12 research outputs found
Dark Energy: The Shadowy Reflection of Dark Matter?
In this article, we review a series of recent theoretical results regarding a
conventional approach to the dark energy (DE) concept. This approach is
distinguished among others for its simplicity and its physical relevance. By
compromising General Relativity (GR) and Thermodynamics at cosmological scale,
we end up with a model without DE. Instead, the Universe we are proposing is
filled with a perfect fluid of self-interacting dark matter (DM), the volume
elements of which perform hydrodynamic flows. To the best of our knowledge, it
is the first time in a cosmological framework that the energy of the cosmic
fluid internal motions is also taken into account as a source of the universal
gravitational field. As we demonstrate, this form of energy may compensate for
the DE needed to compromise spatial flatness, while, depending on the
particular type of thermodynamic processes occurring in the interior of the DM
fluid (isothermal or polytropic), the Universe depicts itself as either
decelerating or accelerating (respectively). In both cases, there is no
disagreement between observations and the theoretical prediction of the distant
supernovae (SNe) Type Ia distribution. In fact, the cosmological model with
matter content in the form of a thermodynamically-involved DM fluid not only
interprets the observational data associated with the recent history of
Universe expansion, but also confronts successfully with every major
cosmological issue (such as the age and the coincidence problems). In this way,
depending on the type of thermodynamic processes in it, such a model may serve
either for a conventional DE cosmology or for a viable alternative one.Comment: Review article, 38 pages, 5 figures, accepted for publication in
Entrop
Alfven modes driven non-linearly by metric perturbations in anisotropic magnetized cosmologies
We consider anisotropic magnetized cosmologies filled with conductive plasma
fluid and study the implications of metric perturbations that propagate
parallel to the ambient magnetic field. It is known that in the first order
(linear) approximation with respect to the amplitude of the perturbations no
electric field and density perturbations arise. However, when we consider the
non-linear coupling of the metric perturbations with their temporal
derivatives, certain classes of solutions can induce steeply increasing in time
electric field perturbations. This is verified both numerically and
analytically. The source of these perturbations can be either high-frequency
quantum vacuum fluctuations, driven by the cosmological pump field, in the
early stages of the evolution of the Universe or astrophysical processes or a
non-linear isotropization process of an initially anisotropic cosmological
spacetime.Comment: 7 pages, RevTex, 3 figures ps, accepted for publication to IJMP
Gravito-magnetic instabilities in anisotropically expanding fluids
Gravitational instabilities in a magnetized Friedman - Robertson - Walker
(FRW) Universe, in which the magnetic field was assumed to be too weak to
destroy the isotropy of the model, are known and have been studied in the past.
Accordingly, it became evident that the external magnetic field disfavors the
perturbations' growth, suppressing the corresponding rate by an amount
proportional to its strength. However, the spatial isotropy of the FRW Universe
is not compatible with the presence of large-scale magnetic fields. Therefore,
in this article we use the general-relativistic (GR) version of the
(linearized) perturbed magnetohydrodynamic equations with and without
resistivity, to discuss a generalized Jeans criterion and the potential
formation of density condensations within a class of homogeneous and
anisotropically expanding, self-gravitating, magnetized fluids in curved
space-time. We find that, for a wide variety of anisotropic cosmological
models, gravito-magnetic instabilities can lead to sub-horizonal, magnetized
condensations. In the non-resistive case, the power spectrum of the unstable
cosmological perturbations suggests that most of the power is concentrated on
large scales (small k), very close to the horizon. On the other hand, in a
resistive medium, the critical wave-numbers so obtained, exhibit a delicate
dependence on resistivity, resulting in the reduction of the corresponding
Jeans lengths to smaller scales (well bellow the horizon) than the
non-resistive ones, while increasing the range of cosmological models which
admit such an instability.Comment: 10 pages RevTex, 4 figures, accepted for publication in the
International Journal of Modern Physics
Dynamo effects in magnetized ideal-plasma cosmologies
The excitation of cosmological perturbations in an anisotropic cosmological
model and in the presence of a homogeneous magnetic field has been studied,
using the ideal magnetohydrodynamic (MHD) equations. In this case, the system
of partial differential equations which governs the evolution of the magnetized
cosmological perturbations can be solved analytically. Our results verify that
fast-magnetosonic modes propagating normal to the magnetic field, are excited.
But, what's most important, is that, at late times, the magnetic-induction
contrast grows, resulting in the enhancement of the ambient magnetic field.
This process can be particularly favored by condensations, formed within the
plasma fluid due to gravitational instabilities.Comment: 7 pages, RevTex, accepted for publication to IJMP
Magnetohydrodynamics and Plasma Cosmology
We study the linear magnetohydrodynamic (MHD) equations, both in the
Newtonian and the general-relativistic limit, as regards a viscous magnetized
fluid of finite conductivity and discuss instability criteria. In addition, we
explore the excitation of cosmological perturbations in anisotropic spacetimes,
in the presence of an ambient magnetic field. Acoustic, electromagnetic (e/m)
and fast-magnetosonic modes, propagating normal to the magnetic field, can be
excited, resulting in several implications of cosmological significance.Comment: 9 pages, RevTeX, To appear in the Proceedings of the Peyresq X
Meeting, IJTP Conference Serie
Graviton production in the scaling of a long-cosmic-string network
In a previous paper [1] we considered the possibility that (within the
early-radiation epoch) there has been (also) a short period of a significant
presence of cosmic strings. During this radiation-plus-strings stage the
Universe matter-energy content can be modelled by a two-component fluid,
consisting of radiation (dominant) and a cosmic-string fluid (subdominant). It
was found that, during this stage, the cosmological gravitational waves (CGWs)
- that had been produced in an earlier (inflationary) epoch - with comoving
wave-numbers below a critical value (which depends on the physics of the
cosmic-string network) were filtered, leading to a distorsion in the expected
(scale-invariant) CGW power spectrum. In any case, the cosmological evolution
gradually results in the scaling of any long-cosmic-string network and, hence,
after a short time-interval, the Universe enters into the late-radiation era.
