117 research outputs found
Is There A String Theory Landscape
We examine recent claims of a large set of flux compactification solutions of
string theory. We conclude that the arguments for AdS solutions are plausible.
The analysis of meta-stable dS solutions inevitably leads to situations where
long distance effective field theory breaks down. We then examine whether these
solutions are likely to lead to a description of the real world. We conclude
that one must invoke a strong version of the anthropic principle. We explain
why it is likely that this leads to a prediction of low energy supersymmetry
breaking, but that many features of anthropically selected flux
compactifications are likely to disagree with experiment.Comment: 39 pages, Latex, ``Terminology surrounding the anthropic principle
revised to conform with accepted usage. More history of the anthropic
principle included. Various references added.
CPT and Other Symmetries in String/M Theory
We initiate a search for non-perturbative consistency conditions in M theory.
Some non-perturbative conditions are already known in Type I theories; we
review these and search for others. We focus principally on possible anomalies
in discrete symmetries. It is generally believed that discrete symmetries in
string theories are gauge symmetries, so anomalies would provide evidence for
inconsistencies. Using the orbifold cosmic string construction, we give some
evidence that the symmetries we study are gauged. We then search for anomalies
in discrete symmetries in a variety of models, both with and without
supersymmetry. In symmetric orbifold models we extend previous searches, and
show in a variety of examples that all anomalies may be canceled by a
Green-Schwarz mechanism. We explore some asymmetric orbifold constructions and
again find that all anomalies may be canceled this way. Then we turn to Type
IIB orientifold models where it is known that even perturbative anomalies are
non-universal. In the examples we study, by combining geometric discrete
symmetries with continuous gauge symmetries, one may define non-anomalous
discrete symmetries already in perturbation theory; in other cases, the
anomalies are universal. Finally, we turn to the question of CPT conservation
in string/M theory. It is well known that CPT is conserved in all string
perturbation expansions; here in a number of examples for which a
non-perturbative formulation is available we provide evidence that it is
conserved exactly.Comment: 52 pages.1 paragraph added in introduction to clarify assumption
Fuzzy Spheres in AdS/CFT Correspondence and Holography from Noncommutativity
We show that the existent fuzzy S^2 and S^4 models are natural candidates for
the quantum geometry on the corresponding spheres in AdS/CFT correspondence.
These models fit nicely the data from the dipole mechanism for the stringy
exclusion principle. In the AdS_2 X S^2 case, we show that a wrapped fractional
membrane can be used to count for the large ground state degeneracy. We also
propose a fuzzy AdS_2 model whose fundamental commutation relation may underlie
the UV/IR connection.Comment: 17 pages, one comment and reference adde
On 't Hooft's S-matrix Ansatz for quantum black holes
The S-matrix Ansatz has been proposed by 't Hooft to overcome difficulties
and apparent contradictions of standard quantum field theory close to the black
hole horizon. In this paper we revisit and explore some of its aspects. We
start by computing gravitational backreaction effects on the properties of the
Hawking radiation and explain why a more powerful formalism is needed to encode
them. We then use the map bulk-boundary fields to investigate the nature of
exchange algebras satisfied by operators associated with ingoing and outgoing
matter. We propose and comment on some analogies between the non covariant form
of the S-matrix amplitude and liquid droplet physics to end up with
similarities with string theory amplitudes via an electrostatic analogy. We
finally recall the difficulties that one encounters when trying to incorporate
non linear gravity effects in 't Hooft's S-matrix and observe how the inclusion
of higher order derivatives might help in the black hole microstate counting.Comment: 22 Pages. Latex Fil
Semiclassical collapse of a sphere of dust
The semiclassical collapse of a homogeneous sphere of dust is studied. After
identifying the independent dynamical variables, the system is canonically
quantised and coupled equations describing matter (dust) and gravitation are
obtained. The conditions for the validity of the adiabatic (Born--Oppenheimer)
and semiclassical approximations are derived. Further on neglecting
back--reaction effects, it is shown that in the vicinity of the horizon and
inside the dust the Wightman function for a conformal scalar field coupled to a
monopole emitter is thermal at the characteristic Hawking temperature.Comment: LaTeX, 25 pages, no figures, final version accepted for publication
in Class. and Quantum Gra
Tachyon-Dilaton-induced Inflation as an alpha'-resummed String Background
Within the framework of a novel functional method on the world-sheet of the
string, we discuss simple but re-summed (in the Regge slope) inflationary
scenarios in the context of closed Bosonic strings, living in four target-space
dimensions, in the presence of non-trivial tachyon, dilaton and graviton
cosmological backgrounds. The inflationary solutions are argued to guarantee
the vanishing of the corresponding Weyl anomaly coefficients in a given
world-sheet renormalization scheme, thereby ensuring conformal invariance of
the corresponding sigma-model to all orders in the Regge slope. The key
property is the requirement of "homogeneity" of the corresponding Weyl anomaly
coefficients. Inflation entails appropriate relations between the dilaton and
tachyon field configurations, whose form can lead to either a de Sitter vacuum,
incompatible though (due to the cosmic horizons) with the perturbative string
scattering amplitudes, or to cosmic space-times involving brief inflationary
periods, interpolating smoothly between power-law and/or Minkowski Universes.
