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Forsberg, Mats
Publications (10 of 11) Show all publications
Bradley, M., Forsberg, M. & Keresztes, Z. (2017). Gravitational Waves in Locally Rotationally Symmetric (LRS) Class II Cosmologies. Universe, 3(4), Article ID 69.
Open this publication in new window or tab >>Gravitational Waves in Locally Rotationally Symmetric (LRS) Class II Cosmologies
2017 (English)In: Universe, ISSN 2218-1997, Vol. 3, no 4, article id 69Article in journal (Refereed) Published
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.                

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-143433 (URN)10.3390/universe3040069 (DOI)000419173800003 ()
Available from: 2017-12-31 Created: 2017-12-31 Last updated: 2018-06-09Bibliographically approved
Bradley, M., Forsberg, M., Keresztes, Z., Gergely, L. A. & Dunsby, P. K. (2015). Perturbations of Kantowski-Sachs models. In: Robert T Jantzen, Kjell Rosquist, Remo Ruffini (Ed.), Proceedings of the Thirteenth Marcel Grossman Meeting on General Relativity: . Paper presented at The Thirteenth Marcel Grossman Meeting on General Relativity, July 1 - 7 2012, Stockholm, Sweden (pp. 2547-2549). Singapore: World Scientific
Open this publication in new window or tab >>Perturbations of Kantowski-Sachs models
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2015 (English)In: 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, Published 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.

Place, publisher, year, edition, pages
Singapore: World Scientific, 2015
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:umu:diva-116120 (URN)9789814612142 (ISBN)
Conference
The Thirteenth Marcel Grossman Meeting on General Relativity, July 1 - 7 2012, Stockholm, Sweden
Available from: 2016-02-08 Created: 2016-02-08 Last updated: 2018-06-07Bibliographically approved
Keresztes, Z., Forsberg, M., Bradley, M., Dunsby, P. K. & Gergely, L. Á. (2014). Perturbations of Kantowski-Sachs Models with a Cosmological Constant. In: Alfonso García-Parrado, Filipe C. Mena, Filipe Moura, Estelita Vaz (Ed.), Progress in Mathematical Relativity, Gravitation and Cosmology: Proceedings, Spanish Relativity Meeting ERE2012, University of Minho, Guimarães, Portugal, September 3-7, 2012. Paper presented at Spanish Relativity Meeting ERE2012, University of Minho, Guimarães, Portugal, September 3-7, 2012 (pp. 289-293). Heidelberg: Springer Berlin/Heidelberg
Open this publication in new window or tab >>Perturbations of Kantowski-Sachs Models with a Cosmological Constant
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2014 (English)In: 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, Published 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.

Place, publisher, year, edition, pages
Heidelberg: Springer Berlin/Heidelberg, 2014
Series
Springer Proceedings in Mathematics and Statistics, ISSN 2194-1009 ; 60
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:umu:diva-93379 (URN)10.1007/978-3-642-40157-2_40 (DOI)978-3-642-40156-5 (ISBN)
Conference
Spanish Relativity Meeting ERE2012, University of Minho, Guimarães, Portugal, September 3-7, 2012
Available from: 2014-09-18 Created: 2014-09-18 Last updated: 2018-06-07Bibliographically approved
Bradley, M., Dunsby, P. K., Forsberg, M. & Keresztes, Z. (2012). Density growth in Kantowski-Sachs cosmologies with cosmological constant. Classical and quantum gravity, 29(9), 095023
Open this publication in new window or tab >>Density growth in Kantowski-Sachs cosmologies with cosmological constant
2012 (English)In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 29, no 9, p. 095023-Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Bristol: Institute of Physics (IOP), 2012
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-54575 (URN)10.1088/0264-9381/29/9/095023 (DOI)000303105600024 ()
Available from: 2012-04-30 Created: 2012-04-30 Last updated: 2018-06-08Bibliographically approved
Forsberg, M., Bradley, M. & Dunsby, P. K. .. (2011). Density perturbations in Kantowski-Sachs models with a cosmological constant. In: Víctor Aldaya, Carlos Barceló and José Luis Jaramillo (Ed.), Spanish Relativity Meeting (ERE 2010): Gravity as a Crossroad in Physics: . Paper presented at Spanish Relativity Meeting (ERE 2010): Gravity as a Crossroad in Physics, Granada, 2010. Institute of Physics (IOP)
Open this publication in new window or tab >>Density perturbations in Kantowski-Sachs models with a cosmological constant
2011 (English)In: 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, Published 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.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2011
Series
Journal of Physics: Conference Series ; 314
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-47616 (URN)10.1088/1742-6596/314/1/012026 (DOI)
Conference
Spanish Relativity Meeting (ERE 2010): Gravity as a Crossroad in Physics, Granada, 2010
Available from: 2011-09-26 Created: 2011-09-26 Last updated: 2019-04-25Bibliographically approved
Forsberg, M. (2010). Gravitational perturbations in plasmas and cosmology. (Doctoral dissertation). Umeå: Institutionen för Fysik
Open this publication in new window or tab >>Gravitational perturbations in plasmas and cosmology
2010 (English)Doctoral 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.

