3,031 research outputs found
Supersymmetric Large Extra Dimensions and the Cosmological Constant Problem
This article briefly summarizes and reviews the motivations for - and the
present status of - the proposal that the small size of the observed Dark
Energy density can be understood in terms of the dynamical relaxation of two
large extra dimensions within a supersymmetric higher-dimensional theory.Comment: Talk presented to Theory Canada I, Vancouver, June 2005. References
added in V
Axionic D3-D7 Inflation
We study the motion of a D3 brane moving within a Type IIB string vacuum
compactified to 4D on K3 x T_2/Z_2 in the presence of D7 and O7 planes. We work
within the effective 4D supergravity describing how the mobile D3 interacts
with the lightest bulk moduli of the compactification, including the effects of
modulus-stabilizing fluxes. We seek inflationary solutions to the resulting
equations, performing our search numerically in order to avoid resorting to
approximate parameterizations of the low-energy potential. We consider
uplifting from D-terms and from the supersymmetry-breaking effects of anti-D3
branes. We find examples of slow-roll inflation (with anti-brane uplifting)
with the mobile D3 moving along the toroidal directions, falling towards a
D7-O7 stack starting from the antipodal point. The inflaton turns out to be a
linear combination of the brane position and the axionic partner of the K3
volume modulus, and the similarity of the potential along the inflaton
direction with that of racetrack inflation leads to the prediction n_s \le 0.95
for the spectral index. The slow roll is insensitive to most of the features of
the effective superpotential, and requires a one-in-10^4 tuning to ensure that
the torus is close to square in shape. We also consider D-term inflation with
the D3 close to the attractive D7, but find that for a broad (but not
exhaustive) class of parameters the conditions for slow roll tend to
destabilize the bulk moduli. In contrast to the axionic case, the best
inflationary example of this kind requires the delicate adjustment of potential
parameters (much more than the part-per-mille level), and gives inflation only
at an inflection point of the potential (and so suffers from additional
fine-tuning of initial conditions to avoid an overshoot problem).Comment: 29 pages, 5 figure
Warped Spectroscopy: Localization of Frozen Bulk Modes
We study the 10D equation of motion of dilaton-axion fluctuations in type IIB
string compactifications with three-form flux, taking warping into account.
Using simplified models with physics comparable to actual compactifications, we
argue that the lightest mode localizes in long warped throats and takes a mass
of order the warped string scale. Also, Gukov-Vafa-Witten superpotential is
valid for the lightest mass mode; however, the mass is similar to the
Kaluza-Klein scale, so the dilaton-axion should be integrated out of the
effective theory in this long throat regime (leaving a constant
superpotential). On the other hand, there is a large hierarchy between
flux-induced and KK mass scales for moderate or weak warping. This hierarchy
agrees with arguments given for trivial warping. Along the way, we also
estimate the effect of the other 10D supergravity equations of motion on the
dilaton-axion fluctuation, since these equations act as constraints. We argue
that they give negligible corrections to the simplest approximation.Comment: 24pp + appendices, 6 figs, JHEP3 class; v2. corrected reference; v3.
added clarifications; v4. corrected typo
Fields Annihilation and Particles Creation in DBI inflation
We consider a model of DBI inflation where two stacks of mobile branes are
moving ultra relativistically in a warped throat. The stack closer to the tip
of the throat is annihilated with the background anti-branes while inflation
proceeds by the second stack. The effects of branes annihilation and particles
creation during DBI inflation on the curvature perturbations power spectrum and
the scalar spectral index are studied. We show that for super-horizon scales,
modes which are outside the sound horizon at the time of branes collision, the
spectral index has a shift to blue spectrum compared to the standard DBI
inflation. For small scales the power spectrum approaches to its background DBI
inflation value with the decaying superimposed oscillatory modulations.Comment: First revision: minor changes, the background spectral index is
corrected, new references are added. Second revision: minor changes, new
references are added, accepted for publication in JCA
Dynamical Fine Tuning in Brane Inflation
We investigate a novel mechanism of dynamical tuning of a flat potential in
the open string landscape within the context of warped brane-antibrane
inflation in type IIB string theory. Because of competing effects between
interactions with the moduli stabilizing D7-branes in the warped throat and
anti-D3-branes at the tip, a stack of branes gives rise to a local minimum of
the potential, holding the branes high up in the throat. As branes successively
tunnel out of the local minimum to the bottom of the throat the potential
barrier becomes lower and is eventually replaced by a flat inflection point,
around which the remaining branes easily inflate. This dynamical flattening of
the inflaton potential reduces the need to fine tune the potential by hand, and
