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

We study the dynamics of a quintessence model based on two interacting scalar fields. The model can account for the (recent) accelerated expansion of the Universe suggested by astronomical observations. Acceleration can be permanent or temporary and, for both scenarios, it is possible to obtain suitable values for the cosmological parameters while satisfying the nucleosynthesis constraint on the quintessence energy density. We argue that the model dynamics can be made consistent with a stable zero-energy relaxing supersymmetric vacuum.Recent observations of type Ia supernovae, together with Cosmic Microwave Background (CMB) and cluster mass distribution data [1][2][3] indicate that the Universe is flat, in agreement with the inflationary prediction, accelerating and that the energy density of (baryonic plus dark) matter is smaller than the critical density. Thus, observations suggest that the dynamics of the Universe at present is dominated by a negative pressure component, the main candidates being a cosmological constant and a slowly-varying vacuum energy, usually referred to as dark energy or quintessence [4,5] (an evolving vacuum energy was discussed somewhat earlier, e.g. [6]). The main difference between the cosmological constant and quintessence scenarios is that, for quintessence, the equation of state parameter, w Q ≡ p/ρ, varies with time and approaches a present value w Q < −0.6, whilst for the cosmological constant, it remains fixed at w Λ = −1.Several quintessence models have been put foward, most of them based on a scalar field which was subdominant in the early Universe and, more recently, has started to dominate the energy density of non-relativistic matter. Theoretical suggestions include a scalar field endowed with exponential [7][8][9][10] or inverse power law potentials [11], the string theory dilaton in the context of gaugino condensation [12], an axion field with an almost massless quark [13], scalar-tensor theories of gravity [14], or one of the fields arising from the compactification process in the multidimensional Einstein-Yang-Mills system [15]. Some of these models address the "cosmic coincidence" problem i.e. the question of explaining why the vacuum energy or scalar field dominates the Universe only recently. In tracker models, the tracker field rolls down a potential according to an attractor-like solution to the equations of motion, causing the energy density of the quintessence field to track the equation of state of the background energy component independently of initial conditions [11]. However, in these models, the overall scale of the potential has to be fine-tuned in order for the quintessence energy to overtake the matter density at present.In k-essence models [16], as a result of the dynamics, tracking of the background energy density can only occur in the radiation epoch; at the onset of matterdomination, the k-essence field energy density drops sharply, increasing again and overtaking the matter energy density at roughly the current epoch. At least in the original propo...

We use big bang nucleosynthesis bounds on the variation of the gravitational coupling to derive constraints on the strength of the deviation from the gravitational inverse-square law due to tensor and vector unparticle exchange.

The mechanism of thermal leptogenesis is investigated in the high-energy regime of braneworld cosmology. Within the simplest seesaw framework with hierarchical heavy Majorana neutrinos, we study the implications of the modified Friedmann equation on the realization of this mechanism. In contrast with the usual leptogenesis scenario of standard cosmology, where low-energy neutrino data favors a mildly strong washout regime, we find that leptogenesis in the braneworld regime is successfully realized in a weak washout regime. Furthermore, a quasi-degenerate light neutrino mass spectrum is found to be compatible with this scenario. For an initially vanishing heavy Majorana neutrino abundance, thermal leptogenesis in the brane requires the decaying heavy Majorana neutrino mass to be M1 > 10^10 GeV and the fundamental five-dimensional gravity scale 10^12 < M5 < 10^16 GeV, which corresponds to a transition from brane to standard cosmology at temperatures 10^8 < Tt < 10^14 GeV.Comment: 7 pages, 3 figures, a few comments and references added. Final version to appear in Phys. Rev.

The mechanism of gravitational baryogenesis, based on the CP T -violating gravitational interaction between the derivative of the Ricci scalar curvature and the baryon-number current, is investigated in the context of the Gauss-Bonnet braneworld cosmology. We study the constraints on the fundamental five-dimensional gravity scale, the effective scale of B-violation and the decoupling temperature, for the above mechanism to generate an acceptable baryon asymmetry during the radiation-dominated era. The scenario of gravitational leptogenesis, where the lepton-number violating interactions are associated with the neutrino mass seesaw operator, is also considered.