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Sökning: WFRF:(Andreasson Håkan)

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  • Föregående 1[2]34567...9Nästa
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11.
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  • 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. ; 23, s. 3659-3677
  • Tidskriftsartikel (refereegranskat)abstract
    • The stability features of steady states of the spherically symmetric EinsteinVlasov 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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14.
  • 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. - 0264-9381. ; 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
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16.
  • Andreasson, Håkan, 1966- (författare)
  • Black hole formation from a complete past for the einstein–vlasov system
  • 2014
  • Ingår i: International conference on Relativity and Gravitation, 2012, Prague, Czech Republic, 25-29 June 2012. - 0930-8989 .- 1867-4941. - 978-3-319-06760-5 ; 157, s. 11-18
  • Konferensbidrag (refereegranskat)abstract
    • Anatural question in general relativity is to find initial data for the Einstein equations whose past evolution is regular and whose future evolution contains a black hole. In [1] initial data of this kind is constructed for the spherically symmetric Einstein–Vlasov system. One consequence of the result is that there exists a class of initial data for which the ratio of the Hawking mass (Formula presented) (r) and the area radius r is arbitrarily small everywhere, such that a black hole forms in the evolution. This result is analogous to the result [2] for a scalar field. Another consequence is that there exist black hole initial data such that the solutions exist for all Schwarzschild time t ∈ (−∞,∞). In the present article we review the results in [1].
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17.
  • Andreasson, Håkan, 1966- (författare)
  • Black Hole Formation from a Complete Regular Past for Collisionless Matter
  • 2012
  • Ingår i: Annales Henri Poincaré. - 1424-0637. ; 13:7, s. 1511-1536
  • Tidskriftsartikel (refereegranskat)abstract
    • Initial data for the spherically symmetric Einstein-Vlasov system is constructed whose past evolution is regular and whose future evolution contains a black hole. This is the first example of initial data with these properties for the Einstein-matter system with a "realistic" matter model. One consequence of the result is that there exists a class of initial data for which the ratio of the Hawking mass mIS= mIS (r) and the area radius r is arbitrarily small everywhere, such that a black hole forms in the evolution. This result is in a sense analogous to the result (Christodoulou Commun Pure Appl Math 44:339-373, 1991) for a scalar field. Another consequence is that there exist black hole initial data such that the solutions exist for all Schwarzschild time .
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18.
  • Andreasson, Håkan, 1966- (författare)
  • Black hole formation from a complete regular past for vlasov matter
  • 2013
  • Ingår i: XVIIth International Congress on Mathematical Physics: Aalborg, Denmark, 6-11 August 2012. - 978-981444924-3 ; s. 365-372
  • Konferensbidrag (refereegranskat)abstract
    • A natural question in general relativity is to find initial data for the Einstein equations whose past evolution is regular and whose future evolution contains a black hole. In [1] initial data of this kind is constructed for the spherically symmetric Einstein-Vlasov system. One consequence of the result is that there exists a class of initial data for which the ratio of the Hawking mass (forumal presented) and the area radius r is arbitrarily small everywhere, such that a black hole forms in the evolution. Another consequence is that there exist black hole initial data such that the solutions exist for all Schwarzschild time t ? (-?, ?). In the present article we review the results in [1]. © 2014 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
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19.
  • Andreasson, Håkan, 1966-, et al. (författare)
  • Bounds on M/R for charged objects with positive cosmological constant
  • 2012
  • Ingår i: Classical and Quantum Gravity. - 0264-9381. ; 29:9
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider charged spherically symmetric static solutions of the Einstein-Maxwell equations with a positive cosmological constant Lambda. If r denotes the area radius, m(g) and q the gravitational mass and charge of a sphere with area radius r respectively, we find that for any solution which satisfies the condition p + 2p(perpendicular to) +/- = 0 and p(perpendicular to) are the radial and tangential pressures respectively, rho >= 0 is the energy density, and for which 0
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20.
  • Andreasson, Håkan, 1966-, et al. (författare)
  • Bounds on M/R for static objects with a positive cosmological constant
  • 2009
  • Ingår i: Classical and Quantum Gravity. ; 26
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider spherically symmetric static solutions of the Einstein equations with a positive cosmological constant Λ, which are regular at the centre, and we investigate the influence of Λ on the bound of M/R, where M is the ADM mass and R is the area radius of the boundary of the static object. We find that for any solution which satisfies the energy condition p + 2p ⊥ ≤ ρ, where p ≥ 0 and p ⊥ are the radial and tangential pressures respectively, and ρ ≥ 0 is the energy density, and for which 0 ≤ ΛR 2 ≤ 1, the inequality holds. If Λ = 0, it is known that infinitely thin shell solutions uniquely saturate the inequality, i.e. the inequality is sharp in that case. The situation is quite different if Λ > 0. Indeed, we show that infinitely thin shell solutions do not generally saturate the inequality except in the two degenerate situations ΛR 2 = 0 and ΛR 2 = 1. In the latter situation there is also a constant density solution, where the exterior spacetime is the Nariai solution, which saturates the inequality; hence, the saturating solution is non-unique. In this case the cosmological horizon and the black hole horizon coincide. This is analogous to the charged situation where there is numerical evidence that uniqueness of the saturating solution is lost when the inner and outer horizons of the Reissner-Nordström solution coincide. © 2009 IOP Publishing Ltd.
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