Place, publisher, year, edition, pages
Umeå: Institutionen för Fysik, 2010. p. 48
Keywords
general relativity, cosmology, gravitational waves, plasma, electromagnetic waves, nonlinear waves
National Category
Fusion, Plasma and Space Physics Other Physics Topics Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:umu:diva-38387 (URN)978-91-7459-125-5 (ISBN)
Public defence
2011-01-04, Naturvetarhuset, N430, Umeå Universitet, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2010-12-14 Created: 2010-12-10 Last updated: 2018-06-08Bibliographically approved
Forsberg, M., Papadopoulos, D. & Brodin, G. (2010). Influence of strong field vacuum polarization on gravitational-electromagnetic wave interaction. Physical Review D. Particles and fields, 82(2), 024001-024001-7
Open this publication in new window or tab >>Influence of strong field vacuum polarization on gravitational-electromagnetic wave interaction
2010 (English)In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 82, no 2, p. 024001-024001-7Article in journal (Refereed) Published
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.

Identifiers
urn:nbn:se:umu:diva-38385 (URN)10.1103/PhysRevD.82.024001 (DOI)000279474100001 ()
Available from: 2010-12-10 Created: 2010-12-10 Last updated: 2018-06-08Bibliographically approved
Brodin, G., Forsberg, M., Marklund, M. & Eriksson, D. (2010). Interaction between gravitational waves and plasma waves in the Vlasov description. Journal of Plasma Physics (76), 345-353
Open this publication in new window or tab >>Interaction between gravitational waves and plasma waves in the Vlasov description
2010 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, no 76, p. 345-353Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Cambridge University Press, 2010
Identifiers
urn:nbn:se:umu:diva-38386 (URN)10.1017/S0022377809990535 (DOI)000278169300013 ()
Available from: 2010-12-10 Created: 2010-12-10 Last updated: 2018-06-08Bibliographically approved
Forsberg, M. & Brodin, G. (2010). Linear theory of gravitational wave propagation in a magnetized, relativistic Vlasov plasma. Physical Review D, 82(12), Article ID 124029.
Open this publication in new window or tab >>Linear theory of gravitational wave propagation in a magnetized, relativistic Vlasov plasma
2010 (English)In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 82, no 12, article id 124029Article in journal (Refereed) Published
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.

Keywords
magnetosonic waves, parametric-excitation, radiation, detector, fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-109560 (URN)10.1103/PhysRevD.82.124029 (DOI)000286743000005 ()
Available from: 2015-10-07 Created: 2015-09-30 Last updated: 2018-06-07Bibliographically approved
Forsberg, M. & Brodin, G. (2008). Harmonic generation of gravitational wave induced Alfven waves. Physical Review D. Particles and fields, 77(2), 024050-024050-6
Open this publication in new window or tab >>Harmonic generation of gravitational wave induced Alfven waves
2008 (English)In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 77, no 2, p. 024050-024050-6Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Physical Society, 2008
Identifiers
urn:nbn:se:umu:diva-10375 (URN)10.1103/PhysRevD.77.024050 (DOI)
Available from: 2008-08-27 Created: 2008-08-27 Last updated: 2018-06-09Bibliographically approved
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