| 1. |
- Aghapour, Sajad, et al.
(författare)
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Helicity, spin, and infra-zilch of light : A Lorentz covariant formulation
- 2021
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Ingår i: Annals of Physics. - : Elsevier BV. - 0003-4916 .- 1096-035X. ; 431
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a novel conserved Lorentz covariant tensor, termed the helicity tensor, is introduced in Maxwell theory. The conservation of the helicity tensor expresses the conservation laws contained in the helicity array, introduced by Cameron et al. (2012), including helicity, spin, as well as the spin-flux or infra zilch. The Lorentz covariance of the helicity tensor is in contrast to previous formulations of the helicity hierarchy of conservation laws, which required the non-Lorentz covariant transverse gauge. The helicity tensor is shown to arise as a Noether current for a variational symmetry of a duality-symmetric Lagrangian for Maxwell theory. This symmetry transformation generalizes the duality symmetry and includes the symmetry underlying the conservation of the spin part of the angular momentum.
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| 2. |
- Aghapour, Sajad, et al.
(författare)
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The zilch electromagnetic conservation law revisited
- 2020
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Ingår i: Journal of Mathematical Physics. - : AIP Publishing. - 0022-2488 .- 1089-7658. ; 61:12
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Tidskriftsartikel (refereegranskat)abstract
- It is shown that the zilch conservation law arises as the Noether current corresponding to a variational symmetry of a duality-symmetric Maxwell Lagrangian. The action of the corresponding symmetry generator on the duality-symmetric Lagrangian, while non-vanishing, is a total divergence as required by the Noether theory. The variational nature of the zilch conservation law was previously known only for some of the components of the zilch tensor, notably the optical chirality. By contrast, our analysis is fully covariant and is, therefore, valid for all components of the zilch tensor. The analysis is presented here for both the real and complex versions of duality-symmetric Maxwell Lagrangians.
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| 3. |
- Bentivegna, Eloisa, et al.
(författare)
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Black-hole lattices as cosmological models
- 2018
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 35:17
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Tidskriftsartikel (refereegranskat)abstract
- The search for solutions of Einstein's equations representing relativistic cosmological models with a discrete matter content has been remarkably fruitful in the last decade. In this review we discuss the progress made in the study of a specific subclass of discrete cosmologies, black-hole lattice models. In particular, we illustrate the techniques used for the construction of these spacetimes, and examine their resulting physical properties. This includes their large-scale dynamics, the dressing of mass due to the interaction between individual black holes, along with features of direct observational interest such as the distance-to-redshift relation. This collection of results provides a novel perspective on the physical effects of averaging in general relativity, as well as on the emergence of gravitational structures from solutions with isolated objects.
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| 4. |
- Bini, Donato, et al.
(författare)
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Effects of friction forces on the motion of objects in smoothly matched interior/exterior spacetimes
- 2013
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Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 30:2, s. 025009-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the non-geodesic motion of a test particle inside a fluid in equilibrium, as described by the interior Schwarzschild solution, in the presence of a Poynting-Robertson-like dissipative force. The accelerated motion in the interior spacetime is (regularly) connected to the geodesic one in the corresponding external Schwarzschild background, mimicking physical conditions often occurring in spacetime regions close to stellar or compact objects. A similar analysis is repeated for the case in which the interior Schwarzschild solution is given by a Pant-Sah perfect-fluid solution. This latter case, supporting a polytropic equation of state for the interior fluid source, appears to be more appropriate for modeling situations of either stellar or cosmological interest.
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| 5. |
- Bini, Donato, et al.
(författare)
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Particle motion in a photon gas : friction matters
- 2012
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Ingår i: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 44:10, s. 2669-2680
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Tidskriftsartikel (refereegranskat)abstract
- The motion of a particle in the Tolman metric generated by a photon gas source is discussed. Both the case of geodesic motion and motion with nonzero friction, due to photon scattering effects, are analyzed. In the Minkowski limit, the particle moves along a straight line segment with a decelerated motion, reaching the endpoint at zero speed. The curved case shows a qualitatively different behavior; the geodesic motion consists of periodic orbits, confined within a specific radial interval. Under the effect of frictional drag, this radial interval closes up in time and in all our numerical simulations the particle ends up in the singularity at the center.
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| 6. |
- Bini, Donato, et al.
