117 research outputs found
Large-Field Inflation with Multiple Axions and the Weak Gravity Conjecture
In this note, we discuss the implications of the weak gravity conjecture
(WGC) for general models of large-field inflation with a large number of axions
. We first show that, from the bottom-up perspective, such models admit a
variety of different regimes for the enhancement of the effective axion decay
constant, depending on the amount of alignment and the number of instanton
terms that contribute to the scalar potential. This includes regimes of no
enhancement, power-law enhancement and exponential enhancement with respect to
. As special cases, we recover the Pythagorean enhancement of -flation,
the and enhancements derived by Bachlechner, Long and McAllister
and the exponential enhancement by Choi, Kim and Yun. We then analyze which
top-down constraints are put on such models from the requirement of consistency
with quantum gravity. In particular, the WGC appears to imply that the
enhancement of the effective axion decay constant must not grow parametrically
with for . On the other hand, recent works proposed that axions
might be able to violate this bound under certain circumstances. Our general
expression for the enhancement allows us to translate this possibility into a
condition on the number of instantons that couple to the axions. We argue that,
at large , models consistent with quantum gravity must either allow
super-Planckian field excursions or have an enormous, possibly even
exponentially large, number of dominant instanton terms in the scalar
potential.Comment: 30+8 pages, 13 figures, 1 table. v2: several remarks and references
added, version to appear in JHE
Weakly Coupled de Sitter Vacua with Fluxes and the Swampland
It was recently argued that the swampland distance conjecture rules out dS
vacua at parametrically large field distances. We point out that this
conclusion can in principle be avoided in the presence of large fluxes that are
not bounded by a tadpole cancellation condition. We then study this possibility
in the concrete setting of classical type IIA flux compactifications with
(anti-)O6-planes, (anti-)D6-branes and/or KK monopoles and show that,
nonetheless, parametrically controlled dS vacua are strongly constrained. In
particular, we find that such dS vacua are ruled out at parametrically large
volume and/or parametrically small string coupling. We also find obstructions
in the general case where the parametrically large field is an arbitrary field
combination.Comment: 27 pages. v2: references added, improved discussion in section 3.2.
v3: minor changes, JHEP versio
Curvature-induced Resolution of Anti-brane Singularities
We study AdS vacua of massive type IIA string theory compactified on a
3-sphere with flux and anti-D6-branes. In such backgrounds, the
anti-brane backreaction is known to generate a singularity in the energy
density, whose interpretation has not been understood so far. We first consider
supersymmetric solutions of this setup and give an analytic proof that the flux
singularity is resolved there by a polarization of the anti-D6-branes into a
D8-brane, which wraps a finite 2-sphere inside of the compact space. To this
end, we compute the potential for a spherical probe D8-brane on top of a
background with backreacting anti-D6-branes and show that it has a local
maximum at zero radius and a local minimum at a finite radius of the 2-sphere.
The polarization is triggered by a term in the potential due to the AdS
curvature and does therefore not occur in non-compact setups where the 7d
external spacetime is Minkowski. We furthermore find numerical evidence for the
existence of non-supersymmetric solutions in our setup. This is supported by
the observation that the general solution to the equations of motion has a
continuous parameter that is suggestive of a modulus and appears to control
supersymmetry breaking. Analyzing the polarization potential for the
non-supersymmetric solutions, we find that the flux singularities are resolved
there by brane polarization as well.Comment: 25 pages, 9 figures. v2: minor changes, discussion of scalar masses
adde
de Sitter-eating O-planes in supercritical string theory
It has been proposed that flux compactifications of supercritical string
theories (i.e., with spacetime dimension ) have dS vacua, with large
acting as a control parameter for corrections to the classical spacetime
effective action. In this paper, we provide a detailed analysis of the
self-consistency of such models, focussing on and backreaction
corrections. We first show that all supercritical AdS, Minkowski and dS vacua
in this setting have curvature and/or field strengths
in the string frame. This may be in tension with suppressing
corrections unless the coefficients of the higher-derivative terms have a
sufficiently strong large- suppression. We then argue that an additional and
more severe problem arises in the dS case due to the backreaction of O-planes.
In particular, we argue using a combination of geometric bounds and
string-theory constraints that the O-plane backreaction is large in
supercritical dS models. This implies that a large part of the naive classical
geometry is eaten up by singular holes and thus indicates a breakdown of the
classical description. Our finding resonates with several other recent results
suggesting that string theory does not admit dS vacua in regimes where string
and backreaction corrections are under control. As byproducts of our analysis,
we derive a number of technical results that are useful beyond the specific
applications in this paper. In particular, we compute the leading backreaction
corrections to the smeared solution in a general flux compactification from
to dimensions for an arbitrary distribution of O-planes and D-branes. We
further argue for a general estimate for Green's functions on compact manifolds
(and therefore for the backreaction corrections) in terms of their diameter,
volume and dimension.Comment: 59 pages, 3 figures. v2: minor changes, JHEP versio
Cosmological Constant, Near Brane Behavior and Singularities
We show that the classical cosmological constant in type II flux
compactifications can be written as a sum of terms from the action of localized
sources plus a specific contribution from non-trivial background fluxes.
Exploiting two global scaling symmetries of the classical supergravity action,
we find that the flux contribution can in many interesting cases be set to zero
such that the cosmological constant is fully determined by the boundary
conditions of the fields in the near-source region. This generalizes and makes
more explicit previous arguments in the literature. We then discuss the problem
of putting \bar{D3}-branes at the tip of the Klebanov-Strassler throat glued to
a compact space in type IIB string theory so as to engineer a de Sitter
solution. Our result for the cosmological constant and a simple global argument
indicate that inserting a fully localized and backreacting \bar{D3}-brane into
such a background yields a singular energy density for the NSNS and RR 3-form
field strengths at the \bar{D3}-brane. This argument does not rely on partial
smearing of the \bar{D3}-brane or a linearization of field equations, but on a
few general assumptions that we also discuss carefully.Comment: 30 pages, no figures, v2: Minor modifications and references added.
Version to appear in JHE
A note on O6 intersections in AdS flux vacua
The DGKT-CFI construction of AdS flux vacua in type IIA string theory has
interesting features such as classical moduli stabilization and a parametric
scale separation between the Hubble scale and the Kaluza-Klein scale. A
possible worry regarding the consistency of these vacua is that pathologies
could arise due to intersections of the O6-planes, which are not well
understood in the 10D solution. In this note, we show that such intersections
are sometimes, but not always, absent if one compactifies on smooth Calabi-Yau
manifolds instead of toroidal orbifolds. In particular, we show that the
blow-up of the orbifold yields a single O6-plane which wraps
a smooth submanifold without any (self-)intersections. On the other hand,
blowing up the orbifold yields an O6-plane which
self-intersects.Comment: 16 pages, 2 figure
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