513 research outputs found

### Brane induced gravity from asymmetric warped compactification

We show that brane induced gravity can be realized as a low energy effective
theory of brane worlds with asymmetric warped compactification. A
self-accelerating universe without cosmological constant on the brane can be
realized in a model where one side of the bulk has finite volume, but the other
side has infinite volume. The spin-2 perturbations for brane induced gravity
and asymmetric warped compactification models have the same spectrum at low
energies. For a de Sitter brane, the spin-2 graviton has mass in the range
$0<m^2 \leq 2H^2$, with $m^2=2H^2$ in the self-accelerating universe.Comment: 4 pages, 3 figures, typos corrected, a reference adde

### Structure formation in modified gravity models alternative to dark energy

We study structure formation in phenomenological models in which the
Friedmann equation receives a correction of the form
$H^{\alpha}/r_c^{2-\alpha}$, which realize an accelerated expansion without
dark energy. In order to address structure formation in these model, we
construct simple covariant gravitational equations which give the modified
Friedmann equation with $\alpha=2/n$ where $n$ is an integer. For $n=2$, the
underlying theory is known as a 5D braneworld model (the DGP model). Thus the
models interpolate between the DGP model ($n=2, \alpha=1$) and the LCDM model
in general relativity ($n \to \infty, \alpha \to 0$). Using the covariant
equations, cosmological perturbations are analyzed. It is shown that in order
to satisfy the Bianchi identity at a perturbative level, we need to introduce a
correction term $E_{\mu \nu}$ in the effective equations. In the DGP model,
$E_{\mu \nu}$ comes from 5D gravitational fields and correct conditions on
$E_{\mu \nu}$ can be derived by solving the 5D perturbations. In the general
case $n>2$, we have to assume the structure of a modified theory of gravity to
determine $E_{\mu \nu}$. We show that structure formation is different from a
dark energy model in general relativity with identical expansion history and
that quantitative features of the difference crucially depend on the conditions
on $E_{\mu \nu}$, that is, the structure of the underlying theory of modified
gravity. This implies that it is essential to identify underlying theories in
order to test these phenomenological models against observational data and,
once we identify a consistent theory, structure formation tests become
essential to distinguish modified gravity models from dark energy models in
general relativity.Comment: 12 pages, 3 figure

### Radion and Large Scale Anisotropy on the Brane

We investigate the effect of the radion on cosmological perturbations in the
brane world. The S^1/Z_2 compactified 5D Anti-de Sitter spacetime bounded by
positive and negative tension branes is considered. The radion is the relative
displacement of the branes in this model. We find two different kinds of the
radion at the linear perturbation order for a cosmological brane. One describes
a "fluctuation" of the brane which does not couple to matter on the brane. The
other describes a "bend" of the brane which couples to the matter. The bend
determines the curvature perturbation on the brane. At large scales, the radion
interacts with anisotopic perturbations in the bulk. By solving the bulk
anisotropic perturbations, large scale metric perturbations and anisotropies of
the Cosmic Microwave Background (CMB) on the positive tension brane are
calculated. We find an interesting fact that the radion contributes to the CMB
anisotropies. The observational consequences of these effects are discussed.Comment: Improved version to appear in PRD, Results and conclusions unchange

### Non-Gaussianity of quantum fields during inflation

In this review, we discuss how non-Gaussianity of cosmological perturbations
arises from inflation. After introducing the in-in formalism to calculate the
$n$-point correlation function of quantum fields, we present the computation of
the bispectrum of the curvature perturbation generated in general single field
inflation models. The shapes of the bispectrum are compared with the local-type
non-Gaussianity that arises from non-linear dynamics on super-horizon scales.Comment: 17 pages, 2 figures, references added, matches published version,
invited review for CQG issue on non-linear cosmolog

### Are there ghosts in the self-accelerating brane universe?

We study the spectrum of gravitational perturbations about a vacuum de Sitter
brane with the induced 4D Einstein-Hilbert term, in a 5D Minkowski spacetime
(DGP model). We consider solutions that include a self-accelerating univese,
where the accelerating expansion of the universe is realized without
introducing a cosmological constant on the brane. The mass of the discrete mode
for the spin-2 graviton is calculated for various $Hr_c$, where $H$ is the
Hubble parameter and $r_c$ is the cross-over scale determined by the ratio
between the 5D Newton constant and the 4D Newton constant. We show that, if we
introduce a positive cosmological constant on the brane ($Hr_c >1$), the spin-2
graviton has mass in the range $0 < m^2 < 2H^2$ and there is a normalisable
brane fluctuation mode with mass $m^2=2 H^2$. Although the brane fluctuation
mode is healthy, the spin-2 graviton has a helicity-0 excitation that is a
ghost. If we allow a negative cosmological constant on the brane, the brane
bending mode becomes a ghost for $1/2 < Hr_c <1$. This confirms the results
obtained by the boundary effective action that there exists a scalar ghost mode
for $Hr_c >1/2$. In a self-accelerating universe $Hr_c=1$, the spin-2 graviton
has mass $m^2=2H^2$, which is known to be a special case for massive gravitons
in de Sitter spacetime where the graviton has no helicity-0 excitation and so
no ghost. However, in DGP model, there exists a brane fluctuation mode with the
same mass and there arises a mixing between the brane fluctuation mode and the
spin-2 graviton. We argue that this mixing presumably gives a ghost in the
self-accelerating universe by continuity across $Hr_c=1$, although a careful
calculation of the effective action is required to verify this rigorously.Comment: 5 pages, 1 figure, significant revisions, conclusion on the self-
accelerating universe change

### Non-Gaussianities in DBI inflation with angular motion

We study DBI spinflation models with angular potentials that are derived in
string theory. We analyse the background dynamics with different parameter sets
and study the impact of changing each parameter on inflationary dynamics. It is
known that the conversion of the entropy perturbation into the curvature
perturbation gives multi-field DBI inflation models a possibility of satisfying
the observational constraints by relaxing the stringent microphysical
constraint that disfavours single field DBI inflation models. we show that our
model is excluded by the Planck satellite observations even with the conversion
mechanism regardless of the parameter set.Comment: 23 pages, 11 figures, matches published versio

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