876 research outputs found
Chaotic Inflation with Time-Variable Space Dimensions
Assuming the space dimension is not constant but decreases during the
expansion of the Universe, we study chaotic inflation with the potential
. Our investigations are based on a model Universe with variable
space dimensions. We write down field equations in the slow-roll approximation,
and define slow-roll parameters by assuming the number of space dimensions
decreases continuously as the Universe expands. The dynamical character of the
space dimension shifts the initial and final value of the inflaton field to
larger values. We obtain an upper limit for the space dimension at the Planck
length. This result is in agreement with previous works for the effective time
variation of the Newtonian gravitational constant in a model Universe with
variable space dimensions.Comment: 19 pages, To be published in Int.J.Mod.Phys.D. Minor changes to match
accepted versio
Some aspects of thermal inflation: the finite temperature potential and topological defects
Currently favoured extensions of the Standard Model typically contain `flaton
fields' defined as fields with large vacuum expectation values (vevs) and
almost flat potentials. If a flaton field is trapped at the origin in the early
universe, one expects `thermal inflation' to take place before it rolls away to
the true vacuum, because the finite-temperature correction to the potential
will hold it at the origin until the temperature falls below 1\TeV or so. In
the first part of the paper, that expectation is confirmed by an estimate of
the finite temperature corrections and of the tunneling rate to the true
vacuum, paying careful attention to the validity of the approximations that are
used. The second part of the paper considers topological defects which may be
produced at the end of an era of thermal inflation. If the flaton fields
associated with the era are GUT higgs fields, then its end corresponds to the
GUT phase transition. In that case monopoles (as well as GUT higgs particles)
will have to be diluted by a second era of thermal inflation. Such an era will
not affect the cosmology of GUT strings, for which the crucial parameter is the
string mass per unit length. Because of the flat Higgs potential, the GUT
symmetry breaking scale required for the strings to be a candidate for the
origin of large scale structure and the cmb anisotropy is about three times
bigger than usual, but given the uncertainties it is still compatible with the
one required by the unification of the Standard Model gauge couplings. The
cosmology of textures and of global monopoles is unaffected by the flatness of
the potential.Comment: 40 pages, LaTeX with epsf macro, 1 figure, preprint number correcte
Observational constraints on the spectral index of the cosmological curvature perturbation
We evaluate the observational constraints on the spectral index , in the
context of the CDM hypothesis which represents the simplest viable
cosmology. We first take to be practically scale-independent. Ignoring
reionization, we find at a nominal 2- level . If
we make the more realisitic assumption that reionization occurs when a fraction
to 1 of the matter has collapsed, the 2- lower bound is
unchanged while the 1- bound rises slightly. These constraints are
compared with the prediction of various inflation models. Then we investigate
the two-parameter scale-dependent spectral index, predicted by running-mass
inflation models, and find that present data allow significant scale-dependence
of , which occurs in a physically reasonable regime of parameter space.Comment: ReVTeX, 15 pages, 5 figures and 3 tables, uses epsf.sty Improved
treatment of reionization and small bug fixed in the constant n case; more
convenient parameterization and better treatment of the n dependence in the
CMB anisotropy for the running mass case; conclusions basically unchanged;
references adde
Generating the curvature perturbation at the end of inflation
The dominant contribution to the primordial curvature perturbation may be
generated at the end of inflation. Taking the end of inflation to be sudden,
formulas are presented for the spectrum, spectral tilt and non-gaussianity.
They are evaluated for a minimal extension of the original hybrid inflation
model.Comment: 5 pages. v3: as it will appear in JCA
The abundance of relativistic axions in a flaton model of Peccei-Quinn symmetry
Flaton models of Peccei-Quinn symmetry have good particle physics motivation,
and are likely to cause thermal inflation leading to a well-defined cosmology.
They can solve the problem, and generate viable neutrino masses.
Canonical flaton models predict an axion decay constant F_a of order 10^{10}
GeV and generic flaton models give F_a of order 10^9 GeV as required by
observation. The axion is a good candidate for cold dark matter in all cases,
because its density is diluted by flaton decay if F_a is bigger than 10^{12}
GeV. In addition to the dark matter axions, a population of relativistic axions
is produced by flaton decay, which at nucleosynthesis is equivalent to some
number \delta N_\nu of extra neutrino species. Focussing on the canonical
model, containing three flaton particles and two flatinos, we evaluate all of
the flaton-flatino-axion interactions and the corresponding axionic decay
rates. They are compared with the dominant hadronic decay rates, for both DFSZ
and KSVZ models. These formulas provide the basis for a precise calculation of
the equivalent \delta N_\nu in terms of the parameters (masses and couplings).
The KSVZ case is probably already ruled out by the existing bound \delta
N_\nu\lsim 1. The DFSZ case is allowed in a significant region of parameter
space, and will provide a possible explanation for any future detection of
nonzero
Cosmological consequences of particle creation during inflation
Particle creation during inflation is considered. It could be important for
species whose interaction is of gravitational strength or weaker. A complete
but economical formalism is given for spin-zero and spin-half particles, and
the particle abundance is estimated on the assumption that the particle mass in
the early universe is of order the Hubble parameter . It is roughly the same
for both spins, and it is argued that the same estimate should hold for higher
spin particles in particular the gravitino. The abundance is bigger than that
from the usual particle collision mechanism if the inflationary energy scale is
of order , but not if it is much lower.Comment: 17 pages, no Figure
Running-mass models of inflation, and their observational constraints
If the inflaton sector is described by softly broken supersymmetry, and the
inflaton has unsuppressed couplings, the inflaton mass will run strongly with
scale. Four types of model are possible. The prediction for the spectral index
involves two parameters, while the COBE normalization involves a third, all of
them calculable functions of the relevant masses and couplings. A crude
estimate is made of the region of parameter space allowed by present
observation.Comment: Latex file, 20 pages, 11 figures, uses epsf.sty. Comment on the
observation of the spectral index scale dependence added; Fig. 3-6 improve
Cosmology and the Korteweg-de Vries Equation
The Korteweg-de Vries (KdV) equation is a non-linear wave equation that has
played a fundamental role in diverse branches of mathematical and theoretical
physics. In the present paper, we consider its significance to cosmology. It is
found that the KdV equation arises in a number of important scenarios,
including inflationary cosmology, the cyclic universe, loop quantum cosmology
and braneworld models. Analogies can be drawn between cosmic dynamics and the
propagation of the solitonic wave solution to the equation, whereby quantities
such as the speed and amplitude profile of the wave can be identified with
cosmological parameters such as the spectral index of the density perturbation
spectrum and the energy density of the universe. The unique mathematical
properties of the Schwarzian derivative operator are important to the analysis.
A connection with dark solitons in Bose-Einstein condensates is briefly
discussed.Comment: 7 pages; References adde
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