Example of a stable wormhole in general relativity

We study a static, spherically symmetric wormhole model whose metric coincides with that of the so-called Ellis wormhole but the material source of gravity consists of a perfect fluid with negative density and a source-free radial electric or magnetic field. For a certain class of fluid equations of state, it has been shown that this wormhole model is linearly stable under both spherically symmetric perturbations and axial perturbations of arbitrary multipolarity. A similar behavior is predicted for polar nonspherical perturbations. It thus seems to be the first example of a stable wormhole model in the framework of general relativity (at least without invoking phantom thin shells as wormhole sources).Comment: 6 pages, no figure

Responses of the Brans-Dicke field due to gravitational collapses

We study responses of the Brans-Dicke field due to gravitational collapses of scalar field pulses using numerical simulations. Double-null formalism is employed to implement the numerical simulations. If we supply a scalar field pulse, it will asymptotically form a black hole via dynamical interactions of the Brans-Dicke field. Hence, we can observe the responses of the Brans-Dicke field by two different regions. First, we observe the late time behaviors after the gravitational collapse, which include formations of a singularity and an apparent horizon. Second, we observe the fully dynamical behaviors during the gravitational collapse and view the energy-momentum tensor components. For the late time behaviors, if the Brans-Dicke coupling is greater (or smaller) than -1.5, the Brans-Dicke field decreases (or increases) during the gravitational collapse. Since the Brans-Dicke field should be relaxed to the asymptotic value with the elapse of time, the final apparent horizon becomes time-like (or space-like). For the dynamical behaviors, we observed the energy-momentum tensors around $\omega$ ~ -1.5. If the Brans-Dicke coupling is greater than -1.5, the $T_{uu}$ component can be negative at the outside of the black hole. This can allow an instantaneous inflating region during the gravitational collapse. If the Brans-Dicke coupling is less than -1.5, the oscillation of the $T_{vv}$ component allows the apparent horizon to shrink. This allows a combination that violates weak cosmic censorship. Finally, we discuss the implications of the violation of the null energy condition and weak cosmic censorship.Comment: 28 pages, 14 figure

Thermocapillary rupture in falling liquid films at moderate Reynolds numbers

An experimental study of the flow of a water film over a heated surface for Re = 15-50 was performed. The influence of the development of thermocapillary instability on the wave amplitudes, the deformation of the surface of the liquid film, and the formation of the first stable dry spot on the heater are investigated. It is shown that the interaction of waves with thermocapillary structures can lead to an increase in the critical heat flux corresponding to the rupture of the liquid film, as compared with the data known in the literature

Homogeneous singularities inside collapsing wormholes

We analyze analytically and numerically the origin of the singularity in the course of the collapse of a wormhole with the exotic scalar field Psi with negative energy density, and with this field Psi together with the ordered magnetic field H. We do this under the simplifying assumptions of the spherical symmetry and that in the vicinity of the singularity the solution of the Einstein equations depends only on one coordinate (the homogeneous approximation). In the framework of these assumptions we found the principal difference between the case of the collapse of the ordinary scalar field Phi with the positive energy density together with an ordered magnetic field H and the collapse of the exotic scalar field Psi together with the magnetic field H. The later case is important for the possible astrophysical manifestation of the wormholes.Comment: 10 pages, 5 figures each of which has a),b),c),and d) sub-figures. To be published in "Physical review. D, Particles, fields, gravitation, and cosmology