4,547 research outputs found

### Nflation: multi-field inflationary dynamics and perturbations

We carry out numerical investigations of the dynamics and perturbations in
the Nflation model of Dimopoulos et al. (2005). This model features large
numbers of scalar fields with different masses, which can cooperate to drive
inflation according to the assisted inflation mechanism. We extend previous
work to include random initial conditions for the scalar fields, and explore
the predictions for density perturbations and the tensor-to-scalar ratio. The
tensor-to-scalar ratio depends only on the number of e-foldings and is
independent of the number of fields, their masses, and their initial
conditions. It therefore always has the same value as for a single massive
field. By contrast, the scalar spectral index has significant dependence on
model parameters. While normally multi-field inflation models make predictions
for observable quantities which depend also on the unknown field initial
conditions, we find evidence of a `thermodynamic' regime whereby the predicted
spectral index becomes independent of initial conditions if there are enough
fields. Only in parts of parameter space where the mass spectrum of the fields
is extremely densely packed is the model capable of satisfying the tight
observational constraints from WMAP3 observations.Comment: 6 pages RevTeX4, 4 figures included. Updated to match PRD accepted
version. Analysis and conclusions unchanged. New references, especially
astro-ph/0510441 which was first to give the general r=8/N resul

### Stability of multi-field cosmological solutions

We explore the stability properties of multi-field solutions of assisted
inflation type, where several fields collectively evolve to the same
configuration. In the case of noninteracting fields, we show that the condition
for such solutions to be stable is less restrictive than that required for
tracking in quintessence models. Our results, which do not rely on the
slow-roll approximation, further indicate that to linear order in homogeneous
perturbations the fields are in fact unaware of each other's existence. We end
by generalizing our results to some cases of interacting fields and to other
background solutions and dynamics, including the high-energy braneworld.Comment: 6 pages; v2: typos corrected, version accepted by PR

### Viable inflationary models ending with a first-order phase transition

We investigate the parameter space of hybrid inflation models where inflation
terminates via a first-order phase transition causing nucleation of bubbles.
Such models experience a tension from the need to ensure nearly scale invariant
density perturbations, while avoiding a near scale-invariant bubble size
distribution which would conflict observations. We perform an exact analysis of
the different regimes of the models, where the energy density of the inflaton
field ranges from being negligible as compared to the vacuum energy to
providing most of the energy for inflation. Despite recent microwave anisotropy
results favouring a spectral index less than one, we find that there are still
viable models that end with bubble production and can match all available
observations. As a by-product of our analysis, we also provide an up-to-date
assessment of the viable parameter space of Linde's original second-order
hybrid model across its full parameter range.Comment: 9 pages, 7 figures. Revised version: corrections to description of
the historical development of the models. v3: Minor corrections to match
version accepted by PR

### A dark energy view of inflation

Traditionally, inflationary models are analyzed in terms of parameters such
as the scalar spectral index ns and the tensor to scalar ratio r, while dark
energy models are studied in terms of the equation of state parameter w.
Motivated by the fact that both deal with periods of accelerated expansion, we
study the evolution of w during inflation, in order to derive observational
constraints on its value during an earlier epoch likely dominated by a dynamic
form of dark energy. We find that the cosmic microwave background and
large-scale structure data is consistent with w_inflation=-1 and provides an
upper limit of 1+w <~ 0.02. Nonetheless, an exact de Sitter expansion with a
constant w=-1 is disfavored since this would result in ns=1.Comment: 5 pages, 4 figures; v2: minor modifications to match published
versio

### Cosmology on Compact and Stable Supergravity Background

We propose a cosmological model of D3-brane universe on compact and stable
supergravity background of wrapped D7-branes in type IIB string theory
previously argued to be dual to pure N=1 SU(N) gauge theory in four dimensions.
A model universe of order Planck size near the UV boundary dynamically flows
toward the IR with constant total energy density and accelerating expansion
followed by smooth transition to decelerating expansion and collides with the
wrapped D7-branes at the IR boundary. The model addresses the horizon and
flatness problems with most of the expansion produced during the decelerating
expansion phase. The inflationary scenario is used to generate sources of
inhomogeneities in the cosmic microwave background radiation and seeds for
large scale structure formation from quantum fluctuations which exit the Hubble
radius early during the accelerating expansion phase and the model addresses
the inhomogeneity problem with red tilt in the power spectrum. We propose that
the kinetic energy of the model universe is converted to matter and radiation
by the collision followed by formation of baryons that stabilizes the model
universe against gravitational force from the background at a finite distance
from the IR boundary with the wrapped D7-branes serving as sources of color.
Friedmann evolution then takes over with a positive cosmological constant term
coming from the remaining potential energy density which is interpreted as dark
energy. The magnitude of dark energy density is smaller than the total energy
density during the flow by a ratio of the scale factor when the model universe
appears in the UV to the scale factor at the moment of collision and stays
constant while the matter-radiation density falls during Friedmann expansion.Comment: 30 page

### Detectable primordial non-gaussianities and gravitational waves in k-inflation

An inflationary single field model with a non-trivial kinetic term for the
inflaton is discussed. It is shown that it is possible to have large primordial
non-gaussianities and large tensor-to-scalar ratio in a simple concrete model
with just a scalar field and a generalized kinetic term for the inflaton field.
This is potentially interesting in the prospect of new forthcoming
observations.Comment: 4 pages, 1 figure, REVTEX, to appear in PR

### The WMAP normalization of inflationary cosmologies

We use the three-year WMAP observations to determine the normalization of the
matter power spectrum in inflationary cosmologies. In this context, the
quantity of interest is not the normalization marginalized over all parameters,
but rather the normalization as a function of the inflationary parameters n and
r with marginalization over the remaining cosmological parameters. We compute
this normalization and provide an accurate fitting function. The statistical
uncertainty in the normalization is 3 percent, roughly half that achieved by
COBE. We use the k-l relation for the standard cosmological model to identify
the pivot scale for the WMAP normalization. We also quote the inflationary
energy scale corresponding to the WMAP normalization.Comment: 4 pages RevTex4 with two figure

### N-flation: Non-Gaussianity in the horizon-crossing approximation

We analyze the cosmic non-gaussianity produced in inflation models with multiple uncoupled fields with monomial potentials, such as Nflation. Using the horizon-crossing approximation to compute the non-gaussianity, we show that when each field has the same form of potential, the prediction is independent the number of fields, their initial conditions, and the spectrum of masses/couplings. It depends only on the number of e-foldings after the horizon crossing of observable perturbations. We also provide a further generalization to the case where the fields can have monomial potentials with different powers. Unless the horizon-crossing approximation is substantially violated, the predicted non-gaussianity is too small to ever be observed

### Nflation: observable predictions from the random matrix mass spectrum

We carry out numerical investigations of the perturbations in Nflation models
where the mass spectrum is generated by random matrix theory. The
tensor-to-scalar ratio and non-gaussianity are already known to take the
single-field values, and so the density perturbation spectral index is the main
parameter of interest. We study several types of random field initial
conditions, and compute the spectral index as a function of mass spectrum
parameters. Comparison with microwave anisotropy data from the Wilkinson
Microwave Anisotropy Probe shows that the model is currently viable in the
majority of its parameter space.Comment: 5 pages RevTeX with 4 figures. Minor corrections to match version to
appear in Physical Review

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