116 research outputs found
Desensitizing Inflation from the Planck Scale
A new mechanism to control Planck-scale corrections to the inflationary eta
parameter is proposed. A common approach to the eta problem is to impose a
shift symmetry on the inflaton field. However, this symmetry has to remain
unbroken by Planck-scale effects, which is a rather strong requirement on
possible ultraviolet completions of the theory. In this paper, we show that the
breaking of the shift symmetry by Planck-scale corrections can be
systematically suppressed if the inflaton field interacts with a conformal
sector. The inflaton then receives an anomalous dimension in the conformal
field theory, which leads to sequestering of all dangerous high-energy
corrections. We analyze a number of models where the mechanism can be seen in
action. In our most detailed example we compute the exact anomalous dimensions
via a-maximization and show that the eta problem can be solved using only
weakly-coupled physics.Comment: 34 pages, 3 figures
D3-brane Potentials from Fluxes in AdS/CFT
We give a comprehensive treatment of the scalar potential for a D3-brane in a
warped conifold region of a compactification with stabilized moduli. By
studying general ultraviolet perturbations in supergravity, we systematically
incorporate `compactification effects' sourced by supersymmetry breaking in the
compact space. Significant contributions to the D3-brane potential, including
the leading term in the infrared, arise from imaginary anti-self-dual (IASD)
fluxes. For an arbitrary Calabi-Yau cone, we determine the most general IASD
fluxes in terms of scalar harmonics, then compute the resulting D3-brane
potential. Specializing to the conifold, we identify the operator dual to each
mode of flux, and for chiral operators we confirm that the potential computed
in the gauge theory matches the gravity result. The effects of four-dimensional
curvature, including the leading D3-brane mass term, arise directly from the
ten-dimensional equations of motion. Furthermore, we show that gaugino
condensation on D7-branes provides a local source for IASD flux. This flux
precisely encodes the nonperturbative contributions to the D3-brane potential,
yielding a promising ten-dimensional representation of four-dimensional
nonperturbative effects. Our result encompasses all significant contributions
to the D3-brane potential discussed in the literature, and does so in the
single coherent framework of ten-dimensional supergravity. Moreover, we
identify new terms with irrational scaling dimensions that were inaccessible in
prior works. By decoupling gravity in a noncompact configuration, then
systematically reincorporating compactification effects as ultraviolet
perturbations, we have provided an approach in which Planck-suppressed
contributions to the D3-brane effective action can be computed.Comment: 70 page
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
A Phase Transition between Small and Large Field Models of Inflation
We show that models of inflection point inflation exhibit a phase transition
from a region in parameter space where they are of large field type to a region
where they are of small field type. The phase transition is between a universal
behavior, with respect to the initial condition, at the large field region and
non-universal behavior at the small field region. The order parameter is the
number of e-foldings. We find integer critical exponents at the transition
between the two phases.Comment: 21 pages, 8 figure
Anomalous Dimensions and Non-Gaussianity
We analyze the signatures of inflationary models that are coupled to strongly
interacting field theories, a basic class of multifield models also motivated
by their role in providing dynamically small scales. Near the squeezed limit of
the bispectrum, we find a simple scaling behavior determined by operator
dimensions, which are constrained by the appropriate unitarity bounds.
