705 research outputs found
Tachyons in Compact Spaces
We discuss condensations of closed string tachyons localized in compact
spaces. Time evolution of an on-shell condensation is naturally related to the
worldsheet RG flow. Some explicit tachyonic compactifications of Type II string
theory is considered, and some of them are shown to decay into supersymmetric
theories known as the little string theories.Comment: 14 page
Dark energy from primordial inflationary quantum fluctuations
We show that current cosmic acceleration can be explained by an almost
massless scalar field experiencing quantum fluctuations during primordial
inflation. Provided its mass does not exceed the Hubble parameter today, this
field has been frozen during the cosmological ages to start dominating the
universe only recently. By using supernovae data, completed with baryonic
acoustic oscillations from galaxy surveys and cosmic microwave background
anisotropies, we infer the energy scale of primordial inflation to be around a
few TeV, which implies a negligible tensor-to-scalar ratio of the primordial
fluctuations. Moreover, our model suggests that inflation lasted for an
extremely long period. Dark energy could therefore be a natural consequence of
cosmic inflation close to the electroweak energy scale.Comment: 5 pages, 2 figures, uses RevTeX. Physical discussion extended,
misprints corrected, references added. Matches published versio
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
Higher Order Corrections to the Primordial Gravitational Wave Spectrum and its Impact on Parameter Estimates for Inflation
We study the impact of the use of the power series expression for the
primordial tensor spectrum on parameter estimation from future direct detection
gravitational wave experiments. The spectrum approximated by the power series
expansion may give large deviation from the true (fiducial) value when it is
normalized at CMB scale because of the large separation between CMB and direct
detection scales. We derive the coefficients of the higher order terms of the
expansion up to the sixth order within the framework of the slow-roll
approximation and investigate how well the inclusion of higher order terms
improves the analytic prediction of the spectrum amplitude by comparing with
numerical results. Using the power series expression, we consider future
constraints on inflationary parameters expected from direct detection
experiments of the inflationary gravitational wave background and show that the
truncation of the higher order terms can lead to incorrect evaluation of the
parameters. We present two example models; a quadratic chaotic inflation model
and mixed inflaton and curvaton model with a quartic inflaton potential.Comment: 25 pages, 7 figures, revised version accepted by JCA
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
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
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
Non-linear isocurvature perturbations and non-Gaussianities
We study non-linear primordial adiabatic and isocurvature perturbations and
their non-Gaussianity. After giving a general formulation in the context of an
extended delta N-formalism, we analyse in detail two illustrative examples. The
first is a mixed curvaton-inflaton scenario in which fluctuations of both the
inflaton and a curvaton (a light isocurvature field during inflation)
contribute to the primordial density perturbation. The second example is that
of double inflation involving two decoupled massive scalar fields during
inflation. In the mixed curvaton-inflaton scenario we find that the bispectrum
of primordial isocurvature perturbations may be large and comparable to the
bispectrum of adiabatic curvature perturbations.Comment: 24 pages, typos corrected, references adde
Ricci flows and expansion in axion-dilaton cosmology
We study renormalization-group flows by deforming a class of conformal
sigma-models. We consider overall scale factor perturbation of Einstein spaces
as well as more general anisotropic deformations of three-spheres. At leading
order in alpha, renormalization-group equations turn out to be Ricci flows. In
the three-sphere background, the latter is the Halphen system, which is exactly
solvable in terms of modular forms. We also analyze time-dependent deformations
of these systems supplemented with an extra time coordinate and time-dependent
dilaton. In some regimes time evolution is identified with
renormalization-group flow and time coordinate can appear as Liouville field.
The resulting space-time interpretation is that of a homogeneous isotropic
Friedmann-Robertson-Walker universe in axion-dilaton cosmology. We find as
general behaviour the superposition of a big-bang (polynomial) expansion with a
finite number of oscillations at early times. Any initial anisotropy disappears
during the evolution.Comment: 22 page
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