2,049 research outputs found
Breeding Habits of the Northern Dace
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/119116/1/ecy1929101161.pd
Gauss-Bonnet brane gravity with a confining potential
A brane scenario is envisaged in which the -dimensional bulk is endowed
with a Gauss-Bonnet term and localization of matter on the brane is achieved by
means of a confining potential. The resulting Friedmann equations on the brane
are modified by various extra terms that may be interpreted as the X-matter,
providing a possible phenomenological explanation for the accelerated expansion
of the universe. The age of the universe in this scenario is studied and shown
to be consistent with the present observational data.Comment: 14 pages, 4 figures, to appear in PR
The inflationary prediction for primordial non-gaussianity
We extend the \delta N formalism so that it gives all of the stochastic
properties of the primordial curvature perturbation \zeta if the initial field
perturbations are gaussian. The calculation requires only the knowledge of some
family of unperturbed universes. A formula is given for the normalisation \fnl
of the bispectrum of \zeta, which is the main signal of non-gaussianity.
Examples of the use of the formula are given, and its relation to cosmological
perturbation theory is explained.Comment: Revtex Latex file. 4 pages, no figures. v4: minor changes, typos
corrected, references added and updated. Version published in Physical Review
Letter
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
Non-linear isocurvature perturbations and non-Gaussianities
We study non-linear primordial adiabatic and isocurvature perturbations and
their non-Gaussianity. After giving a general formulation in the context of an
extended delta N-formalism, we analyse in detail two illustrative examples. The
first is a mixed curvaton-inflaton scenario in which fluctuations of both the
inflaton and a curvaton (a light isocurvature field during inflation)
contribute to the primordial density perturbation. The second example is that
of double inflation involving two decoupled massive scalar fields during
inflation. In the mixed curvaton-inflaton scenario we find that the bispectrum
of primordial isocurvature perturbations may be large and comparable to the
bispectrum of adiabatic curvature perturbations.Comment: 24 pages, typos corrected, references adde
Bulk Gravitational Field and Cosmological Perturbations on the Brane
We investigate the effect of the bulk gravitational field on the cosmological
perturbations on a brane embedded in the 5D Anti-de Sitter (AdS) spacetime. The
effective 4D Einstein equations for the scalar cosmological perturbations on
the brane are obtained by solving the perturbations in the bulk. Then the
behaviour of the corrections induced by the bulk gravitational field to the
conventional 4D Einstein equation are determined. Two types of the corrections
are found. First we investigate the corrections which become significant at
scales below the AdS curvature scales and in the high energy universe with the
energy density larger than the tension of the brane. The evolution equation for
the perturbations on the brane is found and solved. Another type of the
corrections is induced on the brane if we consider the bulk perturbations which
do not contribute to the metric perturbations but do contribute to the matter
perturbations. At low energies, they have imaginary mass m^2=-(2/3) \k^2 in
the bulk where \k is the 3D comoving wave number of the perturbations. They
diverge at the horizon of the AdS spacetime. The induced density perturbations
behave as sound waves with sound velocity in the low energy
universe. At large scales, they are homogeneous perturbations that depend only
on time and decay like radiation. They can be identified as the perturbations
of the dark radiation. They produce isocurvature perturbations in the matter
dominated era. Their effects can be observed as the shifts of the location and
the height of the acoustic peak in the CMB spectrum.Comment: 35 pages, 1 figur
Exactly solvable model for cosmological perturbations in dilatonic brane worlds
We construct a model where cosmological perturbations are analytically solved
based on dilatonic brane worlds. A bulk scalar field has an exponential
potential in the bulk and an exponential coupling to the brane tension. The
bulk scalar field yields a power-law inflation on the brane. The exact
background metric can be found including the back-reaction of the scalar field.
Then exact solutions for cosmological perturbations which properly satisfy the
junction conditions on the brane are derived. These solutions provide us an
interesting model to understand the connection between the behavior of
cosmological perturbations on the brane and the geometry of the bulk. Using
these solutions, the behavior of an anisotropic stress induced on the
inflationary brane by bulk gravitational fields is investigated.Comment: 30 pages, typos corrected, reference adde
Bulk gravitational field and dark radiation on the brane in dilatonic brane world
We discuss the connection between the dark radiation on the brane and the
bulk gravitational field in a dilatonic brane world model proposed by Koyama
and Takahashi where the exact solutions for the five dimensional cosmological
perturbations can be obtained analytically. It is shown that the dark radiation
perturbation is related to the non-normalizable Kaluza-Klein (KK) mode of the
bulk perturbations. For the de Sitter brane in the anti-de Sitter bulk, the
squared mass of this KK mode is where is the Hubble parameter on
the brane. This mode is shown to be connected to the excitation of small black
hole in the bulk in the long wavelength limit. The exact solution for an
anisotropic stress on the brane induced by this KK mode is found, which plays
an important role in the calculation of cosmic microwave background radiation
anisotropies in the brane world.Comment: 11 page
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
Density perturbations in the brane-world
In Randall-Sundrum-type brane-world cosmologies, density perturbations
generate Weyl curvature in the bulk, which in turn backreacts on the brane via
stress-energy perturbations. On large scales, the perturbation equations
contain a closed system on the brane, which may be solved without solving for
the bulk perturbations. Bulk effects produce a non-adiabatic mode, even when
the matter perturbations are adiabatic, and alter the background dynamics. As a
consequence, the standard evolution of large-scale fluctuations in general
relativity is modified. The metric perturbation on large-scales is not constant
during high-energy inflation. It is constant during the radiation era, except
at most during the very beginning, if the energy is high enough.Comment: Additional arguments and minor corrections; version accepted by Phys.
Rev.
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