114 research outputs found
Gravitational waves and cosmic magnetism; a cosmological approach
We present the formalism for the covariant treatment of gravitational
radiation in a magnetized environment and discuss the implications of the field
for gravity waves in the cosmological context. Our geometrical approach brings
to the fore the tension properties of the magnetic force lines and reveals
their intricate interconnection to the spatial geometry of a magnetised
spacetime. We show how the generic anisotropy of the field can act as a source
of gravitational wave perturbations and how, depending on the spatial curvature
distortion, the magnetic tension can boost or suppress waves passing through a
magnetized region.Comment: Minor changes. References added. To appear in Class. Quantum Gra
Large-scale magnetic fields in cosmology
Despite the widespread presence of magnetic fields, their origin, evolution
and role are still not well understood. Primordial magnetism sounds appealing
but is not problem free. The magnetic implications for the large-scale
structure of the universe still remain an open issue. This paper outlines the
advantages and shortcomings of early-time magnetogenesis and the typical role
of B-fields in linear structure-formation scenarios.Comment: Invited Talk (36th EPS Conference on Plasma Physics, 2009
Friedmann-like universes with torsion
We consider spatially homogeneous and isotropic cosmologies with non-zero
torsion. Given the high symmetry of these universes, we adopt a specific form
for the torsion tensor that preserves the homogeneity and isotropy of the
spatial surfaces. Employing both covariant and metric-based techniques, we
derive the torsional versions of the continuity, the Friedmann and the
Raychaudhuri equations. These formulae demonstrate how, by playing the role of
the spatial curvature, or that of the cosmological constant, torsion can
drastically change the evolution of the classic homogeneous and isotropic
Friedmann universes. In particular, torsion alone can lead to exponential
expansion. For instance, in the presence of torsion, the Milne and the
Einstein-de Sitter universes evolve like the de Sitter model. We also show
that, by changing the expansion rate of the early universe, torsion can affect
the primordial nucleosynthesis of helium-4. We use this sensitivity to impose
strong cosmological bounds on the relative strength of the associated torsion
field, requiring that its ratio to the Hubble expansion rate lies in the narrow
interval () around zero. Interestingly, the introduction
of torsion can \textit{reduce} the production of primordial helium-4, unlike
other changes to the standard thermal history of an isotropic universe.
Finally, turning to static spacetimes, we find that there exist torsional
analogues of the classic Einstein static universe, with all three types of
spatial geometry. These models can be stable when the torsion field and the
universe's spatial curvature have the appropriate profiles.Comment: Revised article. Section on BBN limits on torsion added. References
added and update
Gauge-invariant magnetic perturbations in perfect-fluid cosmologies
We develop further our extension of the Ellis-Bruni covariant and
gauge-invariant formalism to the general relativistic treatment of density
perturbations in the presence of cosmological magnetic fields. We present
detailed analysis of the kinematical and dynamical behaviour of perturbed
magnetized FRW cosmologies containing fluid with non-zero pressure. We study
the magnetohydrodynamical effects on the growth of density irregularities
during the radiation era. Solutions are found for the evolution of density
inhomogeneities on small and large scales in the presence of pressure, and some
new physical effects are identified.Comment: Revised version (some minor changes - few equations added). 26 pages.
No figures. To appear in Classical and Quantum Gravit
Charged multifluids in general relativity
The exact 1+3 covariant dynamical fluid equations for a multi-component
plasma, together with Maxwell's equations are presented in such a way as to
make them suitable for a gauge-invariant analysis of linear density and
velocity perturbations of the Friedmann-Robertson-Walker model. In the case
where the matter is described by a two component plasma where thermal effects
are neglected, a mode representing high-frequency plasma oscillations is found
in addition to the standard growing and decaying gravitational instability
picture. Further applications of these equations are also discussed.Comment: 14 pages (example added), to appear in Class. Quantum Gra
Magnetic tension and gravitational collapse
The gravitational collapse of a magnetised medium is investigated by studying
qualitatively the convergence of a timelike family of non-geodesic worldlines
in the presence of a magnetic field. Focusing on the field's tension we
illustrate how the winding of the magnetic forcelines due to the fluid's
rotation assists the collapse, while shear-like distortions in the distribution
of the field's gradients resist contraction. We also show that the relativistic
coupling between magnetism and geometry, together with the tension properties
of the field, lead to a magneto-curvature stress that opposes the collapse.
This tension stress grows stronger with increasing curvature distortion, which
means that it could potentially dominate over the gravitational pull of the
matter. If this happens, a converging family of non-geodesic lines can be
prevented from focusing without violating the standard energy conditions.Comment: Typos corrected. Published versio
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