We consider the scenario emerging from the dynamics of a generalized d-brane in a (d + 1, 1) spacetime. The equation of state describing this system is given in terms of the energy density, ρ, and pressure, p, by the relationship p = −A/ρ α , where A is a positive constant and 0 < α ≤ 1. We discuss the conditions under which homogeneity arises and show that this equation of state describes the evolution of a universe evolving from a phase dominated by non-relativistic matter to a phase dominated by a cosmological constant via an intermediate period where the effective equation of state is given by p = αρ.PACS numbers: 98.80.Cq, 98.65.Es

We study N = 1 Supergravity inflation in the context of the braneworld scenario. Particular attention is paid to the problem of the onset of inflation at sub-Planckian field values and the ensued inflationary observables. We find that the so-called η-problem encountered in supergravity inspired inflationary models can be solved in the context of the braneworld scenario, for some range of the parameters involved. Furthermore, we obtain an upper bound on the scale of the fifth dimension, M5 ∼ < 10 −3 MP , in case the inflationary potential is quadratic in the inflaton field, φ. If the inflationary potential is cubic in φ, consistency with observational data requires that M5 ≃ 9.2 × 10 −4 MP .

We consider the generalized Chaplygin gas (GCG) proposal for unification of dark energy and dark matter and show that it admits an unique decomposition into dark energy and dark matter components once phantom-like dark energy is excluded. Within this framework, we study structure formation and show that difficulties associated to unphysical oscillations or blow-up in the matter power spectrum can be circumvented. Furthermore, we show that the dominance of dark energy is related to the time when energy density fluctuations start deviating from the linear δ ∼ a behaviour.PACS numbers:

Self-interacting dark matter has been suggested in order to overcome the difficulties of the Cold Dark Matter model on galactic scales. We argue that a scalar gauge singlet coupled to the Higgs boson, leading to an invisibly decaying Higgs, is an interesting candidate for this selfinteracting dark matter particle.

We study the dynamics of flat Friedmann-Robertson-Walker (FRW) cosmologies in the presence of a triplet of massive vector fields with SO(3) global symmetry. We find an E 3 -symmetric ansatz for the vector fields that is compatible with the E 3 -invariant FRW metric and propose a method to make invariant ansätze for more general cosmological models. We use techniques of dynamical systems to study qualitatively the behaviour of the model and find, in particular, that the effective equation of state of the system changes gradually from a radiation-dominated to a matter-dominated form and that the scale of the transition depends on the mass of the gauge fields. † Work supported in part by a JNICT graduate scholarship BD/138/90-RM and by STRIDE/FEDER Project JN.91.02.‡ Work supported by a GTAE (Grupo Teórico de Altas Energias) grant.

We study the effect of higher-curvature terms in the string low-energy effective actions on the cosmological solutions of the theory, up to corrections quartic in the curvatures, for the bosonic and heterotic strings as well as for the type II superstring. We find that cosmological solutions exist for all string types but they always disappear when the dilaton field is included, a conclusion that can be avoided if string-loop effects are taken into account.

Working in the context of a proposal for collisional dark matter, we derive bounds on the Higgs boson coupling g ′ to a stable light scalar particle, which we refer to as phion (φ), required to solve problems with small scale structure formation which arise in collisionless dark matter models. We discuss the behaviour of the phion in the early universe for different ranges of its mass. We find that a phion in the mass range of 100 MeV is excluded and that a phion in the mass range of 1 GeV requires a large coupling constant, g ′ ∼ > 2, and m h ∼ < 130 GeV in order to avoid overabundance, in which case the invisible decay mode of the Higgs boson would be dominant.PACS numbers: 95.35.+d, 98.62., 14.80.B

We use the Type Ia Supernova gold sample data of Riess et al in order to constrain three models of dark energy. We study the Cardassian model, the Dvali-Turner gravity modified model and the generalized Chaplygin gas model of dark energy -dark matter unification. In our best fit analysis for these three dark energy proposals we consider flat model and the non-flat model priors. We also discuss the degeneracy of the models with the XCDM model through the computation of the so-called jerk parameter. 98.80.Es