1,947 research outputs found
Cosmological evolution of regularized branes in 6D warped flux compactifications
We study the cosmological evolution of extended branes in 6D warped flux
compactification models. The branes are endowed with the three ordinary spatial
dimensions, which are assumed to be homogeneous and isotropic, as well as an
internal extra dimension compactified on a circle. We embed these codimension 1
branes in a static bulk 6D spacetime, whose geometry is a solution of 6D
Einstein-Maxwell or Einstein-Maxwell-dilaton theories, corresponding to a
warped flux compactification. The brane matter consists of a complex scalar
field which is coupled to the bulk U(1) gauge field. In both models, we show
that there is critical point which the brane cannot cross as it moves in the
bulk. We study the cosmological behaviour, especially when the brane approaches
this critical point or one of the two conical singularities. In the present
setup where the bulk geometry is fixed, we find that the brane cosmology does
not coincide with the standard one in the low energy limit.Comment: 14 pages, 4 figures, references and discussions added, to appear in
PR
Electrical conductivity and tortuosity of solid foam: Effect of pore connections
International audienceNumerical and analytical methods at both micro-and mesoscales are used to study how the electrical resistivity and the high frequency tortuosity of solid foam are modied by the presence of membranes that partially or totally close the cell windows connecting neighbor pores. Finite element method (FEM) simulations are performed on two pores connected by a single-holed membrane and on well-ordered Kelvin foam. For two pores connected by a single-holed membrane, we show that the equation for pore access resistance obtained by Sahu and Zwolak (Phys. Rev. E 98, 012404, 2018) can predict, after a few modications, the electrical resistivity at the membrane scale for a large range of membrane apertures. In the second part, considering these analytical results, we build a pore-network model by using two kinds of conductances at the pore scale-inter-pore conductance and intra-pore conductance. Local inter-pore resistances govern foam electrical conductivity at small membrane aperture size, but when the membrane aperture has the same order of magnitude as the pore size, the intra-pore resistances are no longer negligible. An important success of this pore-network model is that it can be used to study the eects of percolation on the foam electrical conductivity by using pore-network simulations on larger samples containing a few thousands of pores and having dierent proportions of closed membrane randomly distributed over the sample. The tortuosity is found to be drastically larger than one in foam containing membranes with small apertures or a signicant fraction of closed membranes
Bulk gravitons from a cosmological brane
We investigate the emission of gravitons by a cosmological brane into an Anti
de Sitter five-dimensional bulk spacetime. We focus on the distribution of
gravitons in the bulk and the associated production of `dark radiation' in this
process. In order to evaluate precisely the amount of dark radiation in the
late low-energy regime, corresponding to standard cosmology, we study
numerically the emission, propagation and bouncing off the brane of bulk
gravitons.Comment: 27 pages, 5 figures, minor corrections. Final versio
Solutions of gauge invariant cosmological perturbations in long-wavelength limit
We investigate gauge invariant cosmological perturbations in a spatially flat
Friedman-Robertson-Walker universe with scalar fields. It is well known that
the evolution equation for the gauge invariant quantities has exact solutions
in the long-wavelength limit. We find that these gauge invariant solutions can
be obtained by differentiating the background solution with respect to
parameters contained in the background system. This method is very useful when
we analyze the long-wavelength behavior of cosmological perturbation with
multiple scalar fields.Comment: 17 pages, will appear in Classical and Quantum Gravit
Path Integral Quantization of Cosmological Perturbations
We derive the first order canonical formulation of cosmological perturbation
theory in a Universe filled by a few scalar fields. This theory is quantized
via well-defined Hamiltonian path integral. The propagator which describes the
evolution of the initial (for instance, vacuum) state, is calculated.Comment: 16 pages, ETH-TH/94-0
Living on a dS brane: Effects of KK modes on inflation
We develop a formalism to study non-local higher-dimensional effects in
braneworld scenarios from a four-dimensional effective theory point of view and
check it against the well-known Garriga-Tanaka result in the appropriate limit.
We then use this formalism to study the spectrum of density perturbations
during inflation as seen from the lower-dimensional effective theory. In
particular, we find that the gravitational potential is greatly enhanced at
short wavelengths. The consequences to the curvature perturbations are
nonetheless very weak and will lead to no characteristic signatures on the
power spectrum.Comment: 21 pages, no figure
Primordial gravitational waves in inflationary braneworld
We study primordial gravitational waves from inflation in Randall-Sundrum
braneworld model. The effect of small change of the Hubble parameter during
inflation is investigated using a toy model given by connecting two de Sitter
branes. We analyze the power spectrum of final zero-mode gravitons, which is
generated from the vacuum fluctuations of both initial Kaluza-Klein modes and
zero-mode. The amplitude of fluctuations is confirmed to agree with the
four-dimensional one at low energies, whereas it is enhanced due to the
normalization factor of zero-mode at high energies. We show that the
five-dimensional spectrum can be well approximated by applying a simple mapping
to the four-dimensional fluctuation amplitude.Comment: 16 pages, 4 figures, typos correcte
Bulk inflaton shadows of vacuum gravity
We introduce a -dimensional vacuum description of five-dimensional
bulk inflaton models with exponential potentials that makes analysis of
cosmological perturbations simple and transparent. We show that various
solutions, including the power-law inflation model recently discovered by
Koyama and Takahashi, are generated from known -dimensional vacuum
solutions of pure gravity. We derive master equations for all types of
perturbations, and each of them becomes a second order differential equation
for one master variable supplemented by simple boundary conditions on the
brane. One exception is the case for massive modes of scalar perturbations. In
this case, there are two independent degrees of freedom, and in general it is
difficult to disentangle them into two separate sectors.Comment: 22 pages, 4 figures, revtex; v2: references adde
- âŠ