The latter situation is characterized by well-defined scattering amplitudes,
and is thus compatible with a perturbative string framework. It is this
scenario that we consider a self-consistent ground state in our framework,
which is based on local field redefinitions of background fields.Comment: 35 pages Latex, three eps figures incorporate
Noncommutative Inspired Black Holes in Extra Dimensions
In a recent string theory motivated paper, Nicolini, Smailagic and Spallucci
(NSS) presented an interesting model for a noncommutative inspired,
Schwarzschild-like black hole solution in 4-dimensions. The essential effect of
having noncommutative co-ordinates in this approach is to smear out matter
distributions on a scale associated with the turn-on of noncommutativity which
was taken to be near the 4-d Planck mass. In particular, NSS took this smearing
to be essentially Gaussian. This energy scale is sufficiently large that in 4-d
such effects may remain invisible indefinitely. Extra dimensional models which
attempt to address the gauge hierarchy problem, however, allow for the
possibility that the effective fundamental scale may not be far from 1
TeV, an energy regime that will soon be probed by experiments at both the LHC
and ILC. In this paper we generalize the NSS model to the case where flat,
toroidally compactified extra dimensions are accessible at the Terascale and
examine the resulting modifications in black hole properties due to the
existence of noncommutativity. We show that while many of the
noncommutativity-induced black hole features found in 4-d by NSS persist, in
some cases there can be significant modifications due the presence of extra
dimensions. We also demonstrate that the essential features of this approach
are not particularly sensitive to the Gaussian nature of the smearing employed
by NSS.Comment: 30 pages, 12 figures; slight text modifications and references adde
Racetrack Inflation
We develop a model of eternal topological inflation using a racetrack
potential within the context of type IIB string theory with KKLT volume
stabilization. The inflaton field is the imaginary part of the K\"ahler
structure modulus, which is an axion-like field in the 4D effective field
theory. This model does not require moving branes, and in this sense it is
simpler than other models of string theory inflation. Contrary to
single-exponential models, the structure of the potential in this example
allows for the existence of saddle points between two degenerate local minima
for which the slow-roll conditions can be satisfied in a particular range of
parameter space. We conjecture that this type of inflation should be present in
more general realizations of the modular landscape. We also consider
`irrational' models having a dense set of minima, and discuss their possible
relevance for the cosmological constant problem.Comment: 23 pages 7 figures. The final version with minor modifications, to
appear in JHE
Bounds on masses of bulk fields in string compactifications
In string compactification on a manifold X, in addition to the string scale
and the normal scales of low-energy particle physics, there is a Kaluza-Klein
scale 1/R associated with the size of X. We present an argument that generic
string models with low-energy supersymmetry have, after moduli stabilization,
bulk fields with masses which are parametrically lighter than 1/R. We discuss
the implications of these light states for anomaly mediation and gaugino
mediation scenarios.Comment: 15 page
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