13 research outputs found
Asymmetric embedding in brane cosmology
We derive a system of cosmological equations for a braneworld with induced
curvature which is a junction between several bulk spaces. The permutation
symmetry of the bulk spaces is not imposed, and the values of the fundamental
constants, and even the signatures of the extra dimension, may be different on
different sides of the brane. We then consider the usual partial case of two
asymmetric bulk spaces and derive an exact closed system of scalar equations on
the brane. We apply this result to the cosmological evolution on such a brane
and describe its various partial cases.Comment: 10 page
Linearized gravity on the Randall-Sundrum two-brane background with curvature terms in the action for the branes
We study gravitational perturbations in the Randall-Sundrum two-brane
background with scalar-curvature terms in the action for the branes, allowing
for positive as well as negative bulk gravitational constant. In the zero-mode
approximation, we derive the linearized gravitational equations, which have the
same form as in the original Randall-Sundrum model but with different
expressions for the effective physical constants. We develop a generic method
for finding tachyonic modes in the theory, which, in the model under
consideration, may exist only if the bulk gravitational constant is negative.
In this case, if both brane gravitational constants are nonzero, the theory
contains one or two tachyonic mass eigenvalues in the gravitational sector. If
one of the brane gravitational constants is set to zero, then either a single
tachyonic mass eigenvalue is present or tachyonic modes are totally absent
depending on the relation between the nonzero brane gravitational constant and
brane separation. In the case of negative bulk gravitational constant, the
massive gravitational modes have ghost-like character, while the massless
gravitational mode is not a ghost in the case where tachyons are absent.Comment: 23 pages, revtex, published versio
Semiclassical gravitational effects near a singular magnetic flux
We consider the backreaction of the vacuum polarization effect for a massive
charged scalar field in the presence of a singular magnetic massless string on
the background metric. Using semiclassical approach, we find the first-order
(in units) metric modifications and the corresponding gravitational
potential and deficit angle. It is shown that, in certain region of values of
coupling constant and magnetic flux, the gravitational potential and deficit
angle can be positive as well as negative over all distances from the string
and can even change its sign. Unlike the case of massless scalar field, the
gravitational corrections were found to have short-range behavior.Comment: 14 pages, 4 figures, journal versio
Cosmic Mimicry: Is LCDM a Braneworld in Disguise ?
For a broad range of parameter values, braneworld models display a remarkable
property which we call cosmic mimicry. Cosmic mimicry is characterized by the
fact that, at low redshifts, the Hubble parameter in the braneworld model is
virtually indistinguishable from that in the LCDM cosmology. An important point
to note is that the \Omega_m parameters in the braneworld model and in the LCDM
cosmology can nevertheless be quite different. Thus, at high redshifts (early
times), the braneworld asymptotically expands like a matter-dominated universe
with the value of \Omega_m inferred from the observations of the local matter
density. At low redshifts (late times), the braneworld model behaves almost
exactly like the LCDM model but with a renormalized value of the cosmological
density parameter \Omega_m^{LCDM}. The redshift which characterizes cosmic
mimicry is related to the parameters in the higher-dimensional braneworld
Lagrangian. Cosmic mimicry is a natural consequence of the scale-dependence of
gravity in braneworld models. The change in the value of the cosmological
density parameter is shown to be related to the spatial dependence of the
effective gravitational constant in braneworld theory. A subclass of mimicry
models lead to an older age of the universe and also predict a redshift of
reionization which is lower than z_{reion} \simeq 17 in the LCDM cosmology.
These models might therefore provide a background cosmology which is in better
agreement both with the observed quasar abundance at z \gsim 4 and with the
large optical depth to reionization measured by the Wilkinson Microwave
Anisotropy Probe.Comment: 22 pages, 4 figures. A subsection and references added; main results
remain unchanged. Accepted for publication in JCA
Gravitational instability on the brane: the role of boundary conditions
An outstanding issue in braneworld theory concerns the setting up of proper
boundary conditions for the brane-bulk system. Boundary conditions (BC's)
employing regulatory branes or demanding that the bulk metric be nonsingular
have yet to be implemented in full generality. In this paper, we take a
different route and specify boundary conditions directly on the brane thereby
arriving at a local and closed system of equations (on the brane). We consider
a one-parameter family of boundary conditions involving the anisotropic stress
of the projection of the bulk Weyl tensor on the brane and derive an exact
system of equations describing scalar cosmological perturbations on a generic
braneworld with induced gravity. Depending upon our choice of boundary
conditions, perturbations on the brane either grow moderately (region of
stability) or rapidly (instability). In the instability region, the evolution
of perturbations usually depends upon the scale: small scale perturbations grow
much more rapidly than those on larger scales. This instability is caused by a
peculiar gravitational interaction between dark radiation and matter on the
brane. Generalizing the boundary conditions obtained by Koyama and Maartens, we
find for the Dvali-Gabadadze-Porrati model an instability, which leads to a
dramatic scale-dependence of the evolution of density perturbations in matter
and dark radiation. A different set of BC's, however, leads to a more moderate
and scale-independent growth of perturbations. For the mimicry braneworld,
which expands like LCDM, this class of BC's can lead to an earlier epoch of
structure formation.Comment: 35 pages, 9 figures, an appendix and references added, version to be
published in Classical and Quantum Gravit