953 research outputs found
tt* Geometry and Closed String Tachyon Potential
We propose a closed string tachyon action including kinetic and potential
terms for non-supersymmetric orbifolds. The action is given in terms of
solutions to equations which captures the geometry of vacua of the
corresponding N=2 worldsheet theory. In certain cases the solutions are well
studied. In case of tachyons of , solutions to affine toda
equations determine the action. We study the particular case of in detail and find that the Tachyon action is determined in terms of a
solution to Painleve III equation.Comment: 10 page
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
Closed String Tachyons and Semi-Classical Instabilities
We conjecture that the end point of bulk closed string tachyon decay at any
non-zero coupling, is the annihilation of space time by Witten's bubble of
nothing, resulting in a topological phase of the theory. In support of this we
present a variety of situations in which there is a correspondence between the
existence of perturbative tachyons in one regime and the semi-classical
annihilation of space-time. Our discussion will include many recently
investigated scenarios in string theory including Scherk-Schwarz
compactifications, Melvin magnetic backgrounds, and noncompact orbifolds. We
use this conjecture to investigate a possible web of dualities relating the
eleven-dimensional Fabinger-Horava background with nonsupersymmetric string
theories. Along the way we point out where our conjecture resolves some of the
puzzles associated with bulk closed string tachyon condensation.Comment: 31 pages, 5 figures 3 figures added, typos corrected and references
added. Discussion of Type 0/Heterotic s-duality extended and some other
points clarified Revision of discussion on Fabinger-Horava string
descendents, section on Scherk-Schwarz compactification of Horava-Witten
removed, some references adde
Rolling Closed String Tachyons and the Big Crunch
We study the low-energy effective field equations that couple gravity, the
dilaton, and the bulk closed string tachyon of bosonic closed string theory. We
establish that whenever the tachyon induces the rolling process, the string
metric remains fixed while the dilaton rolls to strong coupling. For negative
definite potentials we show that this results in an Einstein metric that
crunches the universe in finite time. This behavior is shown to be rather
generic even if the potentials are not negative definite. The solutions are
reminiscent of those in the collapse stage of a cyclic universe cosmology where
scalar field potentials with negative energies play a central role.Comment: 13 pages, 2 figures, LaTeX. Replaced version: one reference adde
Evolution of FLRW spacetime after the birth of a cosmic string
We consider the evolution of an initially FLRW universe after the formation
of a long, straight, cosmic string with arbitrary tension and mass per unit
length. The birth of the string sources scalar and tensor-type perturbations in
the background metric and both density and velocity perturbations in the
background fluid, which compensate for the string mass and maintain energy
conservation. The former generate the deficit angle within the light cone of
the string and a gravitational shock front at the cosmological horizon, whereas
the latter are confined within the sound cone. We study the properties of the
metric within each region of the resulting spacetime and give the explicit
coordinate transformations which demonstrate non-violation of causality. This
paper generalizes the work of previous studies for the Nambu-Goto string.Comment: 16 pages, 2 figures, published versio
Normalization of Off-shell Boundary State, g-function and Zeta Function Regularization
We consider the model in two dimensions with boundary quadratic deformation
(BQD), which has been discussed in tachyon condensation. The partition function
of this model (BQD) on a cylinder is determined, using the method of zeta
function regularization. We show that, for closed channel partition function, a
subtraction procedure must be introduced in order to reproduce the correct
results at conformal points. The boundary entropy (g-function) is determined
from the partition function and the off-shell boundary state. We propose and
consider a supersymmetric generalization of BQD model, which includes a
boundary fermion mass term, and check the validity of the subtraction
procedure.Comment: 21 pages, LaTeX, comments and 3 new references adde
Non-Gaussianity from isocurvature perturbations
We develop a formalism to study non-Gaussianity in both curvature and
isocurvature perturbations. It is shown that non-Gaussianity in the
isocurvature perturbation between dark matter and photons leaves distinct
signatures in the CMB temperature fluctuations, which may be confirmed in
future experiments, or possibly, even in the currently available observational
data. As an explicit example, we consider the QCD axion and show that it can
actually induce sizable non-Gaussianity for the inflationary scale, H_{inf} =
O(10^9 - 10^{11})GeV.Comment: 24 pages, 6 figures; references added; version to appear in JCA
Closed String Tachyon Condensation on Twisted Circles
We study IIA/B string theory compactified on twisted circles. These models
possess closed string tachyons and reduce to type 0B/A theory in a special
limit. Using methods of gauged linear sigma models and mirror symmetry we
construct a conformal field theory which interpolates between these models and
flat space via an auxiliary Liouville direction. Interpreting motion in the
Liouville direction as renormalization group flow, we argue that the end point
of tachyon condensation in all these models (including 0B/A theory) is
supersymmetric type II theory. We also find a zero-slope limit of these models
which is best described in a T-dual picture as a type II NS-NS fluxbrane. In
this limit tachyon condensation is an interesting and well posed problem in
supergravity. We explicitly determine the tachyon as a fluctuation of
supergravity fields, and perform a rudimentary numerical analysis of the
relevant flows.Comment: 21 pages plus appendices (12 pages), harvmac, 1 fig, v2: minor
changes and references added, v3: minor changes version published in JHE
Inhomogeneous non-Gaussianity
We propose a method to probe higher-order correlators of the primordial
density field through the inhomogeneity of local non-Gaussian parameters, such
as f_NL, measured within smaller patches of the sky. Correlators between
n-point functions measured in one patch of the sky and k-point functions
measured in another patch depend upon the (n+k)-point functions over the entire
sky. The inhomogeneity of non-Gaussian parameters may be a feasible way to
detect or constrain higher-order correlators in local models of
non-Gaussianity, as well as to distinguish between single and multiple-source
scenarios for generating the primordial density perturbation, and more
generally to probe the details of inflationary physics.Comment: 16 pages, 2 figures; v2: Minor changes and references added. Matches
the published versio
Black hole production in tachyonic preheating
We present fully non-linear simulations of a self-interacting scalar field in
the early universe undergoing tachyonic preheating. We find that density
perturbations on sub-horizon scales which are amplified by tachyonic
instability maintain long range correlations even during the succeeding
parametric resonance, in contrast to the standard models of preheating
dominated by parametric resonance. As a result the final spectrum exhibits
memory and is not universal in shape. We find that throughout the subsequent
era of parametric resonance the equation of state of the universe is almost
dust-like, hence the Jeans wavelength is much smaller than the horizon scale.
If our 2D simulations are accurate reflections of the situation in 3D, then
there are wide regions of parameter space ruled out by over-production of black
holes. It is likely however that realistic parameter values, consistent with
COBE/WMAP normalisation, are safetly outside this black hole over-production
region.Comment: 6pages, 7figures, figures correcte
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