Large-scale magnetic fields in cosmology (2009)
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...
On the Stability of Static Ghost Cosmologies (2009)
Barrow, John D, Tsagas, Christos G
We consider the linear stability of a static universe filled with a non-interacting mixture of isotropic radiation and a ghost scalar field. Unlike the conventional Einstein static model, this...
Large-scale peculiar motions and cosmic acceleration (2009)
Observers with small, but finite, peculiar velocities relative to the Hubble flow can measure a negative deceleration parameter, although the universe may be actually decelerating. The effect is...
Slow decay of magnetic fields in open Friedmann universes (2008)
Barrow, John D., Tsagas, Christos G.
We describe how magnetic fields in Friedmann universes can experience superadiabatic growth without departing from conventional electromagnetism. The reason is the relativistic coupling between...
Generalized Ohm's law for relativistic plasmas (2007)
Kandus, Alejandra, Tsagas, Christos G.
We generalise the relativistic expression of Ohm's law by studying a multi-fluid system of charged species using the 1+3 covariant formulation of general relativistic electrodynamics. This is done by...
A Gauge-invariant Analysis of Magnetic Fields in General Relativistic Cosmology (2007)
Christos G. Tsagas, John D. Barrow
We provide a fully general-relativistic treatment of cosmological perturbations in a universe permeated by a large-scale primordial magnetic field using the Ellis-Bruni gauge-invariant formalism. The...
Relativistic cosmology and large-scale structure (2007)
Tsagas, Christos G., Challinor, Anthony, Maartens, Roy
General relativity marked the beginning of modern cosmology and it has since been at the centre of many of the key developments in this field. In the present review, we discuss the...
On constraint-consistency, covariant operators, gauge-invariance, etc (2007)
We look at the covariant techniques and the ideas on constraints and gauge-invariance, which were recently employed in [gr-qc/0702104] to support earlier work by the same authors. That work was...
Galaxy Formation and Cosmic-Ray Acceleration in a Magnetized Universe (2006)
Vlahos, L., Tsagas, Christos G., Papadopoulos, Demetrios
We study the linear magnetohydrodynamic behavior of a Newtonian cosmology with a viscous magnetized fluid of finite conductivity and generalize the Jeans instability criterion. The presence of the...
Magnetic field amplification in cold dark matter anisotropic collapse (2006)
Bruni, Marco, Maartens, Roy, Tsagas, Christos G.
We use the Zel'dovich approximation to analyse the amplification of magnetic fields in gravitational collapse of cold dark matter during the mildly non-linear regime, and identify two key features....
Magnetic Tension and the Geometry of the Universe (2006)
The vector nature of magnetic fields and the geometrical interpretation of gravity introduced by general relativity guarantee a special coupling between magnetism and spacetime curvature. This...
Geometrical Aspects of Cosmic Magnetic Fields (2006)
We discuss how the vector nature of magnetic fields, and the geometrical interpretation of gravity introduced by general relativity, lead to a special coupling between magnetism and spacetime...
Relativistic approach to nonlinear peculiar velocities and the Zeldovich approximation (2006)
Ellis, George F., Tsagas, Christos G.
We study the peculiar motion of nonrelativistic matter in a fully covariant way. The exact nonlinear equations are derived and then applied to the case of pressure-free matter, moving relatively to a...
Barrow, John D., Tsagas, Christos G.
We consider the brane-world generalization of the Gφdel universe and analyze its dynamical interaction with the bulk. The exact homogeneity of the standard Gφdel spacetime no longer holds, unless...
Cosmological Perturbations (2006)
The aim of these lecture notes is to familiarize graduate students and beginning postgraduates with the basic ideas of linear cosmological perturbation theory and of structure formation scenarios. We...
Magnetized gravitational waves (2006)
Maartens, Roy, Tsagas, Christos G., Ungarelli, Carlo
We investigate the influence of cosmic magnetic fields on gravitational wave perturbations and find exact solutions on large scales. We show that a large-scale magnetic field can generate large-scale...
Cosmic magnetism, curvature, and the expansion dynamics (2006)
Matravers, David R., Tsagas, Christos G.
We discuss how a cosmological magnetic field could affect the expansion of the universe through its interaction with the spacetime geometry. The tension of the field lines means that the...
Magnetized cosmological perturbations (2006)
Tsagas, Christos G., Maartens, Roy
A large-scale cosmic magnetic field affects not only the growth of density perturbations, but also rotational instabilities and anisotropic deformation in the density distribution. We give a fully...
Superadiabatic-type magnetic amplification in conventional cosmology (2006)
Tsagas, Christos G., Kandus, Alejandra
We consider the evolution of cosmological magnetic fields in Friedmann-Robertson-Walker models and outline a geometrical mechanism for their superadiabatic amplification on large scales. The...
Resonant amplification of magnetic seed fields by gravitational waves in the early universe (2006)
Inflation is known to produce both gravitational waves and seed magnetic fields on scales well beyond the size of the horizon. The general relativistic study of the interaction between these two...
Averaging anisotropic cosmologies (2006)
Barrow, John D., Tsagas, Christos G.
We examine the effects of spatial inhomogeneities on irrotational anisotropic cosmologies by looking at the average properties of anisotropic pressure-free models. Adopting the Buchert scheme, we...
