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 .

The thawing quintessence model with a nearly flat potential provides a natural mechanism to produce an equation of state parameter, w, close to -1 today. We examine the behavior of such models for the case in which the potential satisfies the slow roll conditions: [(1/V)(dV/dphi)]^2 << 1 and (1/V)(d^2 V/dphi^2) << 1, and we derive the analog of the slow-roll approximation for the case in which both matter and a scalar field contribute to the density. We show that in this limit, all such models converge to a unique relation between 1+w, Omega_phi, and the initial value of (1/V)(dV/dphi). We derive this relation, and use it to determine the corresponding expression for w(a), which depends only on the present-day values for w and Omega_phi. For a variety of potentials, our limiting expression for w(a) is typically accurate to within delta w < 0.005 for w<-0.9. For redshift z < 1, w(a) is well-fit by the Chevallier-Polarski-Linder parametrization, in which w(a) is a linear function of a.Comment: 8 pages, 5 figures, discussion added, references updated, typos corrected, to appear in Phys. Rev.

No abstract

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:

No abstract

We use the most recent Type Ia supernova data in order to study the dark energy-dark matter (XCDM) unification approach in the context of the generalized Chaplygin gas (GCG) model. Rather surprisingly, we find that data allow models with α > 1. We have studied how the GCG adjusts flat and non-flat models, and our results show that GCG is consistent with a flat case up to 68 per cent confidence level. Actually this holds even if one relaxes the flat prior assumption. We have also analysed what one should expect from a future experiment such as the Supernova/Acceleration Probe (SNAP). We find that there is a degeneracy between the GCG model and an XCDM model with a phantom-like dark energy component.

We investigate the constraints on the Cardassian model using the recent results from the Wilkinson microwave anisotropy probe for the locations of the peaks of the cosmic microwave background ͑CMB͒ anisotropy spectrum. We find that the model is consistent with the recent observational data for a certain range of the model parameter n and the cosmological parameters. We find that the Cardassian model is favored compared to the ⌳CDM model for a higher spectral index (n s Ϸ1) together with a lower value of the Hubble parameter h (hр0.71). But for smaller values of n s , both ⌳CDM and Cardassian models are equally favored. Also, irrespective of supernova constraints, CMB data alone predict the current acceleration of the Universe in this model. We have also studied the constraint on 8 , the rms density fluctuations at the 8h Ϫ1 Mpc scale.