7. Zdzislaw A. Golda, Andrzej Woszczyna A field theory approach to cosmological density perturbations Phys. Lett. A , vol. 310, pp. 357362 (2003). [abstract] [preprint] [journal] 
Abstract: Adiabatic perturbations propagate in the expanding universe like scalar massless fields in some effective RobertsonWalker spacetime.

8. Zdzislaw A. Golda, Andrzej Woszczyna Acoustics of early universe. I. Flat versus open universe models Class. Quant. Grav., vol. 18, pp. 543554 (2001). [abstract] [preprint] [journal] 
Abstract: A simple perturbation description unique for all signs of curvature, and based on the gaugeinvariant formalisms is proposed to demonstrate that:
(1) The density perturbations propagate in the flat radiationdominated 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 
9. Zdzislaw A. Golda, Andrzej Woszczyna Acoustics of early universe. II. Lifshitz vs. gaugeinvariant theories J. Math. Phys., vol. 42, pp. 856862 (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 gaugeinvariant results. For a radiation dominated universe we express the metric and density corrections in their exact forms and discuss their acoustic character.

10. Grazyna SiemieniecOzieblo, Andrzej Woszczyna Order reduction in semiclassical cosmology Phys. Rev. D, vol. 59, p. 083504 (1999). [abstract] [preprint] [journal] 
Abstract: We investigate the RobertsonWalker cosmology with Lagrangian $R+\alpha_1\hbar R^2+\alpha_2\hbar R^{\mu\nu}R_{\mu\nu} +L_{rad}$ where $L_{rad}$ means classical source with traceless energymomentum tensor. We weaken the selfconsistence condition (L. Parker, J. Z. Simon, Phys. Rev. {D47}(1993),{1339}). Quantum corrections are expressed as contributions to the effective equation of state. We show that the empty spacetime is stable within the class of radiationfilled expanding universes with no order reduction of the field equations.

11. David H Lyth, Andrzej Woszczyna Large scale perturbations in the open universe Phys. Rev. D, vol. 52, pp. 33383357 (1995). [abstract] [preprint] [journal] 
Abstract: When considering perturbations in an open (Omega<1) universe, cosmologists retain only subcurvature modes (defined as eigenfunctions of the Laplacian whose eigenvalue is less than 1 in units of the curvature scale, in contrast with the supercurvature modes whose eigenvalue is between 1 and 0). Mathematicians have known for almost half a century that all modes must be included to generate the most general HOMOGENEOUS GAUSSIAN RANDOM FIELD, despite the fact that any square integrable FUNCTION can be generated using only the subcurvature modes. The former mathematical object, not the latter, is the relevant one for physical applications. The mathematics is here explained in a language accessible to physicists. Then it is pointed out that if the perturbations originate as a vacuum fluctuation of a scalar field there will be no supercurvature modes in nature. Finally the effect on the cmb of any supercurvature contribution is considered, which generalizes to Omega<1 the analysis given by Grishchuk and Zeldovich in 1978. A formula is given, which is used to estimate the effect. In contrast with the case Omega=1, the effect contributes to all multipoles, not just to the quadrupole. It is important to find out whether it has the same l dependence as the data, by evaluating the formula numerically.

12. Andrzej Woszczyna A dynamical systems approach to the cosmological structure formation  Newtonian universe Mon. Not. R.A.S., vol. 225, p. 701 (1992). [journal] 
Abstract: Abstract 