1,305 research outputs found
Exact Results in N_c=2 IIB Matrix Model
We investigate N_c=2 case of IIB matrix model, which is exactly soluble. We
calculate the partition function exactly and obtain a finite result without
introducing any cut-off. We also evaluate some correlation functions consisting
of Wilson loops.Comment: 8 pages, Late
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
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
Black hole perturbation in the most general scalar-tensor theory with second-order field equations I: The odd-parity sector
We perform a fully relativistic analysis of odd-type linear perturbations
around a static and spherically symmetric solution in the most general
scalar-tensor theory with second-order field equations in four-dimensional
spacetime. It is shown that, as in the case of general relativity, the
quadratic action for the perturbations reduces to the one having only a single
dynamical variable, from which concise formulas for no-ghost and no-gradient
instability conditions are derived. Our result is applicable to all the
theories of gravity with an extra scalar degree of freedom. We demonstrate how
the generic formulas can be applied to some particular examples such as the
Brans-Dicke theory, models, and Galileon gravity.Comment: 10 pages; v2. matches the published version in Phys. Rev. D; v3.
typos in Eqs. (A1), (A3)--(A5) fixed, results unchanged; v4. a typo in Eq.
(24) fixed, results unchanged; v5. a typo in Eq. (14) fixed, results
unchange
Black hole perturbation in nondynamical and dynamical Chern-Simons gravity
Chern-Simons gravitational theories are extensions of general relativity in
which the parity is violated due to the Chern-Simons term. We study linear
perturbations on the static and spherically symmetric background spacetime both
for nondynamical and dynamical Chern-Simons theories. We do not make an
assumption that the background Chern-Simons scalar field vanishes, which has
been adopted in the literature. By eliminating nondynamical variables using
their constraint equations, we derive the reduced second order action from
which a set of closed evolution equations containing only dynamical variables
are immediately obtained and therefore the number of propagating degrees of
freedom as well. It is found that ghost is present both for the nondynamical
case and for the dynamical case unless the background Chern-Simons scalar field
vanishes. It is also found that if the background scalar field vanishes, ghost
degrees of freedom are killed and all the modes propagate at the speed of
light.Comment: 18 pages; matches the published version in Phys. Rev.
Non-Gaussianity, Spectral Index and Tensor Modes in Mixed Inflaton and Curvaton Models
We study non-Gaussianity, the spectral index of primordial scalar
fluctuations and tensor modes in models where fluctuations from the inflaton
and the curvaton can both contribute to the present cosmic density
fluctuations. Even though simple single-field inflation models generate only
tiny non-Gaussianity, if we consider such a mixed scenario, large
non-Gaussianity can be produced. Furthermore, we study the inflationary
parameters such as the spectral index and the tensor-to-scalar ratio in this
kind of models and discuss in what cases models predict the spectral index and
tensor modes allowed by the current data while generating large
non-Gaussianity, which may have many implications for model-buildings of the
inflationary universe.Comment: 40 pages, 16 figures, discussions added, references adde
Non-Gaussianity in the modulated reheating scenario
We investigate the non-Gaussianity of primordial curvature perturbation in
the modulated reheating scenario where the primordial perturbation is generated
due to the spacial fluctuation of the inflaton decay rate to radiation. We use
the formalism to evaluate the trispectrum of curvature perturbation
as well as its bispectrum. We give expressions for three non-linear parameters
and in the modulated reheating scenario. If the
intrinsic non-Gaussianity of scalar field fluctuations and third derivative of
the decay rate with respect to scalar fields are negligibly small, has
at least the same order of magnitude as . We also give general
inequality between and which is true for other
inflationary scenarios as long as primordial non-Gaussianity comes from
super-horizon evolution.Comment: references adde
Cosmological perturbation in f(R,G) theories with a perfect fluid
In order to classify modified gravity models according to their physical
properties, we analyze the cosmological linear perturbations for f(R,G)
theories (R being the Ricci scalar and G, the Gauss-Bonnet term) with a
minimally coupled perfect fluid. For the scalar type perturbations, we identify
in general six degrees of freedom. We find that two of these physical modes
obey the same dispersion relation as the one for a non-relativistic de Broglie
wave. This means that spacetime is either highly unstable or its fluctuations
undergo a scale-dependent super-luminal propagation. Two other modes correspond
to the degrees of freedom of the perfect fluid, and propagate with the sound
speed of such a fluid. The remaining two modes correspond to the entropy and
temperature perturbations of the perfect fluid, and completely decouple from
the other modes for a barotropic equation of state. We then provide a concise
condition on f(R,G) theories, that both f(R) and R+f(G) do fulfill, to avoid
the de Broglie type dispersion relation. For the vector type perturbation, we
find that the perturbations decay in time. For the tensor type perturbation,
the perturbations can be either super-luminal or sub-luminal, depending on the
model. No-ghost conditions are also obtained for each type of perturbation.Comment: 12 pages, uses RevTe
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
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