1,732 research outputs found
Supersymmetry Changing Bubbles in String Theory
We give examples of string compactifications to 4d Minkowski space with
different amounts of supersymmetry that can be connected by spherical domain
walls. The tension of these domain walls is tunably lower than the 4d Planck
scale. The ``stringy'' description of these walls is known in terms of certain
configurations of wrapped Dirichlet and NS branes. This construction allows us
to connect a variety of vacua with 4d N=4,3,2,1 supersymmetry.Comment: 11 pages, harvmac, no figures, reference added, minor correction
Volume Stabilization and the Origin of the Inflaton Shift Symmetry in String Theory
The main problem of inflation in string theory is finding the models with a
flat potential, consistent with stabilization of the volume of the compactified
space. This can be achieved in the theories where the potential has (an
approximate) shift symmetry in the inflaton direction. We will identify a class
of models where the shift symmetry uniquely follows from the underlying
mathematical structure of the theory. It is related to the symmetry properties
of the corresponding coset space and the period matrix of special geometry,
which shows how the gauge coupling depends on the volume and the position of
the branes. In particular, for type IIB string theory on K3xT^2/Z with D3 or D7
moduli belonging to vector multiplets, the shift symmetry is a part of
SO(2,2+n) symmetry of the coset space [SU(1,1)/ U(1)]x[SO(2,2+n)/(SO(2)x
SO(2+n)]. The absence of a prepotential, specific for the stringy version of
supergravity, plays a prominent role in this construction, which may provide a
viable mechanism for the accelerated expansion and inflation in the early
universe.Comment: 12 page
De Sitter stability in quadratic gravity
Quadratic curvature corrections to Einstein-Hilbert action lead in general to
higher-order equations of motion, which can induced instability of some
unperturbed solutions of General Relativity. We study conditions for stability
of de Sitter cosmological solution. We argue that simple form of this condition
known for FRW background in 3+1 dimensions changes seriously if at least one of
these two assumptions is violated. In the present paper the stability
conditions for de Sitter solution have been found for multidimensional FRW
background and for Bianchi I metrics in 3+1 dimensions.Comment: 12 pages with 4 figures; references adde
Interpolating from Bianchi Attractors to Lifshitz and AdS Spacetimes
We construct classes of smooth metrics which interpolate from Bianchi
attractor geometries of Types II, III, VI and IX in the IR to Lifshitz or
geometries in the UV. While we do not obtain these metrics
as solutions of Einstein gravity coupled to a simple matter field theory, we
show that the matter sector stress-energy required to support these geometries
(via the Einstein equations) does satisfy the weak, and therefore also the
null, energy condition. Since Lifshitz or geometries can in
turn be connected to spacetime, our results show that there is no
barrier, at least at the level of the energy conditions, for solutions to arise
connecting these Bianchi attractor geometries to spacetime. The
asymptotic spacetime has no non-normalizable metric deformation turned
on, which suggests that furthermore, the Bianchi attractor geometries can be
the IR geometries dual to field theories living in flat space, with the
breaking of symmetries being either spontaneous or due to sources for other
fields. Finally, we show that for a large class of flows which connect two
Bianchi attractors, a C-function can be defined which is monotonically
decreasing from the UV to the IR as long as the null energy condition is
satisfied. However, except for special examples of Bianchi attractors
(including AdS space), this function does not attain a finite and non-vanishing
constant value at the end points.Comment: 37 pages, 12 figures, The comment regarding the behavior of
C-function for general Bianchi Types appearing in IR or UV clarified, the
relation of Type IX with for made more precise
and a comment regarding type V added in the conclusio
Multi-Throat Brane Inflation
We present a scenario where brane inflation arises more generically. We start
with D3 and anti-D3-branes at the infrared ends of two different throats. This
setup is a natural consequence of the assumption that in the beginning we have
a multi-throat string compactification with many wandering anti-D3-branes. A
long period of inflation is triggered when D3-branes slowly exit the highly
warped infrared region, under a potential generically arising from the moduli
stabilization. In this scenario, the usual slow-roll conditions are not
required, and a large warping is allowed to incorporate the Randall-Sundrum
model.Comment: 11 pages; v3: minor revision, PRD versio
Supersymmetric Three-cycles and (Super)symmetry Breaking
We describe physical phenomena associated with a class of transitions that
occur in the study of supersymmetric three-cycles in Calabi-Yau threefolds. The
transitions in question occur at real codimension one in the complex structure
moduli space of the Calabi-Yau manifold. In type IIB string theory, these
transitions can be used to describe the evolution of a BPS state as one moves
through a locus of marginal stability: at the transition point the BPS particle
