79. G. SiemieniecOziębło, Z. A. Golda Magnetic amplification in cylindrical cosmological structure Astron. Astrophys., vol. 422, pp. 2327 (2004). [abstract] [preprint] [journal] 
Abstract: We derive the amplification of the cosmological magnetic field associated with forming gravitational structure. The selfsimilar 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. 
80. Leszek M. Sokołowski Restrictions on possible forms of classical matter fields carrying no energy Acta Phys. Polon. , vol. B35, pp. 587612 (2004). [abstract] [preprint] [journal] 
Abstract: It is postulated in general relativity that the matter energymomentum 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.

81. Marek Szydlowski, Wojciech Czaja ParticleLike 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. 
82. 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 twodimensional 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 FriedmannRobertsonWalker 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. 
83. Marek Szydlowski, Wojciech Czaja Toward reconstruction of the dynamics of the Universe from distant type Ia supernovae Phys. Rev., vol. D69, p. 083507 (2004). [abstract] [preprint] [journal] 
Abstract: We demonstrate a modelindependent method of estimating qualitative dynamics of an accelerating universe from observations of distant type Ia supernovae. Our method is based on the luminositydistance function, optimized to fit observed distances of supernovae, and the Hamiltonian representation of dynamics for the quintessential universe with a general form of equation of state $p=w(a(z))\rho$. Because of the Hamiltonian structure of FRW dynamics with the equation of state $p = w(a(z)) \rho$, the dynamics is uniquelly determined by the potential function $V(a)$ of the system. The effectiveness of this method in discrimination of model parameters of Cardassian evolution scenario is also given. Our main result is the following, restricting to the flat model with the current value of $\Omega_{m,0}=0.3$, the constraints at $2\sigma$ confidence level to the presence of $\rho^{n}$ modification of the FRW models are $0.50 \lesssim n \lesssim 0.36$. 
84. P. Bizoń, S. J. Szybka, A. Wassserman Periodic selfsimilar wave maps coupled to gravity Phys. Rev. D, vol. 69, p. 064014 (2004). [abstract] [preprint] [journal] 
Abstract: We continue our studies of spherically symmetric selfsimilar solutions in the SU(2) sigma model coupled to gravity. Using mixed numerical and analytical methods we show existence of an unstable periodic solution lying at the boundary between the basins of two generic attractors. 