# Calendar of Physics Talks Vienna

 Self-gravitating branes again
 Speaker: Maria Irakleidou (Vienna University of Technology) Abstract: In view of the absence of direct observational evidence of regular gravitating defects, we raise on theoretical grounds the question of the physical relevance of the Israel matching conditions and their generalizations to higher dimensions and codimensions, the standard cornerstone of the braneworld paradigm and other membrane scenarios. Our reasoning is based on two points: First, the incapability of the conventional matching conditions to accept the Nambu-Goto probe limit (even the geodesic limit of the Israel matching conditions is not acceptable since being the geodesic equation a kinematical fact it should be preserved independent of the gravitational theory or the codimension of the defect, which is not the case for these matching conditions). Second, in the D- dimensional spacetime we live (maybe D = 4), classical defects of any possible codimension could in principle be constructed (even in the lab), and therefore, they should be compatible. The standard matching conditions fail to accept codimension-2 and 3 defects for D = 4 (which represents effectively the spacetime at certain length and energy scales) and most probably fail to accept high enough codimensional defects for any D since there is no corresponding high enough Lovelock density to support them. Here, we indicate that the problem is not the distributional character of the defects, neither the gravitational theory, but mainly the equations of motion of the defects. We propose alternative matching conditions which seem to satisfy all the previous criteria. Instead of varying the brane-bulk action with respect to the bulk metric at the brane position, we vary with respect to the brane embedding fields in a way that takes into account the gravitational back-reaction of the brane to the bulk ("gravitating Nambu-Goto matching conditions"). In the present paper we consider in detail the case of a codimension-2 brane in sixdimensional Einstein-Gauss-Bonnet gravity, prove its consistency for an axially symmetric cosmological configuration and show that the theory possesses richer structure compared to the standard theory. In all the cosmologies found there is the standard LFRW behaviour and extra correction terms. In particular, one of these solutions for a radiation brane and for a range of the integration constants avoids a cosmological singularity (both in density and curvature) and undergoes accelerated expansion near the minimum scale factor. In the presence of an induced gravity term, there naturally appears in the theory the effective cosmological constant scale lambda/(M_6^4 r_c^2), which for a value of the brane tension lambda \sim M_6^4 (e.g. TeV^4) and r_c \sim H_0^{-1} gives the observed value of the cosmological constant (a similar scale fails for other dimensions D). Even if the constraint from the four-dimensional Newton’s constant GN is not easily satisfied, there is still hope, since GN in general depends not only on the parameters of the action but also on the integration constants of the considered solution. Date: Thu, 22.08.2013 Time: 16:00 Duration: 60 min Location: SEM 136 (Freihaus, Wiedner Hauptstrasse 8-10), Institute for Theoretical Physics, Vienna University of Technology Contact: Daniel Grumiller