79. G. Siemieniec-Ozieblo, A. Woszczyna
Acoustic instabilities at the transition from the radiation-dominated to the matter-dominated universe
Astron. Astrophys. , vol. 419, pp. 801-810 (2004).
[abstract] [preprint] [journal]

Abstract:
The transition from acoustic noise in the radiation-dominated universe to the density structures in the matter dominated epoch is considered. The initial state is a stochastic field of sound waves moving in different directions. The construction of the initial state is compatible with the hyperbolic type of propagation equation for density perturbations, and parallel to the theory of stochastic background of gravitational waves. Instantaneous transition between the cosmological epochs is assumed, and Darmois-Israel joining conditions are applied to match solutions for sound waves with growing or decaying modes at the decoupling. As a result a substantial amplification of the low scale structures is obtained.

80. G. Siemieniec-Oziębło, Z. A. Golda
Magnetic amplification in cylindrical cosmological structure
Astron. Astrophys., vol. 422, pp. 23-27 (2004).
[abstract] [preprint] [journal]

Abstract:
We derive the amplification of the cosmological magnetic field associated with forming gravitational structure. The self-similar solutions of magnetohydrodynamic equations are computed both in linear and nonlinear regimes. We find that the relatively fast magnetic field enhancement becomes substantial in the nonlinear phase.

81. Leszek M. Sokołowski
Restrictions on possible forms of classical matter fields carrying no energy
Acta Phys. Polon. , vol. B35, pp. 587-612 (2004).
[abstract] [preprint] [journal]

Abstract:
It is postulated in general relativity that the matter energy-momentum tensor (named the stress tensor) vanishes if and only if all the matter fields vanish. In classical lagrangian field theory the stres tensor is the variational (symmetric) one and a priori it might occur that for some systems the tensor is identically zero for all field configurations whereas evolution of the system is subject to deterministic equations of motion. Such a system would not generate its own gravitational field. To check if such systems may exist we find a relationship between the stress tensor and the Euler operator. We prove that if a system of n interacting scalar fields (n cannot exceed the spacetime dimension d) or a single vector field (if d is even) has the stress tensor such that its divergence is identically zero ("on and off shell"), then the Lagrange equations of motion hold identically too. These systems are unphysical as having no propagation equations at all. Thus nontrivial field equations imply the nontrivial stress tensor. The theorem breaks down if the of the field components n is greater than d. We show that for n>d matter systems without energy and their own gravity (and yet detectable) are in principle admissible. Their equations of motion are degenerate. We also show for which matter systems their stress tensors cannot vanish for all solutions of the field equations.

82. M. Heller, Z. Odrzygóźdź, L. Pysiak, W. Sasin
Quantum Groupoids of the Final Type and Quantization on Orbit Spaces
Demonstratio Mathematica , vol. 37, pp. 671-678 (2004).

Abstract:

83. Marek Szydlowski, Wojciech Czaja
Particle-Like Description in Quintessential Cosmology
Phys. Rev., vol. D69, p. 083518 (2004).
[abstract] [preprint] [journal]

Abstract:
Assuming equation of state for quintessential matter: $p=w(z)\rho$, we analyse dynamical behaviour of the scale factor in FRW cosmologies. It is shown that its dynamics is formally equivalent to that of a classical particle under the action of 1D potential $V(a)$. It is shown that Hamiltonian method can be easily implemented to obtain a classification of all cosmological solutions in the phase space as well as in the configurational space. Examples taken from modern cosmology illustrate the effectiveness of the presented approach. Advantages of representing dynamics as a 1D Hamiltonian flow, in the analysis of acceleration and horizon problems, are presented. The inverse problem of reconstructing the Hamiltonian dynamics (i.e. potential function) from the luminosity distance function $d_{L}(z)$ for supernovae is also considered.

84. Marek Szydlowski, Wojciech Czaja
Stability of FRW cosmology with a generalized Chaplygin gas
Phys. Rev., vol. D69, p. 023506 (2004).
[abstract] [preprint] [journal]

Abstract:
We apply methods of dynamical systems to study the behavior of a universe dominated by the generalized Chaplygin gas. We reduce the dynamics to a two-dimensional Hamiltonian system and study its behavior for various ranges of parameters. The dynamics is studied on the phase plane by using methods of a qualitative analysis of differential equations. The behavior of trajectories at infinity is studied in some convenient coordinates introduced on the phase plane. Hence we show that the Friedmann-Robertson-Walker model with the generalized Chaplygin gas is structurally stable. We clearly find the domains of cosmic acceleration as well as conditions for which the horizon problem is solved. We also define some general class of fluids which generalize the Chaplygin gas. The dynamics of such models in terms of energy conditions is also discussed.