565 research outputs found
Must Cosmological Perturbations Remain Non-Adiabatic After Multi-Field Inflation?
Even if non-adiabatic perturbations are generated in multi-field inflation,
the perturbations will become adiabatic if the universe after inflation enters
an era of local thermal equilibrium, with no non-zero conserved quantities, and
will remain adiabatic as long as the wavelength is outside the horizon, even
when local thermal equilibrium no longer applies. Small initial non-adiabatic
perturbations associated with imperfect local thermal equilibrium remain small
when baryons are created from out-of-equilibrium decay of massive particles, or
when dark matter particles go out of local thermal equilibrium.Comment: 12 pages, typographical errors corrected, acknowledgment added.
Article accepted for publication in Physical Review
Scaling Solutions in Robertson-Walker Spacetimes
We investigate the stability of cosmological scaling solutions describing a
barotropic fluid with and a non-interacting scalar field
with an exponential potential V(\phi)=V_0\e^{-\kappa\phi}. We study
homogeneous and isotropic spacetimes with non-zero spatial curvature and find
three possible asymptotic future attractors in an ever-expanding universe. One
is the zero-curvature power-law inflation solution where
(). Another is the
zero-curvature scaling solution, first identified by Wetterich, where the
energy density of the scalar field is proportional to that of matter with
(). We find that
this matter scaling solution is unstable to curvature perturbations for
. The third possible future asymptotic attractor is a solution with
negative spatial curvature where the scalar field energy density remains
proportional to the curvature with
(). We find that solutions with are
never late-time attractors.Comment: 8 pages, no figures, latex with revte
Exponential potentials and cosmological scaling solutions
We present a phase-plane analysis of cosmologies containing a barotropic
fluid with equation of state , plus a scalar
field with an exponential potential where . In addition to the well-known inflationary
solutions for 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 at late times, are
always unstable (except for the cosmological constant case ). The
relative energy density of the fluid and scalar field depends on the steepness
of the exponential potential, which is constrained by nucleosynthesis to
. 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
Cosmology with positive and negative exponential potentials
We present a phase-plane analysis of cosmologies containing a scalar field
with an exponential potential
where and may be positive or negative. We show that
power-law kinetic-potential scaling solutions only exist for sufficiently flat
() negative
potentials. The latter correspond to a class of ever-expanding cosmologies with
negative potential. However we show that these expanding solutions with a
negative potential are to unstable in the presence of ordinary matter, spatial
curvature or anisotropic shear, and generic solutions always recollapse to a
singularity. Power-law kinetic-potential scaling solutions are the late-time
attractor in a collapsing universe for steep negative potentials (the ekpyrotic
scenario) and stable against matter, curvature or shear perturbations.
Otherwise kinetic-dominated solutions are the attractor during collapse (the
pre big bang scenario) and are only marginally stable with respect to
anisotropic shear.Comment: 8 pages, latex with revtex, 9 figure
Toroidal magnetized iron neutrino detector for a neutrino factory
A neutrino factory has unparalleled physics reach for the discovery and measurement of CP violation in the neutrino sector. A far detector for a neutrino factory must have good charge identification with excellent background rejection and a large mass. An elegant solution is to construct a magnetized iron neutrino detector (MIND) along the lines of MINOS, where iron plates provide a toroidal magnetic field and scintillator planes provide 3D space points. In this paper, the current status of a simulation of a toroidal MIND for a neutrino factory is discussed in light of the recent measurements of large θ13. The response and performance using the 10 GeV neutrino factory configuration are presented. It is shown that this setup has equivalent δCP reach to a MIND with a dipole field and is sensitive to the discovery of CP violation over 85% of the values of δCP
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
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
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