6,549 research outputs found

    A model-independent dark energy reconstruction scheme using the geometrical form of the luminosity-distance relation

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    We put forward a new model-independent reconstruction scheme for dark energy which utilises the expected geometrical features of the luminosity-distance relation. The important advantage of this scheme is that it does not assume explicit ansatzes for cosmological parameters but only some very general cosmological properties via the geometrical features of the reconstructed luminosity-distance relation. Using the recently released supernovae data by the Supernova Legacy Survey together with a phase space representation, we show that the reconstructed luminosity-distance curves best fitting the data correspond to a slightly varying dark energy density with the Universe expanding slightly slower than the Lambda CDM model. However, the Lambda CDM model fits the data at 1 sigma significance level and the fact that our best fitting luminosity-distance curve is lower than that of the corresponding Lambda CDM model could be due to systematics. The transition from an accelerating to a decelerating expansion occurs at a redshift larger than z=0.35. Interpreting the dark energy as a minimally coupled scalar field we also reconstruct the scalar field and its potential. We constrain Ωm0\Omega_{m_0} using the baryon acoustic oscillation peak in the SDSS luminous red galaxy sample and find that the best fit is obtained with Ωm0=0.27\Omega_{m_0}=0.27, in agreement with the CMB data.Comment: 10 pages, 18 figure

    Observational Constraints on Phantom Crossing DGP Gravity

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    We study the observational constraints on the Phantom Crossing DGP model. We demonstrate that the crossing of the phantom divide does not occur within the framework of the original Dvali-Gabadadze-Porrati (DGP) model or the DGP model developed by Dvali and Turner. By extending their model in the framework of an extra dimension scenario, we study a model that realizes crossing of the phantom divide. We investigate the cosmological constraints obtained from the recent observational data of Type Ia Supernovae, Cosmic Microwave Background anisotropies, and Baryon Acoustic Oscillations. The best fit values of the parameters with 1σ\sigma (68%) errors for the Phantom Crossing DGP model are Ωm,0=0.270.02+0.02\Omega_{m,0}=0.27^{+0.02}_{-0.02}, β=0.540.30+0.24\beta=0.54^{+0.24}_{-0.30}. We find that the Phantom Crossing DGP model is more compatible with the observations than the original DGP model or the DGP model developed by Dvali and Turner. Our model can realize late-time acceleration of the universe, similar to that of Λ\LambdaCDM model, without dark energy due to the effect of DGP gravity. In our model, crossing of the phantom divide occurs at a redshift of z0.2z \sim 0.2.Comment: 17 pages, 9 figures, 1 table, Accepted for publication in International Journal of Modern Physics

    Inflation is the generic feature of phantom field-not the big-rip

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    A class of solutions for phantom field corresponding to a generalized k-essence lagrangian has been presented, employing a simple method which provides the scope to explore many such. All the solutions having dynamical state parameter are found to touch the magic line w = -1, asymptotically. The solutions with constant equation of state can represent phantom, quitessence or an ordinary scalar field cosmologies depending on the choice of a couple of parameters of the theory. For w approximately equal to -1, quintessence and phantom models are indistinguishable through the Hubble parameter. Finally, inflation rather than big-rip has been found to be the generic feature of phantom cosmology.Comment: 8 pages, 3 figure

    An all-fibre PM MOPA pumped high-power OPO at 3.82 microns based on large aperture PPMgLN

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    We report a large aperture PPMgLN based OPO generating 21W of average output power at a slope efficiency of 45%, pumped by the output from a polarization maintaining Ytterbium doped fiber MOPA operating at 1060nm producing 58W of average output power and 20ns pulses at a repetition rate of 100kHz. A maximum of 5.5W of optical power was recorded at the idler wavelength of 3.82µm without thermal roll-off. We have experimentally verified that the pulse rise/fall time plays a significant role in the OPO conversion efficiency and that further enhancement in the OPO conversion efficiency will be possible using sub-nanosecond rise and fall times