However, along the transition from an early-radiation epoch to the
late-radiation era through the radiation-plus-strings stage, the
time-dependence of the cosmological scale factor is modified, something that
leads to a discontinuous change of the corresponding scalar curvature, which,
in turn, triggers the quantum-mechanical creation of gravitons. In this paper
we discuss several aspects of such a process, and, in particular, the
observational consequences on the expected gravitational-wave (GW) power
spectrum.Comment: 12 pages, 2 figures, accepted for publication in Physical Review
Charged cosmic strings interacting with gravitational and electromagnetic waves
Under a particular choice of the Ernst potential, we solve analytically the
Einstein-Maxwell equations to derive a new exact solution depending on five
parameters: the mass, the angular-momentum (per unit mass), the
electromagnetic-field strength, k, the parameter-p and the Kerr-NUT parameter,
l. This (Petrov Type D) solution is cylindrically-symmetric and represents the
curved background around a charged, rotating cosmic string, surrounded by
gravitational and electromagnetic waves, under the influence of the Kerr-NUT
parameter. A C-energy study in the radiation zone suggests that both the
incoming and the outgoing radiation is gravitational, strongly focused around
the null direction and preserving its profile. In this case, the absence of the
k-parameter from the C-energy implies that, away from the linear defect the
electromagnetic field is too weak to contribute to the energy-content of the
cylindrically-symmetric space-time under consideration. In order to explain
this result, we have evaluated the Weyl and the Maxwell scalars near the axis
of the linear defect and at the spatial infinity. Accordingly, we have found
that the electromagnetic field is concentrated (mainly) in the vicinity of the
axis, while falling-off prominently at large radial distances. However, as long
as k differs from unity, the non-zero Kerr-NUT parameter enhances those
scalars, both near the axis and at the spatial infinity, introducing some sort
of gravitomagnetic contribution.Comment: 18 pages, Springer_Latex, accepted for publication in General
Relativity and Gravitatio
Gravitational versus Magnetohydrodynamic Waves in Curved Spacetime in the Presence of Large-Scale Magnetic Fields
The general-relativistic (GR) magnetohydrodynamic (MHD) equations for a conductive plasma fluid are derived and discussed in the curved spacetime described by Thorne’s metric tensor, i.e., a family of cosmological models with inherent anisotropy due to the existence of an ambient, large-scale magnetic field. In this framework, it is examined whether the magnetized plasma fluid that drives the evolution of such a model can be subsequently excited by a transient, plane-polarized gravitational wave (GW) or not. To do so, we consider the associated set of the perturbed equations of motion and integrate them numerically in order to study the evolution of instabilities triggered by the GW propagation. In particular, we examine to what extend perturbations of the electric and/or the magnetic field can be amplified due to a potential energy transfer from the GW to the electromagnetic (EM) degrees of freedom. The evolution of the perturbed quantities depends on four free parameters, namely, the conductivity of the fluid, σ; the speed of sound square, 13Csc2≡γ1, which in this model may serve also as a measure of the inherent anisotropy; the GW frequency, ωg; and the associated angle of propagation with respect to the direction of the magnetic field, θ. We find that GW propagation in the anisotropic magnetized medium under consideration does excite several MHD modes; in other words, there is energy transfer from the gravitational to the EM degrees of freedom that can result in the acceleration of charged particles at the spot and in the subsequent damping of the GW
Graviton production in the scaling of a long-cosmic-string network
In a previous paper [K. Kleidis, D. B. Papadopoulos, E. Verdaguer, and L. Vlahos, Phys. Rev. D 78, 024027 (2008).] we considered the possibility that (within the early-radiation epoch) there has been (also) a short period of a significant presence of cosmic strings. During this radiation-plus-strings stage the Universe matter-energy content can be modeled by a two-component fluid, consisting of radiation (dominant) and a cosmic-string fluid (subdominant). It was found that, during this stage, the cosmological gravitational waves that had been produced in an earlier (inflationary) epoch with comoving wave numbers below a critical value (which depends on the physics of the cosmic-string network) were filtered, leading to a distorsion in the expected (scale-invariant) cosmological gravitational wave power spectrum. In any case, the cosmological evolution gradually results in the scaling of any long-cosmic-string network and, hence, after a short time interval, the Universe enters into the late-radiation era. However, along the transition from an early-radiation epoch to the late-radiation era through the radiation-plus-strings stage, the time dependence of the cosmological scale factor is modified, something that leads to a discontinuous change of the corresponding scalar curvature, which, in turn, triggers the quantum-mechanical creation of gravitons. In this paper we discuss several aspects of such a process, and, in particular, the observational consequences on the expected gravitational-wave power spectrum