E.Y. Melkumova

Gravitational and dilaton radiation from a relativistic membrane

Recent scenarios of the TeV-scale brane cosmology suggest a possibility of existence in the early universe of two-dimensional topological defects: relativistic membranes. Like cosmic strings, oscillating membranes could emit gravitational radiation contributing to a stochastic background of gravitational waves. We calculate dilaton and gravitational radiation from a closed toroidal membrane excited along one homology cycle. The spectral-angular distributions of dilaton and gravitational radiation are obtained in a closed form in terms of Bessels functions. The angular distributions are affected by oscillating factors due to an interference of radiation from different segments of the membrane. The dilaton radiation power is dominated by a few lower harmonics of the basic frequency, while the spectrum of the gravitational radiation contains also a substantial contribution from higher harmonics. The radiative lifetime of the membrane is determined by its tension and depends weakly on the ratio of two radii of the torus. Qualitatively it is equal to the ratio of the membrane area at the maximal extension to the gravitational radius of the membrane as a whole.

Piercing of domain walls: new mechanism of gravitational radiation

A bstractDomain wall (DW) moving in media undergoes the friction force due to particle scattering. However certain particles are not scattered, but perforate the wall. As a result, the wall gets excited in the form of the branon wave, while the particle experiences an acceleration jump. This gives rise to generation of gravitational waves which we call “piercing gravitational radiation” (PGR). Though this effect is of higher order in the gravitational constant than the quadrupole radiation from the collapsing DWs, its amplitude is enhanced in the case of relativistic particles or photons because of absence of the velocity factor which is present in the quadrupole formula. We derive the spectral-angular distribution of PGR within the simplified model of the weakly gravitating particle-wall system in Minkowski space-time of arbitrary dimensions. Within this model the radiation amplitude is obtained analytically. The spectral-angular distribution of PGR in such an approach suffers from infrared and ultraviolet divergences as well as from collinear divergence in the case of a massless perforating particle. Different cut-off schemes appropriate in various dimensions are discussed. Our results are applicable both to cosmological DWs and to the braneworld models. PGR can be relevant in the infrared part of the spectrum of the relic gravitons where radiation from the collapsed DWs is damped.

Domain walls: Momentum conservation in absence of asymptotic states

Gravitational potentials of the domain walls in the linearized gravity are growing with distance, so the particle scattering by the wall can not be described in terms of free asymptotic states. In the non-relativistic case this problem is solved using the concept of the potential energy. We show that in the relativistic case one is able to introduce gravitationally dressed momenta the sum of which is conserved up to the momentum flux through the lateral surface of the world tube describing losses due to excitation of the branon waves.

CERENKOV RADIATION FROM COLLISIONS OF STRAIGHT COSMIC (SUPER)STRINGS

Department of Physics, Moscow State University,119899, Moscow, Russia‡E-mail: elenamelk@srd.sinp.msu.ruAbstract. We consider Cerenkov radiation which must arise when randomlyoriented straight cosmic (super)strings move with relativistic velocities withoutintercommutation. String interactions via dilaton, two-form and gravity (gravity beingthe dominant force in the ultra-relativistic regime) leads to formation of superluminalsources which generate Cerenkov radiation of dilatons and axions. Though the effectis of the second order in the couplings of strings to these fields, its total efficiencyis increased by high dependence of the radiation rate on the Lorentz-factor of thecollision.

Form-field bremsstrahlung under collision of p-branes

We calculate classically the radiation of the antisymmetric form field generated in the collision of two non-excited membranes moving with ultrarelativistic velocities in five space-time dimensions. The interaction between branes through the form field is treated perturbatively with the deflection angle as a small parameter. Radiation arises in the second order approximation if the domain of the minimal separation between the branes moves with the superluminal velocity. It exhibits typical Cherenkov cone features. Generalization to p-branes colliding in

Branestrahlung: radiation in the particle-brane collision

D=p+3

Gravitational radiation under perforation of domain walls

dimensions is straightforward.