218 research outputs found
Ultraviolet Property of Noncommutative Wess-Zumino-Witten Model
We construct noncommutative extension of the Wess-Zumino-Witten (WZW) model
and study its ultraviolet property. The \beta-function of the U(N)
noncommutative WZW model resembles that of the ordinary WZW model. The U(1)
noncommutative model has also a nontrivial fixed point.Comment: 7 pages, 1 figur
Non-local Matching Condition and Scale-invariant Spectrum in Bouncing Cosmology
In cosmological scenarios such as the pre-big bang scenario or the ekpyrotic
scenario, a matching condition between the metric perturbations in the pre-big
bang phase and those in the post big-bang phase is often assumed. Various
matching conditions have been considered in the literature. Nevertheless
obtaining a scale invariant CMB spectrum via a concrete mechanism remains
impossible. In this paper, we examine this problem from the point of view of
local causality. We begin with introducing the notion of local causality and
explain how it constrains the form of the matching condition. We then prove a
no-go theorem: independent of the details of the matching condition, a scale
invariant spectrum is impossible as long as the local causality condition is
satisfied. In our framework, it is easy to show that a violation of local
causality around the bounce is needed in order to give a scale invariant
spectrum. We study a specific scenario of this possibility by considering a
nonlocal effective theory inspired by noncommutative geometry around the bounce
and show that a scale invariant spectrum is possible. Moreover we demonstrate
that the magnitude of the spectrum is compatible with observations if the
bounce is assumed to occur at an energy scale which is a few orders of
magnitude below the Planckian energy scale.Comment: 15 pages, 2 figures; v3: clarifications added, changes in references,
version to appear in PR
Locality, Causality and Noncommutative Geometry
We analyse the causality condition in noncommutative field theory and show
that the nonlocality of noncommutative interaction leads to a modification of
the light cone to the light wedge. This effect is generic for noncommutative
geometry. We also check that the usual form of energy condition is violated and
propose that a new form is needed in noncommutative spacetime. On reduction
from light cone to light wedge, it looks like the noncommutative dimensions are
effectively washed out and suggests a reformulation of noncommutative field
theory in terms of lower dimensional degree of freedom. This reduction of
dimensions due to noncommutative geometry could play a key role in explaining
the holographic property of quantum gravity.Comment: 16 pages, LaTeX, 4 figure
Field Equations of Massless Fields in the New Interpretation of the Matrix Model
Recently, some of the authors have introduced a new interpretation of matrix
models in which covariant derivatives on any curved space can be expressed by
large-N matrices. It has been shown that the Einstein equation follows from the
equation of motion of IIB matrix model in this interpretation. In this paper,
we generalize this argument to covariant derivatives with torsion. We find that
some components of the torsion field can be identified with the dilaton and the
-field in string theory. However, the other components do not seem to have
string theory counterparts. We also consider the matrix model with a mass term
or a cubic term, in which the equation of motion of string theory is exactly
satisfied.Comment: 21 page
Non-BPS Solutions of the Noncommutative CP^1 Model in 2+1 Dimensions
We find non-BPS solutions of the noncommutative CP^1 model in 2+1 dimensions.
These solutions correspond to soliton anti-soliton configurations. We show that
the one-soliton one-anti-soliton solution is unstable when the distance between
the soliton and the anti-soliton is small. We also construct time-dependent
solutions and other types of solutions.Comment: 11 pages, minor correction
Low-Energy Dynamics of Noncommutative CP^1 Solitons in 2+1 Dimensions
We investigate the low-energy dynamics of the BPS solitons of the
noncommutative CP^1 model in 2+1 dimensions using the moduli space metric of
the BPS solitons. We show that the dynamics of a single soliton coincides with
that in the commutative model. We find that the singularity in the two-soliton
moduli space, which exists in the commutative CP^1 model, disappears in the
noncommutative model.We also show that the two-soliton metric has the smooth
commutative limit.Comment: AMSLaTeX, 11 page
Colliding Plane Waves in String Theory
We construct colliding plane wave solutions in higher dimensional gravity
theory with dilaton and higher form flux, which appears naturally in the low
energy theory of string theory. Especially, the role of the junction condition
in constructing the solutions is emphasized. Our results not only include the
previously known CPW solutions, but also provide a wide class of new solutions
that is not known in the literature before. We find that late time curvature
singularity is always developed for the solutions we obtained in this paper.
This supports the generalized version of Tipler's theorem in higher dimensional
supergravity.Comment: latex, 25 pages, 1 figur
Hair follicle epidermal stem cells define a niche for tactile sensation
The heterogeneity and compartmentalization of stem cells is a common principle in many epithelia, and is known to function in epithelial maintenance, but its other physiological roles remain elusive. Here we show transcriptional and anatomical contributions of compartmentalized epidermal stem cells in tactile sensory unit formation in the mouse hair follicle. Epidermal stem cells in the follicle upper-bulge, where mechanosensory lanceolate complexes innervate, express a unique set of extracellular matrix (ECM) and neurogenesis-related genes. These epidermal stem cells deposit an ECM protein called EGFL6 into the collar matrix, a novel ECM that tightly ensheathes lanceolate complexes. EGFL6 is required for the proper patterning, touch responses, and αv integrin-enrichment of lanceolate complexes. By maintaining a quiescent original epidermal stem cell niche, the old bulge, epidermal stem cells provide anatomically stable follicle–lanceolate complex interfaces, irrespective of the stage of follicle regeneration cycle. Thus, compartmentalized epidermal stem cells provide a niche linking the hair follicle and the nervous system throughout the hair cycle
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