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  • 1.
    Bradley, Michael
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dunsby, Peter K S
    Dept. of Mathematics and Applied Mathematics, University of Cape Town.
    Forsberg, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Keresztes, Zoltán
    Dept. of Experimental Physics, University of Szeged.
    Density growth in Kantowski-Sachs cosmologies with cosmological constant2012In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 29, no 9, p. 095023-Article in journal (Refereed)
    Abstract [en]

    In this work the growth of density perturbations in Kantowski-Sachs cosmologies with a positive cosmological constant is studied, using the 1+3 and 1+1+2 covariant formalisms. For each wave number we obtain a closed system for scalars formed from quantities that are zero on the background and hence are gauge-invariant. The solutions to this system are then analyzed both analytically andnumerically. In particular the effects of anisotropy and the behaviour close to a bounce in the cosmic scale factor are considered.We find that typically the density gradient in the bouncing direction experiences a local maximum at or slightly after the bounce.

  • 2.
    Bradley, Michael
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Forsberg, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Keresztes, Zoltán
    Gravitational Waves in Locally Rotationally Symmetric (LRS) Class II Cosmologies2017In: Universe, ISSN 2218-1997, Vol. 3, no 4, article id 69Article in journal (Refereed)
    Abstract [en]

    In this work we consider perturbations of homogeneous and hypersurface orthogonal cosmological backgrounds with local rotational symmetry (LRS), using a method based on the 1 + 1 + 2 covariant split of spacetime. The backgrounds, of LRS class II, are characterised by that the vorticity, the twist of the 2-sheets, and the magnetic part of the Weyl tensor all vanish. They include the flat Friedmann universe as a special case. The matter contents of the perturbed spacetimes are given by vorticity-free perfect fluids, but otherwise the perturbations are arbitrary and describe gravitational, shear, and density waves. All the perturbation variables can be given in terms of the time evolution of a set of six harmonic coefficients. This set decouples into one set of four coefficients with the density perturbations acting as source terms, and another set of two coefficients describing damped source-free gravitational waves with odd parity. We also consider the flat Friedmann universe, which has been considered by several others using the 1 + 3 covariant split, as a check of the isotropic limit. In agreement with earlier results we find a second-order wavelike equation for the magnetic part of the Weyl tensor which decouples from the density gradient for the flat Friedmann universes. Assuming vanishing vector perturbations, including the density gradient, we find a similar equation for the electric part of the Weyl tensor, which was previously unnoticed.                

  • 3.
    Bradley, Michael
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Forsberg, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Keresztes, Zoltán
    University of Szeged.
    Gergely, László A´
    University of Szeged.
    Dunsby, Peter K S
    University of Cape Town.
    Perturbations of Kantowski-Sachs models2015In: Proceedings of the Thirteenth Marcel Grossman Meeting on General Relativity / [ed] Robert T Jantzen, Kjell Rosquist, Remo Ruffini, Singapore: World Scientific, 2015, p. 2547-2549Conference paper (Refereed)
    Abstract [en]

    Perturbations of Kantowski-Sachs models with a positive cosmological constant are considered in a harmonic decomposition, in the framework of gauge invariant 1+3 and 1+1+2covariant splits of spacetime. Scalar, vector and tensor modes are allowed, however they remain vorticity-free and of perfect fluid type. The dynamics is encompassed in six evolution equations for six harmonic coefficients.

  • 4.
    Brodin, Gert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Forsberg, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eriksson, Daniel
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Interaction between gravitational waves and plasma waves in the Vlasov description2010In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, no 76, p. 345-353Article in journal (Refereed)
    Abstract [en]

    The nonlinear interaction between electromagnetic, electrostatic and gravitational waves in a Vlasov plasma is reconsidered. By using a orthonormal tetrad description the three-wave coupling coefficients are computed. Comparing with previous results, it is found that the present theory leads to algebraic expression that are much reduced, as compared to those computed using a coordinate frame formalism. Furthermore, here we calculate the back reaction on the gravitational waves, and a simple energy conservation law is deduced in the limit of a cold plasma.

  • 5.
    Forsberg, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gravitational perturbations in plasmas and cosmology2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Gravitational perturbations can be in the form of scalars, vectors or tensors. This thesis focuses on the evolution of scalar perturbations in cosmology, and interactions between tensor perturbations, in the form of gravitational waves, and plasma waves.

    The gravitational waves studied in this thesis are assumed to have small amplitudes and wavelengths much shorter than the background length scale, allowing for the assumption of a flat background metric. Interactions between gravitational waves and plasmas are described by the Einstein-Maxwell-Vlasov, or the Einstein-Maxwell-fluid equations, depending on the level of detail required. Using such models, linear wave excitation of various waves by gravitational waves in astrophysical plasmas are studied, with a focus on resonance effects. Furthermore, the influence of strong magnetic field quantum electrodynamics, leading to detuning of the gravitational wave-electromagnetic wave resonances, is considered. Various nonlinear phenomena, including parametric excitation and wave steepening are also studied in different astrophysical settings.

    In cosmology the evolution of gravitational perturbations are of interest in processes such as structure formation and generation of large scale magnetic fields. Here, the growth of density perturbations in Kantowski-Sachs cosmologies with positive cosmological constant is studied.

