Demetrios B. Papadopoulos

Torsional oscillations of magnetized relativistic stars

Strong magnetic fields in relativistic stars can be a cause of crust fracturing, resulting in the excitation of global torsional oscillations. Such oscillations could become observable in gravitational waves or in high-energy radiation, thus becoming a tool for probing the equation of state of relativistic stars. As the eigenfrequency of torsional oscillation modes is affected by the presence of a strong magnetic field, we study torsional modes in magnetized relativistic stars. We derive the linearized perturbation equations that govern torsional oscillations coupled to the oscillations of a magnetic field, when variations in the metric are neglected (Cowling approximation). The oscillations are described by a single two-dimensional wave equation, which can be solved as a boundary-value problem to obtain eigenfrequencies. We find that, in the non-magnetized case, typical oscillation periods of the fundamental l = 2 torsional modes can be nearly a factor of 2 larger for relativistic stars than previously computed in the Newtonian limit. For magnetized stars, we show that the influence of the magnetic field is highly dependent on the assumed magnetic field configuration, and simple estimates obtained previously in the literature cannot be used for identifying normal modes observationally.

Nonperturbative semiclassical stability of de Sitter spacetime for small metric deviations

We consider the linearized semiclassical Einstein equations for small deviations around de Sitter spacetime including the vacuum polarization effects of conformal fields. Employing the method of order reduction, we find the exact solutions for general metric perturbations (of scalar, vector and tensor type). Our exact (nonperturbative) solutions show clearly that in this case de Sitter is stable with respect to small metric deviations and a late-time attractor. Furthermore, they also reveal a breakdown of perturbative solutions for a sufficiently long evolution inside the horizon. Our results are valid for any conformal theory, even self-interacting ones with arbitrarily strong coupling.

The Cosmic Battery and the inner edge of the accretion disc

The Poynting–Robertson Cosmic Battery proposes that the innermost part of the accretion disc around a black hole is threaded by a large-scale dipolar magnetic field generated in situ, and that the return part of the field diffuses outwards through the accretion disc. This is different from the scenario that the field originates at large distances and is carried inwards by the accretion flow. In view of the importance of large-scale magnetic fields in regulating the processes of accretion and outflows, we study the stability of the inner edge of a magnetized disc in general relativity when the distribution of the magnetic field is the one predicted by the Poynting–Robertson Cosmic Battery. We found that as the field grows, the inner edge of the disc gradually moves outwards. In a fast spinning black hole with a ≳ 0.8M the inner edge moves back in towards the black hole horizon as the field grows beyond some threshold value. In all cases, the inner part of the disc undergoes a dramatic structural change as the field approaches equipartition.

GALAXY FORMATION AND COSMIC-RAY ACCELERATION IN A MAGNETIZED UNIVERSE

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 field favors the anisotropic collapse of the fluid, which in turn leads to further magnetic amplification and to enhanced current-sheet formation in the plane normal to the ambient magnetic field. When the currents exceed a certain threshold, the resulting electrostatic turbulence can dramatically amplify the resistivity of the medium (anomalous resistivity). This could trigger strong electric fields and subsequently the acceleration of ultra-high-energy cosmic rays during the formation of protogalactic structures.

Waves and instabilities in an anisotropic universe

The excitation of low frequency plasma waves in an expanding anisotropic cosmological model that contains a magnetic field frozen into the matter and pointing in the longitudinal direction is discussed. Using the exact equations governing finite-amplitude wave propagation in hydromagnetic media within the framework of the general theory of relativity, we show that a spectrum of magnetized sound waves will be excited and form large-scale “ damped oscillations ” in the expanding universe. The characteristic frequency of the excited waves is slightly shifted away from the sound frequency and the shift depends on the strength of the primordial magnetic field. This magnetic field dependent shift may have an effect on the acoustic peaks of the CMB.

Acceleration and cyclotron radiation induced by gravitational waves

The equations that determine the response of a charged particle moving in a magnetic field to an incident gravi- tational wave (GW) are derived in the linearized approximation to general relativity. We briefly discuss several astrophysical applications of the derived formulae, taking into account the resonance between the wave and the particles motion that occurs at!g = 2, whenever the GW is parallel to the constant magnetic field. In the case where the GW is perpendicular to the constant magnetic field, magnetic resonances appear at!g =and!g = 2. Such a resonant mechanism may be useful in building models of GW-driven cyclotron emitters.

Impulsive Electron Acceleration by Gravitational Waves

We investigate the nonlinear interaction of a strong gravitational wave with the plasma during the collapse of a massive magnetized star and subsequent formation of a black hole or during the merging of neutron star binaries (the central engine). We found that under certain conditions this coupling may result in an efficient energy-space diffusion of particles. We suggest that the atmosphere created around the central engine is filled with three-dimensional magnetic neutral sheets (magnetic nulls). We demonstrate that the passage of strong pulses of gravitational waves through the magnetic neutral sheets accelerates electrons to very high energies. Superposition of many such short-lived accelerators, embedded inside a turbulent plasma, may be the source of the observed impulsive short-lived bursts. We conclude that in several astrophysical events, gravitational pulses may accelerate the tail of the ambient plasma to very high energies and become the driver for many types of astrophysical bursts.

On the adiabatic expansion of the visible space in a higher dimensional cosmology

In the context of higher-dimensional cosmologies, with isotropic visible and internal space and multi-perfect fluid matter, we study the conditions under which adiabatic expansion of the visible external space is possible, when a time-dependent internal space is present. The analysis is based on a reinterpretation of the four-dimensional stress-energy tensor in the presence of the extra dimensions. This modifies the usual adiabatic energy conservation laws for the visible universe, leading to a new type of cosmological evolution which includes large-scale entropy production in four dimensions.

Magnetohydrodynamics and Plasma Cosmology

Abstract 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.

Excitation of MHD waves in magnetized anisotropic cosmologies

The excitation of cosmological perturbations in an anisotropic cosmological model and in the presence of a homogeneous magnetic field was studied, using the resistive magnetohydrodynamic (MHD) equations. We have shown that fast-magnetosonic modes, propagating normal to the magnetic field, grow exponentially and saturate at high values, due to the resistivity. We also demonstrate that Jeans-like instabilities can be enhanced inside a resistive fluid and that the formation of condensations influence the growing magnetosonic waves.