(författare)
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Spinning particles in twisted gravitational wave spacetimes
- 2018
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 98:2
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Tidskriftsartikel (refereegranskat)abstract
- Twisted gravitational waves (TGWs) are nonplanar waves with twisted rays that move along a fixed direction in space. We study further the physical characteristics of a recent class of Ricci-flat solutions of general relativity representing TGWs with wave fronts that have negative Gaussian curvature. In particular, we investigate the influence of TGWs on the polarization of test electromagnetic waves and on the motion of classical spinning test particles in such radiation fields. To distinguish the polarization effects of twisted waves from plane waves, we examine the theoretical possibility of existence of spin-twist coupling and show that this interaction is generally consistent with our results.
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| 7. |
- Bäckdahl, Thomas, 1980-
(författare)
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Multipole moments of axisymmetric spacetimes
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we study multipole moments of axisymmetric spacetimes. Using the recursive definition of the multipole moments of Geroch and Hansen we develop a method for computing all multipole moments of a stationary axisymmetric spacetime without the use of a recursion. This is a generalisation of a method developed by Herberthson for the static case.Using Herberthson’s method we also develop a method for finding a static axisymmetric spacetime with arbitrary prescribed multipole moments, subject to a specified convergence criteria. This method has, in general, a step where one has to find an explicit expression for an implicitly defined function. However, if the number of multipole moments are finite we give an explicit expression in terms of power series.
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| 8. |
- Chicone, C., et al.
(författare)
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Cosmic jets
- 2011
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Ingår i: Physics Letters A. - : Elsevier BV. - 0375-9601 .- 1873-2429. ; 375:12, s. 1427-1430
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Tidskriftsartikel (refereegranskat)abstract
- We discuss time-dependent gravitational fields that accelerate free test particles to the speed of light resulting in cosmic double-jet configurations. It turns out that complete gravitational collapse along a spatial axis together with corresponding expansion along the other two axes leads to the accelerated motion of free test particles up and down parallel to the collapse axis such that a double-jet pattern is asymptotically formed with respect to the collapsed configuration.
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| 9. |
- Chicone, C., et al.
(författare)
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Double-Kasner spacetime : Peculiar velocities and cosmic jets
- 2011
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 83:12, s. 124029-
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Tidskriftsartikel (refereegranskat)abstract
- In dynamic spacetimes in which asymmetric gravitational collapse/expansion is taking place, the timelike geodesic equation appears to exhibit an interesting property: Relative to the collapsing configuration, free test particles undergo gravitational acceleration and form a double-jet configuration parallel to the axis of collapse. We illustrate this aspect of peculiar motion in simple spatially homogeneous cosmological models such as the Kasner spacetime. To estimate the effect of spatial inhomogeneities on cosmic jets, timelike geodesics in the Ricci-flat double-Kasner spacetime are studied in detail. While spatial inhomogeneities can significantly modify the structure of cosmic jets, we find that under favorable conditions the double-jet pattern can initially persist over a finite period of time for sufficiently small inhomogeneities.
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| 10. |
- Clifton, Timothy, et al.
(författare)
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An exact quantification of backreaction in relativistic cosmology
- 2012
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 86:4, s. 043506-
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Tidskriftsartikel (refereegranskat)abstract
- An important open question in cosmology is the degree to which the Friedmann-Lemaitre-Robertson-Walker (FLRW) solutions of Einstein's equations are able to model the large-scale behavior of the locally inhomogeneous observable universe. We investigate this problem by considering a range of exact n-body solutions of Einstein's constraint equations. These solutions contain discrete masses, and so allow arbitrarily large density contrasts to be modeled. We restrict our study to regularly arranged distributions of masses in topological 3-spheres. This has the benefit of allowing straightforward comparisons to be made with FLRW solutions, as both spacetimes admit a discrete group of symmetries. It also provides a time-symmetric hypersurface at the moment of maximum expansion that allows the constraint equations to be solved exactly. We find that when all the mass in the universe is condensed into a small number of objects (less than or similar to 10) then the amount of back-reaction in dust models can be large, with O(1) deviations from the predictions of the corresponding FLRW solutions. When the number of masses is large (greater than or similar to 100), however, then our measures of back-reaction become small (less than or similar to 1%). This result does not rely on any averaging procedures, which are notoriously hard to define uniquely in general relativity, and so provides (to the best of our knowledge) the first exact and unambiguous demonstration of back-reaction in general relativistic cosmological modelling. Discrete models such as these can therefore be used as laboratories to test ideas about back-reaction that could be applied in more complicated and realistic settings.
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