Defect Formation with Bulk Fields

It has recently been realized that brane-antibrane annihilation (a possible explanation for ending inflation) may result in defect formation, due to the dynamics of the tachyon field. Studies of this possibility have generally ignored the interaction of the brane fields with fields in the bulk; recently it has been argued [1] that interactions with bulk fields suppress or even eliminate defect formation. To investigate the impact of bulk fields on brane defect formation, we construct a toy model that captures the essential features of the tachyon condensation with bulk fields. We study the structure of defects in this toy model, and simulate their formation and evolution on the lattice. We find that, while the energetics and interactions of defects are influenced by the size of the extra dimension and the bulk-brane coupling, the bulk-brane coupling does not prevent the formation of a defect network.Comment: 23 pages, 5 figures, revtex4, 2 appendices adde

Origin of matter out of pure curvature

We propose a mechanism for origin of matter in the universe in the framework of Einstein-Gauss-Bonnet gravity in higher dimensions. The recently discovered new static black hole solution by the authors \cite{md2006} with the Kaluza-Klein split up of spacetime as a product of the usual {\ma M}^4 with a space of negative constant curvature is indeed a pure gravitational creation of a black hole which is also endowed with a Maxwell-like {\it gravitational charge} in four-dimensional vacuum spacetime. Further it could be envisioned as being formed from anti-de Sitter spacetime by collapse of radially inflowing charged null dust. It thus establishes the remarkable reciprocity between matter and gravity - as matter produces gravity (curvature), gravity too produces matter.Comment: 8 pages, 1 Fig, Received Honorable Mention in 2007 GRF Essay Competition, Summary of the talk given at Himalayan Relativity Dialogue at Mirik, April 18-20, 200

Kaluza-Klein black hole with negatively curved extra dimensions in string generated gravity models

We obtain a new exact black-hole solution in Einstein-Gauss-Bonnet gravity with a cosmological constant which bears a specific relation to the Gauss-Bonnet coupling constant. The spacetime is a product of the usual 4-dimensional manifold with a $(n-4)$-dimensional space of constant negative curvature, i.e., its topology is locally {\ma M}^n \approx {\ma M}^4 \times {\ma H}^{n-4}. The solution has two parameters and asymptotically approximates to the field of a charged black hole in anti-de Sitter spacetime. The most interesting and remarkable feature is that the Gauss-Bonnet term acts like a Maxwell source for large $r$ while at the other end it regularizes the metric and weakens the central singularity.Comment: 4 pages, 2 figures, final version to appear in Physical Review D as a rapid communicatio

The dynamical Casimir effect in braneworlds

In braneworld cosmology the expanding Universe is realized as a brane moving through a warped higher-dimensional spacetime. Like a moving mirror causes the creation of photons out of vacuum fluctuations, a moving brane leads to graviton production. We show that, very generically, Kaluza-Klein (KK) particles scale like stiff matter with the expansion of the Universe and can therefore not represent the dark matter in a warped braneworld. We present results for the production of massless and KK gravitons for bouncing branes in five-dimensional anti de Sitter space. We find that for a realistic bounce the back reaction from the generated gravitons will be most likely relevant. This letter summarizes the main results and conclusions from numerical simulations which are presented in detail in a long paper [M.Ruser and R. Durrer, Phys. Rev. D 76, 104014 (2007), arXiv:0704.0790]Comment: misprints corrected, matches published versio

New Hints from General Relativity

The search for a quantum theory of gravity has followed two parallel but different paths. One aims at arriving at the final theory starting from a priori assumptions as to its form and building it from the ground up. The other tries to infer as much as possible about the unknown theory from the existing ones and use our current knowledge to constrain the possibilities for the quantum theory of gravity. Probably the biggest success of the second path has been the results of black hole thermodynamics. The subject of this essay is a new, highly promising such result, the application of quasinormal modes in quantum gravity.Comment: This essay received an "honorable mention" in the 2003 Essay Competition of the Gravity Research Foundatio

D3-branes dynamics and black holes

Using the D3-brane as the fundamental tool, we adress two aspects of D-branes physics. The first regards the interaction between two electromagnetic dual D-branes in 10 dimensions. In particular, we give a meaning to {\it both} even and odd spin structure contributions, the latter being non vanishing for non zero relative velocity $v$ (and encoding the Lorentz-like contribution). The second aspect regards the D-brane/black holes correspondence. We show how the 4 dimensional configuration corresponding to a {\it single} D3-brane wrapped on the orbifold T^6/Z_3 represents a regular Reissner-Nordstrom solution of d=4 N=2 supergravityComment: 8 pages, latex, 1 eps figure. Talk presented by M. Bertolini at the conference "Quantum aspects of gauge theories, supergravity and unification" in Corfu`; to appear in the proceeding

Path integral evaluation of Dbrane amplitudes

We extend Polchinski's evaluation of the measure for the one-loop closed string path integral to open string tree amplitudes with boundaries and crosscaps embedded in Dbranes. We explain how the nonabelian limit of near-coincident Dbranes emerges in the path integral formalism. We give a careful path integral derivation of the cylinder amplitude including the modulus dependence of the volume of the conformal Killing group.Comment: Extended version replacing hep-th/9903184, includes discussion of nonabelian limit, Latex, 10 page

Moduli Stabilization with the String Higgs Effect

We review the notion of the Higgs effect in the context of string theory. We find that by including this effect in time dependent backgrounds, one is led to a natural mechanism for stabilizing moduli at points of enhanced gauge symmetry. We consider this mechanism for the case of the radion (size of the extra dimensions) and find that as decompactification of the large spatial dimensions takes place the radion will remain stabilized at the self dual radius. We discuss how this mechanism can be incorporated into models of string cosmology and brane inflation to resolve some outstanding problems. We also address some issues regarding which string states should be included when constructing low energy actions in string cosmology.Comment: 20 pages, references added, typos correcte