7. Zdzisław A. Golda, Andrzej Woszczyna, Karolina Zawada
Canonical gauge-invariant variables
Acta Phys. Pol., B , vol. B36, p. 2133 (2005).
[abstract] [preprint] [journal]

Abstract:
Under an appropriate change of the perturbation variable Lifshitz-Khalatnikov propagation equations for the scalar perturbation reduce to d'Alembert equation. The change of variables is based on the Darboux transform

8. 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.

9. Z. Golda, A.Woszczyna
Dispersion of density waves in the early universe with positive cosmological constant
Class. Quantum Grav., vol. 20, p. 277 (2003).
[abstract] [preprint] [journal]

Abstract:
Density perturbations in the flat (K=0) Robertson-Walker universe with radiation ($p=\epsilon/3$) and positive cosmological constant ($\Lambda>0$) are investigated. The phenomenon of anomalous dispersion of acoustic waves on $\Lambda$ is discussed.

10. Zdzislaw A. Golda, Andrzej Woszczyna
A field theory approach to cosmological density perturbations
Phys. Lett. A , vol. 310, pp. 357-362 (2003).
[abstract] [preprint] [journal]

Abstract:
Adiabatic perturbations propagate in the expanding universe like scalar massless fields in some effective Robertson-Walker space-time.

11. Zdzislaw A. Golda, Andrzej Woszczyna
Acoustics of early universe. I. Flat versus open universe models
Class. Quant. Grav., vol. 18, pp. 543-554 (2001).
[abstract] [preprint] [journal]

Abstract:
A simple perturbation description unique for all signs of curvature, and based on the gauge-invariant formalisms is proposed to demonstrate that: (1) The density perturbations propagate in the flat radiation-dominated universe in exactly the same way as electromagnetic or gravitational waves propagate in the epoch of the matter domination. (2) In the open universe, sounds are dispersed by curvature. The space curvature defines the minimal frequency $\omega_{\rm c}$ below which the propagation of perturbations is forbidden. Gaussian acoustic fields are considered and the curvature imprint in the perturbations spectrum is discussed

12. Zdzislaw A. Golda, Andrzej Woszczyna
Acoustics of early universe. II. Lifshitz vs. gauge-invariant theories
J. Math. Phys., vol. 42, pp. 856-862 (2001).
[abstract] [preprint] [journal]

Abstract:
Appealing to classical methods of order reduction, we reduce the Lifshitz system to a second order differential equation. We demonstrate its equivalence to well known gauge-invariant results. For a radiation dominated universe we express the metric and density corrections in their exact forms and discuss their acoustic character.