Specifically, we analyze two simple and calculable classes of examples:
conformal field theories (CFTs), and large-N CFTs deformed by relevant
time-dependent double-trace operators. Together these two classes of examples
exhibit a wide range of scalings and shapes of the bispectrum, including nearly
equilateral, orthogonal and local non-Gaussianity in different regimes. Along
the way, we compare and contrast the shape and amplitude with previous results
on weakly coupled fields coupled to inflation. This signature provides a
precision test for strongly coupled sectors coupled to inflation via irrelevant
operators suppressed by a high mass scale up to 1000 times the inflationary
Hubble scale.Comment: 40 pages, 10 figure
Chasing Brane Inflation in String-Theory
We investigate the embedding of brane anti-brane inflation into a concrete
type IIB string theory compactification with all moduli fixed. Specifically, we
are considering a D3-brane, whose position represents the inflaton , in a
warped conifold throat in the presence of supersymmetrically embedded D7-branes
and an anti D3-brane localized at the tip of the warped conifold cone. After
presenting the moduli stabilization analysis for a general D7-brane embedding,
we concentrate on two explicit models, the Ouyang and the Kuperstein
embeddings. We analyze whether the forces, induced by moduli stabilization and
acting on the D3-brane, might cancel by fine-tuning such as to leave us with
the original Coulomb attraction of the anti D3-brane as the driving force for
inflation. For a large class of D7-brane embeddings we obtain a negative
result. Cancelations are possible only for very small intervals of
around an inflection point but not globally. For the most part of its motion
the inflaton then feels a steep, non slow-roll potential. We study the
inflationary dynamics induced by this potential.Comment: 34 pages, 4 figures. Final version published in JCA
Brane inflation and the WMAP data: a Bayesian analysis
The Wilkinson Microwave Anisotropy Probe (WMAP) constraints on string
inspired ''brane inflation'' are investigated. Here, the inflaton field is
interpreted as the distance between two branes placed in a flux-enriched
background geometry and has a Dirac-Born-Infeld (DBI) kinetic term. Our method
relies on an exact numerical integration of the inflationary power spectra
coupled to a Markov-Chain Monte-Carlo exploration of the parameter space. This
analysis is valid for any perturbative value of the string coupling constant
and of the string length, and includes a phenomenological modelling of the
reheating era to describe the post-inflationary evolution. It is found that the
data favour a scenario where inflation stops by violation of the slow-roll
conditions well before brane annihilation, rather than by tachyonic
instability. Concerning the background geometry, it is established that log(v)
> -10 at 95% confidence level (CL), where "v" is the dimensionless ratio of the
five-dimensional sub-manifold at the base of the six-dimensional warped
conifold geometry to the volume of the unit five-sphere. The reheating energy
scale remains poorly constrained, Treh > 20 GeV at 95% CL, for an extreme
equation of state (wreh ~ -1/3) only. Assuming the string length is known, the
favoured values of the string coupling and of the Ramond-Ramond total
background charge appear to be correlated. Finally, the stochastic regime
(without and with volume effects) is studied using a perturbative treatment of
the Langevin equation. The validity of such an approximate scheme is discussed
and shown to be too limited for a full characterisation of the quantum effects.Comment: 65 pages, 15 figures, uses iopart. Shortened version, updated
references. Matches publication up to appendix B kept on the arXi
On D3-brane Potentials in Compactifications with Fluxes and Wrapped D-branes
We study the potential governing D3-brane motion in a warped throat region of
a string compactification with internal fluxes and wrapped D-branes. If the
Kahler moduli of the compact space are stabilized by nonperturbative effects, a
D3-brane experiences a force due to its interaction with D-branes wrapping
certain four-cycles. We compute this interaction, as a correction to the warped
four-cycle volume, using explicit throat backgrounds in supergravity. This
amounts to a closed-string channel computation of the loop corrections to the
nonperturbative superpotential that stabilizes the volume. We demonstrate for
warped conical spaces that the superpotential correction is given by the
embedding equation specifying the wrapped four-cycle, in agreement with the
general form proposed by Ganor. Our approach automatically provides a solution
to the problem of defining a holomorphic gauge coupling on wrapped D7-branes in
a background with D3-branes. Finally, our results have applications to
cosmological inflation models in which the inflaton is modeled by a D3-brane
moving in a warped throat.Comment: 45 pages, 1 figure; v2: added reference, clarified notatio
Volume modulus inflection point inflation and the gravitino mass problem
Several models of inflection point inflation with the volume modulus as the
inflaton are investigated. Non-perturbative superpotentials containing two
gaugino condensation terms or one such term with threshold corrections are
considered. It is shown that the gravitino mass may be much smaller than the
Hubble scale during inflation if at least one of the non-perturbative terms has
a positive exponent. Higher order corrections to the Kahler potential have to
be taken into account in such models. Those corrections are used to stabilize
the potential in the axion direction in the vicinity of the inflection point.
Models with only negative exponents require uplifting and in consequence have
the supersymmetry breaking scale higher than the inflation scale. Fine-tuning
of parameters and initial conditions is analyzed in some detail for both types
of models. It is found that fine-tuning of parameters in models with heavy
gravitino is much stronger than in models with light gravitino. It is shown
that recently proposed time dependent potentials can provide a solution to the
problem of the initial conditions only in models with heavy gravitino. Such
potentials can not be used to relax fine tuning of parameters in any model
because this would lead to values of the spectral index well outside the
experimental bounds.Comment: 27 pages, 9 figures, comments and references added, version to be
publishe
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