Cosmology in three dimensions: steps towards the general solution (2006)
Barrow, John D., Shaw, Douglas J., Tsagas, Christos G.
We use covariant and first-order formalism techniques to study the properties of general relativistic cosmology in three dimensions. The covariant approach provides an irreducible decomposition of...
Galaxy Formation and Cosmic-Ray Acceleration in a Magnetized Universe (2006)
Tsagas, Christos G., Papadopoulos, Demetrios
We study the linear magnetohydrodynamic behavior of a Newtonian cosmology with a viscous magnetized fluid of finite conductivity and generalize the Jeans instability criterion. The presence of the...
Geometrical Generation of Cosmic Magnetic Fields within Standard Electromagnetism (2006)
Christos G. Tsagas, Alejandra Kandus
Geometrical Generation of Cosmic Magnetic Fields within Standard Electromagnetism
Geometrical generation of cosmic magnetic fields within standard electromagnetism (2005)
Tsagas,Christos G., Kandus,Alejandra
We study the evolution of cosmological magnetic fields in FRW models with curved spatial sections and outline a geometrical mechanism for their superadiabatic amplification on large scales. The...
Magnetized Tolman-Bondi Collapse (2005)
Germani, Cristiano, Tsagas, Christos G.
We investigate the gravitational implosion of magnetized matter by studying the inhomogeneous collapse of a weakly magnetized Tolman-Bondi spacetime. The role of the field is analyzed by looking at...
Resonant amplification of magnetic seed fields by gravitational waves in the early universe (2005)
Inflation is known to produce both gravitational waves and seed magnetic fields on scales well beyond the size of the horizon. The general relativistic interaction between these two sources at the...
Superadiabatic-type magnetic amplification in conventional cosmology (2005)
Tsagas, Christos G., Kandus, Alejandra
We consider the evolution of cosmological magnetic fields in FRW models and outline a geometrical mechanism for their superadiabatic amplification on large scales. The mechanism operates within...
Primordial magnetic seed field amplification by gravitational waves: comment on gr-qc/0503006 (2005)
We consider the amplification of cosmological magnetic fields by gravitational waves as it was recently presented in [gr-qc/0503006]. That study confined to infinitely conductive environments,...
Structure and stability of the Lukash plane-wave spacetime (2004)
Barrow, John D., Tsagas, Christos G.
We study the vacuum, plane-wave Bianchi $VII{}_{h}$ spacetimes described by the Lukash metric. Combining covariant with orthonormal frame techniques, we describe these models in terms of their...
New Isotropic and Anisotropic Sudden Singularities (2004)
Barrow, John D., Tsagas, Christos G.
We show the existence of an infinite family of finite-time singularities in isotropically expanding universes which obey the weak, strong, and dominant energy conditions. We show what new type of...
Electromagnetic fields in curved spacetimes (2004)
We consider the evolution of electromagnetic fields in curved spacetimes and calculate the exact wave equations of the associated electric and magnetic components. Our analysis is fully covariant,...
Magnetic tension and gravitational collapse (2003)
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....
Barrow, John D., Tsagas, Christos G.
We consider the brane-world generalisation of the Godel universe and analyse its dynamical interaction with the bulk. The exact homogeneity of the standard Godel spacetime no longer holds, unless the...
Dynamics and stability of the Godel universe (2003)
Barrow, John D., Tsagas, Christos G.
We use covariant techniques to describe the properties of the Godel universe and then consider its linear response to a variety of perturbations. Against matter aggregations, we find that the...
The Emergent Universe: An Explicit Construction (2003)
Ellis, George F. R., Murugan, Jeff, Tsagas, Christos G.
We provide a realisation of a singularity-free inflationary universe in the form of a simple cosmological model dominated at early times by a single minimally coupled scalar field with a physically...
A relativistic approach to nonlinear peculiar velocities and the Zeldovich approximation (2002)
Ellis, George F. R., Tsagas, Christos G.
We study the peculiar motion of non-relativistic matter in a fully covariant way. The exact nonlinear equations are derived and then applied to the case of pressure-free matter, moving relatively to...
Gravitational waves and cosmic magnetism; a cosmological approach (2002)
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....
Cosmological Perturbations (2002)
The aim of these lecture notes is to familiarize graduate students and beginning postgraduates with the basic ideas of linear cosmological perturbation theory and of structure formation scenarios. We...
Geometrical Aspects of Cosmic Magnetic Fields (2001)
We discuss how the vector nature of magnetic fields, and the geometrical interpretation of gravity introduced by general relativity, lead to a special coupling between magnetism and spacetime...
Magnetic tension and the geometry of the universe (2000)
The vector nature of magnetic fields and the geometrical interpretation of gravity introduced by general relativity, guarantee a special coupling between magnetism and spacetime curvature. This...
Gauge-invariant magnetic perturbations in perfect-fluid cosmologies (1998)
Tsagas, Christos G., Barrow, John D.
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...
Thesis (Ph. D.)--University of Sussex, 1998.
A Gauge-invariant Analysis of Magnetic Fields in General Relativistic Cosmology (1997)
Tsagas, Christos G., Barrow, John D.
We provide a fully general-relativistic treatment of cosmological perturbations in a universe permeated by a large-scale primordial magnetic field, using the Ellis-Bruni gauge-invariant formalism....