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

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 $\Omega_{m_0}$ using the baryon acoustic oscillation peak in the SDSS luminous red galaxy sample and find that the best fit is obtained with $\Omega_{m_0}=0.27$, in agreement with the CMB data.Comment: 10 pages, 18 figure

Observational Constraints on Phantom Crossing DGP Gravity

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 1$\sigma$ (68%) errors for the Phantom Crossing DGP model are $\Omega_{m,0}=0.27^{+0.02}_{-0.02}$, $\beta=0.54^{+0.24}_{-0.30}$. 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 $\Lambda$CDM model, without dark energy due to the effect of DGP gravity. In our model, crossing of the phantom divide occurs at a redshift of $z \sim 0.2$.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

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

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

Transient Accelerated Expansion and Double Quintessence

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 $w_{eff}$, the relative density of the Dark Energy $\Omega_{Q,0}$ 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 $\Omega_{m,0}\gtrsim 0.35$ and $h\lesssim 0.68$.Comment: Version accepted in Phys. Rev. D, 22 pages, 10 figures, 4 table

Holographic Dark Energy Scenario and Variable Modified Chaplygin Gas

In this letter, we have considered that the universe is filled with normal matter and variable modified Chaplygin gas. Also we have considered the interaction between normal matter and variable modified Chaplygin gas in FRW universe. Then we have considered a correspondence between the holographic dark energy density and interacting variable modified Chaplygin gas energy density. Then we have reconstructed the potential of the scalar field which describes the variable modified Chaplygin cosmology.Comment: 4 latex pages, no figures, RevTeX styl

Constraints on holographic dark energy models using the differential ages of passively evolving galaxies

Using the absolute ages of passively evolving galaxies observed at different redshifts, one can obtain the differential ages, the derivative of redshift $z$ with respect to the cosmic time $t$ (i.e. ${\rm d} z/{\rm d}t$). Thus, the Hubble parameter $H(z)$ can be measured through the relation $H(z)=-({\rm d} z/{\rm d}t)/(1+z)$. By comparing the measured Hubble parameter at different redshifts with the theoretical one containing free cosmological parameters, one can constrain current cosmological models. In this paper, we use this method to present the constraint on a spatially flat Friedman-Robert-Walker Universe with a matter component and a holographic dark energy component, in which the parameter $c$ plays a significant role in this dark energy model. Firstly we consider three fixed values of $c$=0.6, 1.0 and 1.4 in the fitting of data. If we set $c$ free, the best fitting values are $c=0.26$, $\Omega_{\rm m0}=0.16$, $h=0.9998$. It is shown that the holographic dark energy behaves like a quintom-type at the $1\sigma$ level. This result is consistent with some other independent cosmological constrains, which imply that $c<1.0$ is favored. We also test the results derived from the differential ages using another independent method based on the lookback time to galaxy clusters and the age of the universe. It shows that our results are reliable.Comment: 18 pages including 7 figures and 1 tables. Final version for publication in Modern Physics Letters A (MPLA)[minor revision to match the appear version

Dark energy and dark matter from cosmological observations

The present status of our knowledge about the dark matter and dark energy is reviewed. Bounds on the content of cold and hot dark matter from cosmological observations are discussed in some detail. I also review current bounds on the physical properties of dark energy, mainly its equation of state and effective speed of sound.Comment: 12 pages, 4 figures, to appear in Lepton-Photon 2005 proceedings, added figure and typos correcte

Quantum effects can render w<-1 on cosmological scales

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