PSI to turbulence during internal wave beam refraction through the upper ocean pycnocline

A numerical study based on large eddy simulation (LES) is performed to investigate the nonlinear interaction of a semidiurnal (M2) internal wave beam with an upper ocean pycnocline. During refraction through the pycnocline, the wave beam undergoes parametric subharmonic instability (PSI) with formation of waves with (1/2)M2 frequency. The three-dimensional LES enables new results that quantify the route to turbulence through PSI. The subharmonic waves generated from PSI have an order of magnitude smaller vertical scale and are susceptible to wave breaking. Convective instability initiates transition to turbulence, while shear production maintains it. Turbulence at points in the subharmonic wave paths is modulated at (1/2)M2 frequency. The beam suffers substantial degradation owing to PSI, reflected harmonics and ducted waves so that only about 30% of the incoming energy is transported by the main reflected beam.We are pleased to acknowledge support through ONR N000140910287, program manager Terri Paluszkiewicz, ARC DECRA Fellowship DE140100089 for B.G., and helpful discussions with Shaun Johnston, SIO

Non-perturbative unification in the light of LEP results

We consider an alternative to conventional GUTs originally proposed by Maiani, Parisi and Petronzio, where owing to the existence of extra fermion generations at some intermediate scale, the gauge couplings become large at high energies. We first comment on how the non- supersymmetric version of this scenario is ruled out; we then consider the two-loop evolution of the couplings in the supersymmetric extension of this scenario, and check whether such a scenario is feasible in the light of the precies values of couplings now available from LEP.Comment: Latex file 7 pages+1 fig. (ps file appended after the latex file), CERN-TH.6913/9

Modified theory of gravity and the history of cosmic evolution

A continuous transition from early Friedmann-like radiation era through to late time cosmic acceleration passing through a long Friedmann-like matter dominated era followed by a second phase of radiation era has been realized in modified theory of gravity containing a combination of curvature squared term, a linear term, a three-half term and an ideal fluid. Thus the history of cosmic evolution is explained by modified theory of gravity singlehandedly. The second phase of radiation-like era might provide an explanation to the hydrogen and helium reionization at low redshift.Comment: 15 pages, 6 figures, Astrophys Space Sci (2014

BCS Instability and Finite Temperature Corrections to Tachyon Mass in Intersecting D1-Branes

A holographic description of BCS superconductivity is given in arxiv:1104.2843. This model was constructed by insertion of a pair of D8-branes on a D4-background. The spectrum of intersecting D8-branes has tachyonic modes indicating an instability which is identified with the BCS instability in superconductors. Our aim is to study the stability of the intersecting branes under finite temperature effects. Many of the technical aspects of this problem are captured by a simpler problem of two intersecting D1-branes on flat background. In the simplified set-up we compute the one-loop finite temperature corrections to the tree-level tachyon mass using the frame-work of SU(2) Yang-Mills theory in (1 + 1)-dimensions. We show that the one-loop two-point functions are ultraviolet finite due to cancellation of ultraviolet divergence between the amplitudes containing bosons and fermions in the loop. The amplitudes are found to be infrared divergent due to the presence of massless fields in the loops. We compute the finite temperature mass correction to all the massless fields and use these temperature dependent masses to compute the tachyonic mass correction. We show numerically the existence of a transition temperature at which the effective mass of the tree-level tachyons becomes zero, thereby stabilizing the brane configuration.Comment: Article, 95 pages, 59 figures, improved numerics, added reference