becomes degenerate with a supersymmetric two particle state, and after the
transition the lowest energy state carrying the same charges is a
non-supersymmetric two particle state. In the IIA theory, wrapping the cycles
in question with D6-branes leads to a simple realization of the Fayet model:
for some values of the CY modulus gauge symmetry is spontaneously broken, while
for other values supersymmetry is spontaneously broken.Comment: 10 pages, harvmac big; v2, minor change
Self-tuning flat domain walls in 5d gravity and string theory
We present Poincare invariant domain wall (``3-brane'') solutions to some
5-dimensional effective theories which can arise naturally in string theory. In
particular, we find theories where Poincare invariant solutions exist for
arbitrary values of the brane tension, for certain restricted forms of the bulk
interactions. We describe examples in string theory where it would be natural
for the quantum corrections to the tension of the brane (arising from quantum
fluctuations of modes with support on the brane) to maintain the required form
of the action. In such cases, the Poincare invariant solutions persist in the
presence of these quantum corrections to the brane tension, so that no 4d
cosmological constant is generated by these modes.Comment: 21 pages, harvmac bi
The moduli problem at the perturbative level
Moduli fields generically produce strong dark matter -- radiation and baryon
-- radiation isocurvature perturbations through their decay if they remain
light during inflation. We show that existing upper bounds on the magnitude of
such fluctuations can thus be translated into stringent constraints on the
moduli parameter space m_\sigma (modulus mass) -- \sigma_{inf} (modulus vacuum
expectation value at the end of inflation). These constraints are complementary
to previously existing bounds so that the moduli problem becomes worse at the
perturbative level. In particular, if the inflationary scale H_{inf}~10^{13}
GeV, particle physics scenarios which predict high moduli masses m_\sigma >
10-100 TeV are plagued by the perturbative moduli problem, even though they
evade big-bang nucleosynthesis constraints.Comment: 4 pages, 3 figures (revtex) -- v2: an important correction on the
amplitude/transfer of isocurvature modes at the end of inflation, typos
corrected, references added, basic result unchange
Extremal Horizons with Reduced Symmetry: Hyperscaling Violation, Stripes, and a Classification for the Homogeneous Case
Classifying the zero-temperature ground states of quantum field theories with
finite charge density is a very interesting problem. Via holography, this
problem is mapped to the classification of extremal charged black brane
geometries with anti-de Sitter asymptotics. In a recent paper [1], we proposed
a Bianchi classification of the extremal near-horizon geometries in five
dimensions, in the case where they are homogeneous but, in general,
anisotropic. Here, we extend our study in two directions: we show that Bianchi
attractors can lead to new phases, and generalize the classification of
homogeneous phases in a way suggested by holography. In the first direction, we
show that hyperscaling violation can naturally be incorporated into the Bianchi
horizons. We also find analytical examples of "striped" horizons. In the second
direction, we propose a more complete classification of homogeneous horizon
geometries where the natural mathematics involves real four-algebras with three
dimensional sub-algebras. This gives rise to a richer set of possible
near-horizon geometries, where the holographic radial direction is
non-trivially intertwined with field theory spatial coordinates. We find
examples of several of the new types in systems consisting of reasonably simple
matter sectors coupled to gravity, while arguing that others are forbidden by
the Null Energy Condition. Extremal horizons in four dimensions governed by
three-algebras or four-algebras are also discussed.Comment: 58 pages, 1 figure and 1 cartoon. v2: references adde
Gaugino Condensation and Nonperturbative Superpotentials in Flux Compactifications
There are two known sources of nonperturbative superpotentials for K\"ahler
moduli in type IIB orientifolds, or F-theory compactifications on Calabi-Yau
fourfolds, with flux: Euclidean brane instantons and low-energy dynamics in D7
brane gauge theories. The first class of effects, Euclidean D3 branes which
lift in M-theory to M5 branes wrapping divisors of arithmetic genus 1 in the
fourfold, is relatively well understood. The second class has been less
explored. In this paper, we consider the explicit example of F-theory on with flux. The fluxes lift the D7 brane matter fields, and stabilize
stacks of D7 branes at loci of enhanced gauge symmetry. The resulting theories
exhibit gaugino condensation, and generate a nonperturbative superpotential for
K\"ahler moduli. We describe how the relevant geometries in general contain
cycles of arithmetic genus (and how divisors can
contribute to the superpotential, in the presence of flux). This second class
of effects is likely to be important in finding even larger classes of models
where the KKLT mechanism of moduli stabilization can be realized. We also
address various claims about the situation for IIB models with a single
K\"ahler modulus.Comment: 24 pages, harvmac, no figures, references adde
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