    Quantum effects can render w<-1 on cosmological scales

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    We report on a revision of our previous computation of the renormalized expectation value of the stress-energy tensor of a massless, minimally coupled scalar with a quartic self-interaction on a locally de Sitter background. This model is important because it demonstrates that quantum effects can lead to violations of the weak energy condition on cosmological scales - on average, not just in fluctuations - although the effect in this particular model is far too small to be observed. The revision consists of modifying the propagator so that dimensional regularization can be used when the dimension of the renormalized theory is not four. Although the finite part of the stress-energy tensor does not change (in D=4) from our previous result, the counterterms do. We also speculate that a certain, finite and separately conserved part of the stress tensor can be subsumed into a natural correction of the initial state from free Bunch-Davies vacuum.Comment: 9 pages, references adde

    Effects of variable chemical reaction and variable electric conductivity on free convective heat and mass transfer flow along an inclined stretching sheet with variable heat and mass fluxes under the influence of Dufour and Soret effects

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    This paper deals with the effects of variable chemical reaction and variable electric&nbsp;conductivity on free convection and mass transfer flow of a viscous, incompressible and electrically&nbsp;conducting fluid over an inclined stretching sheet with variable heat and mass fluxes under the&nbsp;influence of Dufour and Soret effects. The non-linear boundary layer equations with boundary&nbsp;conditions are transferred into a system of non-linear ordinary differential equations using an&nbsp;established similarity transformation. These non-linear and locally-similar ordinary differential&nbsp;equations are solved numerically by applying Nachtsheim–Swigert shooting iteration technique&nbsp;with sixth-order Runge–Kutta integration scheme. Comparison with previously published work is&nbsp;obtained and excellent agreement is found. The effects of various parameters on the dimensionless&nbsp;velocity, temperature and concentration profiles as well as the local skin-friction coefficient, heat&nbsp;and mass transfer rate from the stretching sheet to the surrounding fluid are presented graphically&nbsp;and in tabulated form for a hydrogen-air mixture. The numerical results showed that chemical&nbsp;reaction parameter K, order of reaction n, Dufour number Df , Soret number Sr and heat (or mass)&nbsp;flux parameter r play a crucial role in the solutions

    Transient Accelerated Expansion and Double Quintessence

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    We consider Double Quintessence models for which the Dark Energy sector consists of two coupled scalar fields. We study in particular the possibility to have a transient acceleration in these models. In both Double Quintessence models studied here, it is shown that if acceleration occurs, it is necessarily transient. We consider also the possibility to have transient acceleration in two one-field models, the Albrecht-Skordis model and the pure exponential. Using separate conservative constraints (marginalizing over the other parameters) on the effective equation of state weffw_{eff}, the relative density of the Dark Energy ΩQ,0\Omega_{Q,0} and the present age of the universe, we construct scenarios with a transient acceleration that has already ended at the present time, and even with no acceleration at all, but a less conservative analysis using the CMB data rules out the last possibility. The scenario with a transient acceleration ended by today, can be implemented for the range of cosmological parameters Ωm,00.35\Omega_{m,0}\gtrsim 0.35 and h0.68h\lesssim 0.68.Comment: Version accepted in Phys. Rev. D, 22 pages, 10 figures, 4 table

    MEDIA OPTIMIZATION FOR BIOPROTEINS PRODUCTION FROM CHEAPER CARBON SOURCE

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    There are high demands for animal and human food supply especially protein, which is an important dietary component. Agricultural wastes, cheap carbon sources- which are rich and have high energy, can be used for producing the value added bioprotein. A lab scale study was carried out to optimize the media composition for bioprotein production from a cheaper carbon source - wheat flour using potential strain, which was selected earlier by screening different microorganisms. The performance of the selected strain was enhanced by media optimization with varied substrate concentration, nitrogen sources and nutrient supplementation according to the central composite design from STATISTICA software. Statistical optimization was carried out to evaluate the polynomial regression model through effect of linear, quadratic and interaction of the factors. The maximum biomass produced was 21.89 g/L with optimum fermentation conditions of wheat flour (4 g/L), nitrogen concentration (0.5 g/L), nutrient concentration (0.1 g/L), and four days of fermentation

    Variable Modified Chaplygin Gas and Accelerating Universe

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    In this letter, I have proposed a model of variable modified Chaplygin gas and shown its role in accelerating phase of the universe. I have shown that the equation of state of this model is valid from the radiation era to quiessence model. The graphical representations of statefinder parameters characterize different phase of evolution of the universe. All results presented in the letter concerns the case k=0k=0.Comment: 7 Latex pages, 5 figures, revtex styl
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