Accelerated Universe from Modified Chaplygin Gas and Tachyonic Fluid

A cosmological model with an exotic fluid is investigated. We show that the equation of state of this ``modified Chaplygin'' gas can describe the current accelerated expansion of the universe. We then reexpress it as FRW cosmological model containing a scalar field $\phi$ and find its self--interacting potential. Moreover motivated by recent works of Sen [sen1, sen2] and Padmanbhan [pad] on tachyon field theory, a map for this exotic fluid as a normal scalar field $\phi$ with Lagrangian ${\cal L}_{\phi} = \frac{\dot{\phi}^2}{2} - U (\phi)$ to the tachyonic field $T$ with Lagrangian ${\cal L}_{T} = - V(T) \sqrt{1 - \dot{T}^2}$ is obtained.Comment: 11 pages, no figure

Modified Chaplygin Gas Cosmology

Modified Chaplygin gas as an exotic fluid has been introduced in [34]. Essential features of the modified Chaplygin gas as a cosmological model are discussed. Observational constraints on the parameters of the model have been included. The relationship between the modified Chaplygin gas and a homogeneous minimally coupled scalar field are reevaluated by constructing its self-interacting potential. In addition, we study the role of the tachyonic field in the modified Chaplygin gas cosmological model and the mapping between scalar field and tachyonic field is also considered.Comment: 11 page

New Parametrization of Neutrino Mixing Matrix

Global fits to neutrino oscillation data are compatible with tri-bimaximal mixing pattern, which predicts $\theta_{23} = \frac{\pi}{4}, \theta_{12} = \sin^{-1} (\frac{1}{\sqrt{3}})$ and $\theta_{13} = 0$. We propose here to parametrize the tri-bimaximal mixing matrix $V_{TBM}$ by its hermitian generator $H_{TBM}$ using the exponential map. Then we use the exponential map to express the deviations from tri-bimaximal pattern by deriving the hermitian matrices $H_{z=0}$ and $H_1$. These deviations might come from the symmetry breaking of the neutrino and charged lepton sectors.Comment: 10 pages, no figures, correted minor typo