E. Yu. Melkumova

Energy-momentum balance in a particle domain wall perforating collision

We investigate the energy-momentum balance in the perforating collision of a point particle with an infinitely thin planar domain wall within the linearized gravity in arbitrary dimensions. Since the metric of the wall increases with distance, the wall and the particle are never free, and their energy-momentum balance involves not only the instantaneous kinetic momenta, but also the non-local contribution of gravitational stresses. However, careful analysis shows that the stresses can be unambiguously divided between the colliding objects leading to definition of the gravitationally dressed momenta. These take into account for gravity in the same way as the potential energy does in the non-relativistic theory, but our treatment is fully relativistic. Another unusual feature of our problem is the non-vanishing flux of the total energy-momentum tensor through the lateral surface of the world tube. In this case the zero divergence of the energy-momentum tensor does not imply conservation of the total momentum defined as the integral over the space-like section of the tube. But one can still define the conservation low infinitesimally, passing to time derivatives of the momenta. Using this definition we establish the momentum balance in terms of the dressed particle and wall momenta.

Axion bremsstrahlung from collisions of global strings

We calculate axion radiation emitted in the collision of two straight global strings. The strings are supposed to be in the unexcited ground state, to be inclined with respect to each other, and to move in parallel planes. Radiation arises when the point of minimal separation between the strings moves faster than light. This effect exhibits a typical Cerenkov nature. Surprisingly, it allows an alternative interpretation as bremsstrahlung under a collision of point charges in 2+1 electrodynamics. This can be demonstrated by suitable world-sheet reparameterizations and dimensional reduction. Cosmological estimates show that our mechanism generates axion production comparable with that from the oscillating string loops and may lead to further restrictions on the axion window.

Cerenkov radiation from moving straight strings

We study Cerenkov radiation from moving straight strings which slide with respect to each other in such a way that the projected intersection point moves faster than light. To calculate this effect we develop classical perturbation theory for the system of Nambu-Goto strings interacting with the dilaton, two-form, and gravity fields. In the first order, one encounters divergent self-action terms which are eliminated by classical renormalization of the string tension. Cerenkov radiation arises in the second order. It is generated by an effective source which contains contributions localized on the string world sheets and bulk contributions quadratic in the first-order fields. In the ultrarelativistic limit radiation exhibits angular peaking on the Cerenkov cone in the forward direction of the fast string in the rest frame of another. The radiation spectrum then extends up to high frequencies proportional to the square of the Lorentz factor of the relative velocity. Gravitational radiation is absent since the

About the singularity criterion in general relativity

1+2

Plasma graviton production in TeV-scale gravity

space-time transverse to the straight string does not allow for gravitons. A rough estimate of the Cerenkov radiation in the cosmological cosmic strings network is presented.

Nambu-Goldstone explosion under brane perforation

The metric in vacuum is found for the case of a spherical charged dust source. It is shown that a “dead zone” occurs in the vacuum, impenetrable to light or test particles. All singularities resulting from theorems about geodesic lines, are located in the “dead zone” and do not represent a physical problem for the theory. The infinite-density type singularities may be absent inside matter-filled zone.

Dilaton and axion bremsstrahlung from collisions of cosmic (super)strings

We develop the theory of interaction of classical plasma with Kaluza-Klein (KK) gravitons in the ADD model of TeV-scale gravity. Plasma is described within the kinetic approach as the system of charged particles and Maxwell field both confined on the brane. Interaction with multidimensional gravity living in the bulk with n compact extra dimensions is introduced within the linearized theory. The KK gravitons emission rates are computed taking into account plasma collective effects through the two-point correlation functions of the fluctuations of the plasma energy-momentum tensor. Apart from known mechanisms (such as bremsstrahlung and gravi-Primakoff effect) we find essentially collective channels such as the coalescence of plasma waves into gravitons which may be manifest in turbulent plasmas. Our results indicate that commonly used rates of the KK gravitons production in stars and supernovae may be underestimated.

Calculation of the Shirokov effect for circular orbits in an axisymmetric static gravitational field

We show that perforation of the three-brane by mass impinging upon it from the five-dimensional bulk excites Nambu-Goldstone spherical wave propagating outwards with the velocity of light. It is speculated that such an effect can give rise to “unmotivated” energy release events in the brane-world cosmological models.

Plasmon-graviton conversion in a magnetic field in TeV-scale gravity

We calculate dilaton and axion radiation generated in the collision of two straight initially unexcited strings and give a rough cosmological estimate of dilaton and axion densities produced via this mechanism in the early universe.

Plasma gravi-bremsstrahlung in TeV-scale gravity

In the post-Newtonian approximation of the metric theory of gravitation we calculate the frequencies of Shirokov oscillations for equatorial circular geodesics in an asymptotically flat static space-time which satisfies the conditions of axial symmetry and symmetry with respect to the plane of the equator.