18,642 research outputs found
Cosmology and Hierarchy in Stabilized Warped Brane Models
We examine the cosmology and hierarchy of scales in models with branes
immersed in a five-dimensional curved spacetime subject to radion
stabilization. When the radion field is time-independent and the inter-brane
spacing is stabilized, the universe can naturally find itself in the
radiation-dominated epoch. This feature is independent of the form of the
stabilizing potential. We recover the standard Friedmann equations without
assuming a specific form for the bulk energy-momentum tensor. In the models
considered, if the observable brane has positive tension, a solution to the
hierarchy problem requires the presence of a negative tension brane somewhere
in the bulk. We find that the string scale can be as low as the electroweak
scale. In the situation of self-tuning branes where the bulk cosmological
constant is set to zero, the brane tensions have hierarchical values. In the
case of a polynomial stabilizing potential no new hierarchy is created.Comment: Version to appear in PL
The Fractal Geometry of the Cosmic Web and its Formation
The cosmic web structure is studied with the concepts and methods of fractal
geometry, employing the adhesion model of cosmological dynamics as a basic
reference. The structures of matter clusters and cosmic voids in cosmological
N-body simulations or the Sloan Digital Sky Survey are elucidated by means of
multifractal geometry. A non-lacunar multifractal geometry can encompass three
fundamental descriptions of the cosmic structure, namely, the web structure,
hierarchical clustering, and halo distributions. Furthermore, it explains our
present knowledge of cosmic voids. In this way, a unified theory of the
large-scale structure of the universe seems to emerge. The multifractal
spectrum that we obtain significantly differs from the one of the adhesion
model and conforms better to the laws of gravity. The formation of the cosmic
web is best modeled as a type of turbulent dynamics, generalizing the known
methods of Burgers turbulence.Comment: 35 pages, 8 figures; corrected typos, added references; further
discussion of cosmic voids; accepted by Advances in Astronom
Mass hierarchy, mass gap and corrections to Newton's law on thick branes with Poincare symmetry
We consider a scalar thick brane configuration arising in a 5D theory of
gravity coupled to a self-interacting scalar field in a Riemannian manifold. We
start from known classical solutions of the corresponding field equations and
elaborate on the physics of the transverse traceless modes of linear
fluctuations of the classical background, which obey a Schroedinger-like
equation. We further consider two special cases in which this equation can be
solved analytically for any massive mode with m^2>0, in contrast with numerical
approaches, allowing us to study in closed form the massive spectrum of
Kaluza-Klein (KK) excitations and to compute the corrections to Newton's law in
the thin brane limit. In the first case we consider a solution with a mass gap
in the spectrum of KK fluctuations with two bound states - the massless 4D
graviton free of tachyonic instabilities and a massive KK excitation - as well
as a tower of continuous massive KK modes which obey a Legendre equation. The
mass gap is defined by the inverse of the brane thickness, allowing us to get
rid of the potentially dangerous multiplicity of arbitrarily light KK modes. It
is shown that due to this lucky circumstance, the solution of the mass
hierarchy problem is much simpler and transparent than in the (thin)
Randall-Sundrum (RS) two-brane configuration. In the second case we present a
smooth version of the RS model with a single massless bound state, which
accounts for the 4D graviton, and a sector of continuous fluctuation modes with
no mass gap, which obey a confluent Heun equation in the Ince limit. (The
latter seems to have physical applications for the first time within braneworld
models). For this solution the mass hierarchy problem is solved as in the
Lykken-Randall model and the model is completely free of naked singularities.Comment: 25 pages in latex, no figures, content changed, corrections to
Newton's law included for smooth version of RS model and an author adde
Kaluza-Klein towers in warped spaces with metric singularities
The version of the warp model that we proposed to explain the mass scale
hierarchy has been extended by the introduction of one or more singularities in
the metric. We restricted ourselves to a real massless scalar field supposed to
propagate in a five dimensional bulk with the extradimension being compactified
on a strip or on a circle. With the same emphasis on the hermiticity and
commutativity properties of the Kakuza Klein operators, we have established all
the allowed boundary conditions to be imposed on the fields. From them, for
given positions of the singularities, one can deduce either mass eigenvalues
building up a Kaluza Klein tower, or a tachyon, or a zero mass state. Assuming
the Planck mass to be the high mass scale and by a choice, unique for all
boundary conditions, of the major warp parameters, the low lying mass
eigenvalues are of the order of the TeV, in this way explaining the mass scale
hierarchy. In our model, the physical masses are related to the Kaluza Klein
eigenvalues, depending on the location of the physical brane which is an
arbitrary parameter of the model. Illustrative numerical calculations are given
to visualize the structure of Kaluza Klein mass eigenvalue towers. Observation
at high energy colliders like LHC of a mass tower with its characteristic
structure would be the fingerprint of the model.Comment: 33 pages, 1 figur
Hydrodynamic approach to the evolution of cosmological structures
A hydrodynamic formulation of the evolution of large-scale structure in the
Universe is presented. It relies on the spatially coarse-grained description of
the dynamical evolution of a many-body gravitating system. Because of the
assumed irrelevance of short-range (``collisional'') interactions, the way to
tackle the hydrodynamic equations is essentially different from the usual case.
The main assumption is that the influence of the small scales over the
large-scale evolution is weak: this idea is implemented in the form of a
large-scale expansion for the coarse-grained equations. This expansion builds a
framework in which to derive in a controlled manner the popular ``dust'' model
(as the lowest-order term) and the ``adhesion'' model (as the first-order
correction). It provides a clear physical interpretation of the assumptions
involved in these models and also the possibility to improve over them.Comment: 14 pages, 3 figures. Version to appear in Phys. Rev.
Brane World Models With Bulk Scalar Fields
We examine several different types of five dimensional stationary spacetimes
with bulk scalar fields and parallel 3-branes. We study different methods for
avoiding the appearance of spacetime singularities in the bulk for models with
and without cosmological expansion. For non-expanding models, we demonstrate
that in general the Randall-Sundrum warp factor is recovered in the asymptotic
bulk region, although elsewhere the warping may be steeper than exponential. We
show that nonsingular expanding models can be constructed as long as the
gradient of the bulk scalar field vanishes at zeros of the warp factor, which
are then analogous to the particle horizons found in expanding models with a
pure AdS bulk. Since the branes in these models are stabilized by bulk scalar
fields, we expect there to be no linearly unstable radion modes. As an
application, we find a specific class of expanding, stationary solutions with
no singularities in the bulk in which the four dimensional cosmological
constant and mass hierarchy are naturally very small.Comment: 16 page
The Ubiquitous Throat
We attempt to quantify the widely-held belief that large hierarchies induced
by strongly-warped geometries are common in the string theory landscape. To
this end, we focus on the arguably best-understood subset of vacua -- type IIB
Calabi-Yau orientifolds with non-perturbative Kaehler stabilization and a
SUSY-breaking uplift (the KKLT setup). Within this framework, vacua with a
realistically small cosmological constant are expected to come from Calabi-Yaus
with a large number of 3-cycles. For appropriate choices of flux numbers, many
of these 3-cycles can, in general, shrink to produce near-conifold geometries.
Thus, a simple statistical analysis in the spirit of Denef and Douglas allows
us to estimate the expected number and length of Klebanov-Strassler throats in
the given set of vacua. We find that throats capable of explaining the
electroweak hierarchy are expected to be present in a large fraction of the
landscape vacua while shorter throats are essentially unavoidable in a
statistical sense.Comment: References added, typos fixed. LaTex, 17 pages, 1 figur
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