11 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.Comment: 13 pages, 1 figur
Curvature perturbations from ekpyrotic collapse with multiple fields
A scale-invariant spectrum of isocurvature perturbations is generated during
collapse in the ekpyrotic scaling solution in models where multiple fields have
steep negative exponential potentials. The scale invariance of the spectrum is
realized by a tachyonic instability in the isocurvature field. This instability
drives the scaling solution to the late time attractor that is the old
ekpyrotic collapse dominated by a single field. We show that the transition
from the scaling solution to the single field dominated ekpyrotic collapse
automatically converts the initial isocurvature perturbations about the scaling
solution to comoving curvature perturbations about the late-time attractor. The
final amplitude of the comoving curvature perturbation is determined by the
Hubble scale at the transition.Comment: 15 pages, 3 figures, a reference added, to be published in CQG, a
remark on the comoving density perturbation correcte
Vector Bundle Moduli Superpotentials in Heterotic Superstrings and M-Theory
The non-perturbative superpotential generated by a heterotic superstring
wrapped once around a genus-zero holomorphic curve is proportional to the
Pfaffian involving the determinant of a Dirac operator on this curve. We show
that the space of zero modes of this Dirac operator is the kernel of a linear
mapping that is dependent on the associated vector bundle moduli. By explicitly
computing the determinant of this map, one can deduce whether or not the
dimension of the space of zero modes vanishes. It is shown that this
information is sufficient to completely determine the Pfaffian and, hence, the
non-perturbative superpotential as explicit holomorphic functions of the vector
bundle moduli. This method is illustrated by a number of non-trivial examples.Comment: 81 pages, LaTeX, corrected typo
Scale-invariance in expanding and contracting universes from two-field models
We study cosmological perturbations produced by the most general
two-derivative actions involving two scalar fields, coupled to Einstein
gravity, with an arbitrary field space metric, that admit scaling solutions.
For contracting universes, we show that scale-invariant adiabatic perturbations
can be produced continuously as modes leave the horizon for any equation of
state parameter . The corresponding background solutions are unstable,
which we argue is a universal feature of contracting models that yield
scale-invariant spectra. For expanding universes, we find that nearly
scale-invariant adiabatic perturbation spectra can only be produced for , and that the corresponding scaling solutions are attractors. The
presence of a nontrivial metric on field space is a crucial ingredient in our
results.Comment: 23 pages, oversight in perturbations calculation corrected,
conclusions for expanding models modifie
Non-Gaussianity from Symmetry
We point out that a light scalar field fluctuating around a symmetry-enhaced
point can generate large non-Gaussianity in density fluctuations. We name such
a particle as an "ungaussiton", a scalar field dominantly produced by the
quantum fluctuations,generating sizable non-Gaussianity in the density
fluctuations. We derive a consistency relation between the bispectrum and the
trispectrum, tau_NL = 10^3 f_NL^(4/3), which can be extended to arbitrary high
order correlation functions. If such a relation is confirmed by future
observations, it will strongly support this mechanism.Comment: 26 pages, 1 figure;v2 discussion and references added. To appear in
JCA
Non-Gaussianity as a Probe of the Physics of the Primordial Universe and the Astrophysics of the Low Redshift Universe
A new and powerful probe of the origin and evolution of structures in the
Universe has emerged and been actively developed over the last decade. In the
coming decade, non-Gaussianity, i.e., the study of non-Gaussian contributions
to the correlations of cosmological fluctuations, will become an important
probe of both the early and the late Universe. Specifically, it will play a
leading role in furthering our understanding of two fundamental aspects of
cosmology and astrophysics: (i) the physics of the very early universe that
created the primordial seeds for large-scale structures, and (ii) the
subsequent growth of structures via gravitational instability and gas physics
at later times. To date, observations of fluctuations in the Cosmic Microwave
Background (CMB) and the Large-Scale Structure of the Universe (LSS) have
focused largely on the Gaussian contribution as measured by the two-point
correlations (or the power spectrum) of density fluctuations. However, an even
greater amount of information is contained in non-Gaussianity and a large
discovery space therefore still remains to be explored. Many observational
probes can be used to measure non-Gaussianity, including CMB, LSS,
gravitational lensing, Lyman-alpha forest, 21-cm fluctuations, and the
abundance of rare objects such as clusters of galaxies and high-redshift
galaxies. Not only does the study of non-Gaussianity maximize the science
return from a plethora of present and future cosmological experiments and
observations, but it also carries great potential for important discoveries in
the coming decade.Comment: 8 pages, 1 figure. Science White Paper submitted to the Cosmology and
Fundamental Physics (CFP) Science Frontier Panel of the Astro 2010 Decadal
Survey (v2,v3,v4) More co-signers and references adde
Non-Gaussianity as a Probe of the Physics of the Primordial Universe and the Astrophysics of the Low Redshift Universe
A new and powerful probe of the origin and evolution of structures in the Universe has emerged and been actively developed over the last decade. In the coming decade, non-Gaussianity, i.e., the study of non-Gaussian contributions to the correlations of cosmological fluctuations, will become an important probe of both the early and the late Universe. Specifically, it will play a leading role in furthering our understanding of two fundamental aspects of cosmology and astrophysics: (i) the physics of the very early universe that created the primordial seeds for large-scale structures, and (ii) the subsequent growth of structures via gravitational instability and gas physics at later times. To date, observations of fluctuations in the Cosmic Microwave Background (CMB) and the Large-Scale Structure of the Universe (LSS) have focused largely on the Gaussian contribution as measured by the two-point correlations (or the power spectrum) of density fluctuations. However, an even greater amount of information is contained in non-Gaussianity and a large discovery space therefore still remains to be explored. Many observational probes can be used to measure non-Gaussianity, including CMB, LSS, gravitational lensing, Lyman-alpha forest, 21-cm fluctuations, and the abundance of rare objects such as clusters of galaxies and high-redshift galaxies. Not only does the study of non-Gaussianity maximize the science return from a plethora of present and future cosmological experiments and observations, but it also carries great potential for important discoveries in the coming decade