
Calendar of Physics Talks Vienna
Selfgravitating 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 NambuGoto
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
codimension2 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 branebulk 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 backreaction
of the brane to the bulk ("gravitating NambuGoto matching conditions"). In
the present paper we consider in detail the case of a codimension2 brane in sixdimensional
EinsteinGaussBonnet 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 fourdimensional Newtonâ€™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 810), Institute for Theoretical Physics, Vienna University of Technology 
Contact:  Daniel Grumiller 
