ar X iv : g r - qc / 0 31 10 16 v 1 5 N ov 2 00 3 Spin - Torsion , Braneworlds and Changing Symmetry in the Universe From the Beginning to the End

In this thesis, we explore three alternatives to the current paradigm of the standard inflationary big bang scenario. The three alternative themes are spin torsion (or Einstein-Cartan-Kibble-Sciama) theories, extra dimensions (braneworld cosmology) and changing global symmetry. In the spin torsion theories, we found new cosmological solutions with a cosmological constant as alternative to the standard scalar field driven inflationary scenario and we conclude that these toy models do not exhibit an inflationary phase. In the theme of extra dimensions, we discuss the dynamics of linearized scalar and tensor perturbations in an almost FriedmannRobertson-Walker braneworld cosmology of Randall-Sundrum type II using the 1+3 covariant approach. We derive a complete set of frame-independent equations for the total matter variables, and a partial set of equations for the non-local variables, which arise from the projection of the Weyl tensor in the bulk. The latter equations are incomplete since there is no propagation equation for the non-local anisotropic stress. For the scalar perturbations, we found solutions that reveal the existence of new modes arising from the two additional non-local degrees of freedom (extra dimensions) under the assumption that the non-local anisotropic stress vanishes. Our solutions should prove useful in setting up initial conditions for numerical codes aimed at exploring the effect of braneworld corrections on the cosmic microwave background (CMB) power spectrum. For the tensor perturbations, we set out the framework of a program to compute the tensor anisotropies in the CMB that are generated in braneworld models. In the simplest approximation, we show the braneworld imprint as a correction to the power spectra for standard temperature and polarization anisotropies and similarly show that the tensor anisotropies are also insensitive to the high energy effects. Finally in the theme of changing global symmetry, we constructed a bounded isothermal solution embedded in an expanding Einstein de Sitter universe and showed that there is a possible phase transition in the far future.