6,204 research outputs found
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 (68%) errors for the Phantom Crossing DGP model are
, . 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
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 .Comment: 17 pages, 9 figures, 1 table, Accepted for publication in
International Journal of Modern Physics
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 3.82”m 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
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
This paper deals with the effects of variable chemical reaction and variable electric conductivity on free convection and mass transfer flow of a viscous, incompressible and electrically conducting fluid over an inclined stretching sheet with variable heat and mass fluxes under the influence of Dufour and Soret effects. The non-linear boundary layer equations with boundary conditions are transferred into a system of non-linear ordinary differential equations using an established similarity transformation. These non-linear and locally-similar ordinary differential equations are solved numerically by applying NachtsheimâSwigert shooting iteration technique with sixth-order RungeâKutta integration scheme. Comparison with previously published work is obtained and excellent agreement is found. The effects of various parameters on the dimensionless velocity, temperature and concentration profiles as well as the local skin-friction coefficient, heat and mass transfer rate from the stretching sheet to the surrounding fluid are presented graphically and in tabulated form for a hydrogen-air mixture. The numerical results showed that chemical reaction parameter K, order of reaction n, Dufour number Df , Soret number Sr and heat (or mass) flux parameter r play a crucial role in the solutions
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
High energy in-band pumped erbium doped pulse fibre laser
We demonstrate an inband, core-pumped master oscillator power amplifier (MOPA) with a maximum pulse energy of 1.56 mJ at a repetition rate of 1.25 kHz, seeded by an actively Q-switched Erbium/Ytterbium-codoped fiber (EYDF) ring laser, producing 150 ns pulses at 1562.5 nm. To maximize energy extraction whilst minimizing signal saturation effects, a 40 ”m Er3+-doped larger mode area (LMA) fiber was used as the gain medium. A 1535 nm single mode fiber laser was used for in-band pumping of the LMA fiber. The output beam quality (M2) was measured to be ~1.6. This is to the best of our knowledge is the highest reported pulse energy for a pulse fiber laser at 1.5 ”m with M2~1.6
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 , the relative density
of the Dark Energy 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 and .Comment: Version accepted in Phys. Rev. D, 22 pages, 10 figures, 4 table
Accelerating Universe as Window for Extra Dimensions
Homogeneous cosmological solutions are obtained in five dimensional space
time assuming equations of state and where p
is the isotropic 3 - pressure and , that for the fifth dimension. Using
different values for the constants k and many known solutions are
rediscovered. Further the current acceleration of the universe has led us to
investigate higher dimensional gravity theory, which is able to explain
acceleration from a theoretical view point without the need of introducing dark
energy by hand. We argue that the terms containing higher dimensional metric
coefficients produce an extra negative pressure that apparently drives an
acceleration of the 3D space, tempting us to suggest that the accelerating
universe seems to act as a window to the existence of extra spatial dimensions.
Interestingly the 5D matter field remains regular while the \emph{effective}
negative pressure is responsible for the inflation. Relaxing the assumptions of
two equations of state we also present a class of solutions which provide early
deceleration followed by a late acceleration in a unified manner. Interesting
to point out that in this case our cosmology apparently mimics the well known
quintessence scenario fuelled by a generalised Chaplygin-type of fluid where a
smooth transition from a dust dominated model to a de Sitter like one takes
place.Comment: 20 pages,3 figure
New Physics and the Landau Pole
In scalar field theories the Landau pole is an ultraviolet singularity in the
running coupling constant that indicates a mass scale at which the theory
breaks down and new physics must intervene. However, new physics at the pole
will in general affect the running of the low energy coupling constant, which
will in turn affect the location of the pole and the related upper limit
(``triviality'' bound) on the low energy coupling constant. If the new physics
is strongly coupled to the scalar fields these effects can be significant even
though they are power suppressed. We explore the possible range of such effects
by deriving the one loop renormalization group equations for an effective
scalar field theory with a dimension 6 operator representing the low energy
effects of the new physics. As an independent check we also consider a
renormalizable model of the high-scale physics constructed so that its low
energy limit coincides with the effective theory.Comment: 26 pages, 5 figure
Development of an Advanced Engineering Polymer from the Modification of Nylon 66 by e-Beam Irradiation
When Nylon 66 was irradiated by an optimum dose of e-beam in presence of polyurethane as impact modifierin combination with triallyl isocyanurate as cross-linker, a superior performance was observed for the irradiatednylon 66. Significant improvement of properties, such as hardness, tensile strength, flexural modulus and impactstrength was obtained on radiation processing of nylon 66 by e-beam. More interestingly, percent water absorptionof such irradiated nylon 66 was reduced substantially. Improvement of mechanical properties and reduction of waterabsorption of irradiated nylon 66 were due to the cross-linking of the polymer system. Increase of cross-linkingwith dose of e-beam was verified by the increased gel content at higher doses. Irradiated nylon 66 showed betterdimensional stability than those achieved with pristine nylon 66. The increase in dimensional stability may beattributed to reduction in crystallinity with increasing dose of e-beam as revealed by DSC studies.Defence Science Journal, Vol. 64, No. 3, May 2014, pp. 281-289, DOI:http://dx.doi.org/10.14429/dsj.64.732
Equation of state description of the dark energy transition between quintessence and phantom regimes
The dark energy crossing of the cosmological constant boundary (the
transition between the quintessence and phantom regimes) is described in terms
of the implicitly defined dark energy equation of state. The generalizations of
the models explicitly constructed to exhibit the crossing provide the insight
into the cancellation mechanism which makes the transition possible.Comment: 3 pages, talk given at TAUP200
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