also leads to successful inflation for a larger range of inflaton initial
conditions, due to trapping in the local minimum.Comment: 23 pages, 9 figures. v2: Updated D3-dependence in potential, small
changes to numerical result
DBI Lifshitz Inflation
A new model of DBI inflation is introduced where the mobile brane, the
inflaton field, is moving relativistically inside a Lifshitz throat with an
arbitrary anisotropic scaling exponent . After dimensional reduction to four
dimension the general covariance is broken explicitly both in the matter and
the gravitational sectors. The general action for the metric and matter field
perturbations are obtained and it is shown to be similar to the classifications
made in the effective field theory of inflation literature.Comment: Version 3: minor typos corrected, the JCAP published versio
String Pair Creations in D-brane Systems
We investigate the criterion, on the Born-Infeld background fields, for the
open string pair creation to occur in D-(anti-)D-brane systems. Although
the pair creation occurs generically in both D-D and D-anti-D
systems for the cases which meet the criterion, it is more drastic in
D-anti-D-brane systems by some exponential factor depending on the
background fields. Various configurations exhibiting pair creations are
obtained via duality transformations. These include the spacelike scissors and
two D-strings (slanted at different angles) passing through each other. We
raise the scissors paradox and suggest a resolution based on the triple
junction in IIB setup.Comment: V2. 1+28 pages, 5 figures in JHEP3, minor changes, added reference
Fibre Inflation: Observable Gravity Waves from IIB String Compactifications
We introduce a simple string model of inflation, in which the inflaton field
can take trans-Planckian values while driving a period of slow-roll inflation.
This leads naturally to a realisation of large field inflation, inasmuch as the
inflationary epoch is well described by the single-field scalar potential . Remarkably, for a broad class of vacua
all adjustable parameters enter only through the overall coefficient , and
in particular do not enter into the slow-roll parameters. Consequently these
are determined purely by the number of \e-foldings, , and so are not
independent: . This implies similar
relations among observables like the primordial scalar-to-tensor amplitude,
, and the scalar spectral tilt, : . is
itself more model-dependent since it depends partly on the post-inflationary
reheat history. In a simple reheating scenario a reheating temperature of
GeV gives , corresponding to and , within reach of future observations. The model is
an example of a class that arises naturally in the context of type IIB string
compactifications with large-volume moduli stabilisation, and takes advantage
of the generic existence there of Kahler moduli whose dominant appearance in
the scalar potential arises from string loop corrections to the Kahler
potential. The inflaton field is a combination of Kahler moduli of a K3-fibered
Calabi-Yau manifold. We believe there are likely to be a great number of models
in this class -- `high-fibre models' -- in which the inflaton starts off far
enough up the fibre to produce observably large primordial gravity waves.Comment: Extended calculations beyond the leading approximations, including
numerical integrations of multi-field evolution; Display an example with ; Simplify the discussion of large fields; Corrected minor errors and
typos; Added references; 41 pages LaTeX, 25 figure
Uniqueness and examples of compact toric Sasaki-Einstein metrics
In [11] it was proved that, given a compact toric Sasaki manifold of positive
basic first Chern class and trivial first Chern class of the contact bundle,
one can find a deformed Sasaki structure on which a Sasaki-Einstein metric
exists. In the present paper we first prove the uniqueness of such Einstein
metrics on compact toric Sasaki manifolds modulo the action of the identity
component of the automorphism group for the transverse holomorphic structure,
and secondly remark that the result of [11] implies the existence of compatible
Einstein metrics on all compact Sasaki manifolds obtained from the toric
diagrams with any height, or equivalently on all compact toric Sasaki manifolds
whose cones have flat canonical bundle. We further show that there exists an
infinite family of inequivalent toric Sasaki-Einstein metrics on for each positive integer .Comment: Statements of the results are modifie
Skin friction Reduction by Introduction of Micro-bubbles into Turbulent Boundary Layer
The phenomenon of drag reduction by the injection of microbubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Two variants namely the Inhomogeneous and MUSIG (MUltiple SIze Group) based on Population balance models are investigated. The simulated results are compared against the experimental findings of Madavan et al [1]. The model employed in the investigation comprises of a twodimensional micro-bubble laden flow wherein the Reynolds averaged Navier-Stokes (RANS) transport equations were used to describe both the phases of the flow. A SST (Shear Stress Transport) turbulence model is used as the turbulent closure for the primary phase and a zero equation turbulence model is used for the micro-bubbles
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