  • Forskningsöversikt (refereegranskat)abstract
    • The purpose of this work is to review the status about stationary solutions of the axially symmetric Einstein-Vlasov system with a focus on open problems of both analytical and numerical nature. For the latter we emphasize that the code used to construct stationary solutions in Ames et al (2016 Class. Quantum Grav. 33 155008; 2019 Phys. Rev. D 99 024012) is open source, see Ames and Logg (2023 J. Open Source Softw. 8 5979). In the analytical setting the open problems include establishing methods for proving existence of axisymmetric stationary solutions which are far from spherically symmetric, both in the general case and for certain special classes of solutions pointed out in the text. In the numerical setting there are intriguing properties of highly relativistic solutions that demand further attention, such as whether a sequence of such stationary solutions can approach a Kerr black hole, or if they necessarily approach the thin ring limit reminiscent of cosmic strings. The question of whether stationary solutions include states with thin-disk like morphologies as seen in many galaxies is also open. Finally, there are opportunities to extend this research to new settings such as the case of massless particles and coupled black hole-matter systems. We believe that some of the open problems highlighted here are of central importance for the understanding of nature.
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7.
  • Andreasson, Bengt Håkan, 1955 (författare)
  • Bilens minskade attraktionskraft
  • 2006
  • Ingår i: Vardagslivets fronter. - Göteborg : Arkipelag. - 9185838705
  • Bokkapitel (övrigt vetenskapligt/konstnärligt)
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8.
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9.
  • Andreasson, Håkan, 1966, et al. (författare)
  • A numerical investigation of the stability of steady states and critical phenomena for the spherically symmetric Einstein–Vlasov system
  • 2006
  • Ingår i: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 23, s. 3659-3677
  • Tidskriftsartikel (refereegranskat)abstract
    • The stability features of steady states of the spherically symmetric Einstein–Vlasov system are investigated numerically. We find support for the conjecture by Zel'dovich and Novikov that the binding energy maximum along a steady state sequence signals the onset of instability, a conjecture which we extend to and confirm for non-isotropic states. The sign of the binding energy of a solution turns out to be relevant for its time evolution in general. We relate the stability properties to the question of universality in critical collapse and find that for Vlasov matter universality does not seem to hold.
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10.
  • Andreasson, Håkan, 1966, et al. (författare)
  • A numerical investigation of the steady states of the spherically symmetric Einstein-Vlasov-Maxwell system
  • 2009
  • Ingår i: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 26:14
  • Tidskriftsartikel (refereegranskat)abstract
    • We construct, by numerical means, static solutions of the spherically symmetric Einstein-Vlasov-Maxwell system and investigate various features of the solutions. This extends a previous investigation (Andreasson and Rein 2007 Class. Quantum Grav. 24 1809) of the chargeless case. We study the possible shapes of the energy density profile as a function of the area radius when the electric charge of an individual particle is varied as a parameter. We find profiles which are multi-peaked, where the peaks are separated either by vacuum or a thin atmosphere, and we find that for a sufficiently large charge parameter the solutions break down at a finite radius. Furthermore, we investigate the inequality root M <= root R/3 + root R/9 + Q(2)/3R, which is derived in Andreasson (2009 Commun. Math. Phys. 288 715) for general matter models, and we find that it is sharp for the Einstein-Vlasov-Maxwell system. Here M is the ADM mass, Q is the charge and R is the area radius of the boundary of the static object. We find two classes of solutions with this property, while there is only one in the chargeless case. In particular we find numerical evidence for the existence of arbitrarily thin shell solutions to the Einstein-Vlasov-Maxwell system. Finally, we consider one-parameter families of steady states, and we find spirals in the mass-radius diagram for all examples of the microscopic equation of state which we consider.
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