537 research outputs found

### Ekpyrotic collapse with multiple fields

A scale invariant spectrum of isocurvature perturbations is generated during collapse in the scaling solution in models where two or more fields have steep negative exponential potentials. The scale invariance of the spectrum is realised by a tachyonic instability in the isocurvature field. We show that this instability is due to the fact that the scaling solution is a saddle point in the phase space. The late time attractor is identified with a single field dominated ekpyrotic collapse in which a steep blue spectrum for isocurvature perturbations is found. Although quantum fluctuations do not necessarily to disrupt the classical solution, an additional preceding stage is required to establish classical homogeneity

### Anisotropic Pressures at Ultra-stiff Singularities and the Stability of Cyclic Universes

We show that the inclusion of simple anisotropic pressures stops the
isotropic Friedmann universe being a stable attractor as an initial or final
singularity is approached when pressures can exceed the energy density. This
shows that the situation with isotropic pressures, studied earlier in the
context of cyclic and ekpyrotic cosmologies, is not generic, and Kasner-like
behaviour occurs when simple pressure anisotropies are present. We find all the
asymptotic behaviours and determine the dynamics when the anisotropic principal
pressures are proportional to the density. We expect distortions and
anisotropies to be significantly amplified through a simple cosmological bounce
in cyclic or ekpyrotic cosmologies when ultra-stiff pressures are present.Comment: 18 pages, 2 figure

### Phase-plane analysis of Friedmann-Robertson-Walker cosmologies in Brans-Dicke gravity

We present an autonomous phase-plane describing the evolution of
Friedmann-Robertson-Walker models containing a perfect fluid (with barotropic
index gamma) in Brans-Dicke gravity (with Brans-Dicke parameter omega). We find
self-similar fixed points corresponding to Nariai's power-law solutions for
spatially flat models and curvature-scaling solutions for curved models. At
infinite values of the phase-plane variables we recover O'Hanlon and Tupper's
vacuum solutions for spatially flat models and the Milne universe for negative
spatial curvature. We find conditions for the existence and stability of these
critical points and describe the qualitative evolution in all regions of the
(omega,gamma) parameter space for 0-3/2. We show that the
condition for inflation in Brans-Dicke gravity is always stronger than the
general relativistic condition, gamma<2/3.Comment: 24 pages, including 9 figures, LaTe

### Exponential potentials and cosmological scaling solutions

We present a phase-plane analysis of cosmologies containing a barotropic
fluid with equation of state $p_\gamma = (\gamma-1) \rho_\gamma$, plus a scalar
field $\phi$ with an exponential potential $V \propto \exp(-\lambda \kappa
\phi)$ where $\kappa^2 = 8\pi G$. In addition to the well-known inflationary
solutions for $\lambda^2
3\gamma$ in which the scalar field energy density tracks that of the barotropic
fluid (which for example might be radiation or dust). We show that the scaling
solutions are the unique late-time attractors whenever they exist. The
fluid-dominated solutions, where $V(\phi)/\rho_\gamma \to 0$ at late times, are
always unstable (except for the cosmological constant case $\gamma = 0$). The
relative energy density of the fluid and scalar field depends on the steepness
of the exponential potential, which is constrained by nucleosynthesis to
$\lambda^2 > 20$. We show that standard inflation models are unable to solve
this `relic density' problem.Comment: 6 pages RevTeX file with four figures incorporated (uses RevTeX and
epsf). Matches published versio

### Cosmologies with Energy Exchange

We provide a simple mathematical description of the exchange of energy
between two fluids in an expanding Friedmann universe with zero spatial
curvature. The evolution can be reduced to a single non-linear differential
equation which we solve in physically relevant cases and provide an analysis of
all the possible evolutions. Particular power-law solutions exist for the
expansion scale factor and are attractors at late times under particular
conditions. We show how a number of problems studied in the literature, such as
cosmological vacuum energy decay, particle annihilation, and the evolution of a
population of evaporating black holes, correspond to simple particular cases of
our model. In all cases we can determine the effects of the energy transfer on
the expansion scale factor. We also consider the situation in the presence of
anti-decaying fluids and so called phantom fluids which violate the dominant
energy conditions.Comment: 12 pages, 1 figur

### Dynamics of Assisted Inflation

We investigate the dynamics of the recently proposed model of assisted
inflation. In this model an arbitrary number of scalar fields with exponential
potentials evolve towards an inflationary scaling solution, even if each of the
individual potentials is too steep to support inflation on its own. By choosing
an appropriate rotation in field space we can write down explicitly the
potential for the weighted mean field along the scaling solution and for fields
orthogonal to it. This demonstrates that the potential has a global minimum
along the scaling solution. We show that the potential close to this attractor
in the rotated field space is analogous to a hybrid inflation model, but with
the vacuum energy having an exponential dependence upon a dilaton field. We
present analytic solutions describing homogeneous and inhomogeneous
perturbations about the attractor solution without resorting to slow-roll
approximations. We discuss the curvature and isocurvature perturbation spectra
produced from vacuum fluctuations during assisted inflation.Comment: 9 pages, 2 figures, latex with revtex and eps

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