1,261 research outputs found
Stochastic Inflation in Compact Extra Dimensions
While moving down the potential on its classical slow roll trajectory, the
inflaton field is subject to quantum jumps, which take it up or down the
potential at random. In "stochastic inflation", the impact of these quantum
jumps is modeled by smoothing out the field over (at least) Hubble-patch sized
domains and treating fluctuations on smaller scales as noise. The inflaton thus
becomes a stochastic process whose values at a given time are calculated using
its probability distribution. We generalize this approach for non-canonic
kinetic terms of Dirac Born Infeld (DBI) type and investigate the resulting
modifications of the field's trajectory. Since models of DBI inflation arise
from string-inspired scenarios in which the scalar field has a geometric
interpretation, we insist that field value restrictions imposed by the model's
string origin must be respected at the quantum level.Comment: Proceedings of the ERE 2010 conference, 4 pages, 1 figur
A Self-Tuning Solution of the Cosmological Constant Problem
We discuss the four-dimensional cosmological constant problem in a
five-dimensional setting. A scalar field coupled to the SM forms dynamically a
smooth brane with four-dimensional Poincare invariance, independently of SM
physics. In this respect, our solution may be regarded as a self-tuning
solution, free of any singularities and fine-tuning problems.Comment: latex, 4 pages, some new material adde
Gravitational backreaction of anti-D branes in the warped compactification
We derive a low-energy effective theory for gravity with anti-D branes, which
are essential to get de Sitter solutions in the type IIB string warped
compactification, by taking account of gravitational backreactions of anti-D
branes. In order to see the effects of the self-gravity of anti-D branes, a
simplified model is studied where a 5-dimensional anti-de Sitter ({\it AdS})
spacetime is realized by the bulk cosmological constant and the 5-form flux,
and anti-D branes are coupled to the 5-form field by Chern-Simon terms. The
{\it AdS} spacetime is truncated by introducing UV and IR cut-off branes like
the Randall-Sundrum model. We derive an effective theory for gravity on the UV
brane and reproduce the familiar result that the tensions of the anti-D branes
give potentials suppressed by the forth-power of the warp factor at the
location of the anti-D branes. However, in this simplified model, the potential
energy never inflates the UV brane, although the anti-D-branes are inflating.
The UV brane is dominated by dark radiation coming from the projection of the
5-dimensional Weyl tensor, unless the moduli fields for the anti-D branes are
stabilized. We comment on the possibility of avoiding this problem in a
realistic string theory compactification.Comment: typos corrected, 11 pages, 3 figure
Lectures on Warped Compactifications and Stringy Brane Constructions
In these lectures, two different aspects of brane world scenarios in 5d
gravity or string theory are discussed. In the first two lectures, work on how
warped compactifications of 5d gravity theories can change the guise of the
hierarchy problem and the cosmological constant problem is reviewed, and a
discussion of several issues which remain unclear in this context is provided.
In the next two lectures, microscopic constructions in string theory which
involve D-branes wrapped on cycles of Calabi-Yau manifolds are described. The
focus is on computing the superpotential in the brane worldvolume field theory.
Such calculations may be a necessary step towards understanding e.g.
supersymmetry breaking and moduli stabilization in stringy realizations of such
scenarios, and are of intrinsic interest as probes of the quantum geometry of
the Calabi-Yau space.Comment: 31 pages, harvmac big. To appear in the proceedings of TASI 1999 and
the 2000 Trieste Spring Workshop on Superstring
Tension is Dimension
We propose a simple universal formula for the tension of a D-brane in terms
of a regularized dimension of the associated conformal field theory statespace.Comment: 18 pages, harvmac (b), one ref added, one typo fixe
Green's Functions and Non-Singlet Glueballs on Deformed Conifolds
We study the Laplacian on Stenzel spaces (generalized deformed conifolds),
which are tangent bundles of spheres endowed with Ricci flat metrics. The
(2d-2)-dimensional Stenzel space has SO(d) symmetry and can be embedded in C^d
through the equation \sum_{i = 1}^d {z_i^2} = \epsilon^2. We discuss the
Green's function with a source at a point on the S^{d-1} zero section of
TS^{d-1}. Its calculation is complicated by mixing between different harmonics
with the same SO(d) quantum numbers due to the explicit breaking by the
\epsilon-deformation of the U(1) symmetry that rotates z_i by a phase. A
similar mixing affects the spectrum of normal modes of warped deformed
conifolds that appear in gauge/gravity duality. We solve the mixing problem
numerically to determine certain bound state spectra in various representations
of SO(d) for the d=4 and d=5 examples.Comment: 52 pages, 3 figure
Lectures on Non-BPS Dirichlet branes
A comprehensive introduction to the boundary state approach to Dirichlet
branes is given. Various examples of BPS and non-BPS Dirichlet branes are
discussed. In particular, the non-BPS states in the duality of Type IIA on K3
and the heterotic string on T4 are analysed in detail.Comment: 46 pages, 5 figures, LaTeX; lectures given at the TMR network school
on `Quantum aspects of gauge theories, supersymmetry and quantum gravity',
Torino, 26 January - 2 February 2000, and at the `Spring workshop on
Superstrings and related matters', Trieste, 27 March - 4 April 2000;
references adde
Flux Compactifications: Stability and Implications for Cosmology
We study the dynamics of the size of an extra-dimensional manifold stabilised
by fluxes. Inspecting the potential for the 4D field associated with this size
(the radion), we obtain the conditions under which it can be stabilised and
show that stable compactifications on hyperbolic manifolds necessarily have a
negative four-dimensional cosmological constant, in contradiction with
experimental observations. Assuming compactification on a positively curved
(spherical) manifold we find that the radion has a mass of the order of the
compactification scale, M_c, and Planck suppressed couplings. We also show that
the model becomes unstable and the extra dimensions decompactify when the
four-dimensional curvature is higher than a maximum value. This in particular
sets an upper bound on the scale of inflation in these models: V_max \sim M_c^2
M_P^2, independently of whether the radion or other field is responsible for
inflation. We comment on other possible contributions to the radion potential
as well as finite temperature effects and their impact on the bounds obtained.Comment: 16 pages, 1 figure, LaTeX; v2: typos fixed and references adde
A New Fractional D2-brane, G_2 Holonomy and T-duality
Recently, a new example of a complete non-compact Ricci-flat metric of G_2
holonomy was constructed, which has an asymptotically locally conical structure
at infinity with a circular direction whose radius stabilises. In this paper we
find a regular harmonic 3-form in this metric, which we then use in order to
obtain an explicit solution for a fractional D2-brane configuration. By
performing a T-duality transformation on the stabilised circle, we obtain the
type IIB description of the fractional brane, which now corresponds to D3-brane
with one of its world-volume directions wrapped around the circle.Comment: Latex, 13 page
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