  • 6.
    Forsberg, Mats
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bradley, Michael
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dunsby, Peter K.S.
    University of Cape Town, South Africa.
    Density perturbations in Kantowski-Sachs models with a cosmological constant2011In: Spanish Relativity Meeting (ERE 2010): Gravity as a Crossroad in Physics / [ed] Víctor Aldaya, Carlos Barceló and José Luis Jaramillo, Institute of Physics (IOP), 2011Conference paper (Refereed)
    Abstract [en]

    The growth of density perturbations in Kantowski-Sachs cosmologies with a positive cosmological constant is studied, using the 1+3 and 1+1+2 covariant formalisms. For each wave number a closed system for eight scalars is obtained. These are formed from quantities that are zero on the background and hence are gauge invariant. As an example a numerical solution describing the evolution of density perturbations on a background that experiences a bounce is presented. Typically the density gradient in the bouncing directions experiences a local maxium at or slightly after the bounce.

  • 7.
    Forsberg, Mats
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Harmonic generation of gravitational wave induced Alfven waves2008In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 77, no 2, p. 024050-024050-6Article in journal (Refereed)
    Abstract [en]

    Here we consider the nonlinear evolution of Alfvén waves that have been excited by gravitational waves from merging binary pulsars. We derive a wave equation for strongly nonlinear and dispersive Alfvén waves. Because of the weak dispersion of the Alfvén waves, significant wave steepening can occur, which in turn implies strong harmonic generation. We find that the harmonic generation is saturated due to dispersive effects, and use this to estimate the resulting spectrum. Finally we discuss the possibility of observing the above process.

  • 8.
    Forsberg, Mats
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Linear theory of gravitational wave propagation in a magnetized, relativistic Vlasov plasma2010In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 82, no 12, article id 124029Article in journal (Refereed)
    Abstract [en]

    We consider propagation of gravitational waves in a magnetized plasma, using the linearized Maxwell-Vlasov equations coupled to Einstein's equations. A set of coupled electromagnetic-gravitational wave equations are derived that can be straightforwardly reduced to a single dispersion relation. We demonstrate that there is a number of different resonance effects that can enhance the influence of the plasma on the gravitational waves.

  • 9.
    Forsberg, Mats
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shukla, Padma K.
    Institut für Theoretische Physik IV and Centre for Plasma Science and Astrophysics, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany.
    Moortgat, J.
    Department of Physics and Astronomy, University of Rochester, Bausch & Lomb Hall, P.O. Box 270171, 600 Wilson Boulevard, Rochester, New York 14627-0171, USA .
    Nonlinear interactions between gravitational radiation and modified Alfvén modes in astrophysical dusty plasmas2006In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 74, p. 064014-064014-5Article in journal (Refereed)
    Abstract [en]

    We present an investigation of nonlinear interactions between gravitational radiation and modified Alfvén modes in astrophysical dusty plasmas. Assuming that stationary charged dust grains form neutralizing background in an electron-ion-dust plasma, we obtain the three-wave coupling coefficients and calculate the growth rates for parametrically coupled gravitational radiation and modified Alfvén-Rao modes. The threshold value of the gravitational wave amplitude associated with convective stabilization is particularly small if the gravitational frequency is close to twice the modified Alfvén wave frequency. The implication of our results to astrophysical dusty plasmas is discussed.

  • 10.
    Forsberg, Mats
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Papadopoulos, Demetrios
    Department of Physics, Section of Astrophysics, Astronomy and Mechanics, Thessaloniki, Greece.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Influence of strong field vacuum polarization on gravitational-electromagnetic wave interaction2010In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 82, no 2, p. 024001-024001-7Article in journal (Refereed)
    Abstract [en]

    The interaction between gravitational and electromagnetic waves in the presence of a static magnetic field is studied. The field strength of the static field is allowed to surpass the Schwinger critical field, such that the QED effects of vacuum polarization and magnetization are significant. Equations governing the interaction are derived and analyzed. It turns out that the energy conversion from gravitational to electromagnetic waves can be significantly altered due to the QED effects. The consequences of our results are discussed.

  • 11.
    Keresztes, Zoltán
    et al.
    University of Szeged.
    Forsberg, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bradley, Michael
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dunsby, Peter K S
    University of Cape Town.
    Gergely, László Á
    University of Szged.
    Perturbations of Kantowski-Sachs Models with a Cosmological Constant2014In: Progress in Mathematical Relativity, Gravitation and Cosmology: Proceedings, Spanish Relativity Meeting ERE2012, University of Minho, Guimarães, Portugal, September 3-7, 2012 / [ed] Alfonso García-Parrado, Filipe C. Mena, Filipe Moura, Estelita Vaz, Heidelberg: Springer Berlin/Heidelberg, 2014, p. 289-293Conference paper (Refereed)
    Abstract [en]

    We investigate perturbations of Kantowski–Sachs models with a positive cosmological constant, using the gauge invariant 1 + 3 and 1 + 1 + 2 covariant splits of spacetime together with a harmonic decomposition. The perturbations are assumed to be vorticity-free and of perfect fluid type, but otherwise include general scalar, vector and tensor modes. In this case the set of equations can be reduced to six evolution equations for six harmonic coefficients.

1 - 11 of 11
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