39 research outputs found
Single-Brane Cosmological Solutions with a Stable Compact Extra Dimension
We consider 5-dimensional cosmological solutions of a single brane. The
correct cosmology on the brane, i.e., governed by the standard 4-dimensional
Friedmann equation, and stable compactification of the extra dimension is
guaranteed by the existence of a non-vanishing \hat{T}^5_5 which is
proportional to the 4-dimensional trace of the energy-momentum tensor. We show
that this component of the energy-momentum tensor arises from the backreaction
of the dilaton coupling to the brane. The same positive features are exhibited
in solutions found in the presence of non-vanishing cosmological constants both
on the brane (\Lambda_{br}) and in the bulk (\Lambda_B). Moreover, the
restoration of the Friedmann equation, with the correct sign, takes place for
both signs of so long as the sign of is opposite
in order to cancel the energy densities of the two cosmological
constants. We further extend our single-brane thin-wall solution to allow a
brane with finite thickness.Comment: 25 pages, Latex file, no figures, comments added, references updated,
final version to appear in Physical Review
Extra Dimensions at the Weak Scale and Deviations from Newtonian Gravity
We consider theories in which the Standard Model gauge fields propagate in
extra dimensions whose size is around the electroweak scale. The Standard Model
quarks and leptons may either be localized to a brane or propagate in the bulk.
This class of theories includes models of Scherk-Schwarz supersymmetry
breaking and universal extra dimensions. We consider the problem of stabilizing
the volume of the extra dimensions. We find that for a large class of
stabilization mechanisms the field which corresponds to fluctuations of the
volume remains light even after stabilization, and has a mass in the
eV range. In particular this is the case if stabilization does not involve
dynamics at scales larger than the cutoff of the higher dimensional Standard
Model, and if the effective theory below the compactification scale is four
dimensional. The mass of this field is protected against large radiative
corrections by the general covariance of the higher dimensional theory and by
the weakness of its couplings, which are Planck suppressed. Its couplings to
matter mediate forces whose strength is comparable to that of gravity and which
can give rise to potentially observable deviations from Newton's Law at
sub-millimeter distances. Current experiments investigating short distance
gravity can probe extra dimensions too small to be accessible to current
collider experiments. In particular for a single extra dimension stabilized by
the Casimir energy of the Standard Model fields compactification radii as small
as 5 inverse TeV are accessible to current sub-millimeter gravity experiments.Comment: Minor corrections, conclusions unchanged. References adde
Variable-Speed-of-Light Cosmology from Brane World Scenario
We argue that the four-dimensional universe on the TeV brane of the
Randall-Sundrum scenario takes the bimetric structure of Clayton and Moffat,
with gravitons traveling faster than photons instead, while the radion varies
with time. We show that such brane world bimetric model can thereby solve the
flatness and the cosmological constant problems, provided the speed of a
graviton decreases to the present day value rapidly enough. The resolution of
other cosmological problems such as the horizon problem and the monopole
problem requires supplementation by inflation, which may be achieved by the
radion field provided the radion potential satisfies the slow-roll
approximation.Comment: 18 pages, LaTeX, revised version to appear in Phys. Rev.
Precision Electroweak Data and Unification of Couplings in Warped Extra Dimensions
Warped extra dimensions allow a novel way of solving the hierarchy problem,
with all fundamental mass parameters of the theory naturally of the order of
the Planck scale. The observable value of the Higgs vacuum expectation value is
red-shifted, due to the localization of the Higgs field in the extra dimension.
It has been recently observed that, when the gauge fields propagate in the
bulk, unification of the gauge couplings may be achieved. Moreover, the
propagation of fermions in the bulk allows for a simple solution to potentially
dangerous proton decay problems. However, bulk gauge fields and fermions pose a
phenomenological challenge, since they tend to induce large corrections to the
precision electroweak observables. In this article, we study in detail the
effect of gauge and fermion fields propagating in the bulk in the presence of
gauge brane kinetic terms compatible with gauge coupling unification, and we
present ways of obtaining a consistent description of experimental data, while
allowing values of the first Kaluza Klein mode masses of the order of a few
TeV.Comment: 32 pages, 7 figures. References adde
Cosmological Non-Linearities as an Effective Fluid
The universe is smooth on large scales but very inhomogeneous on small
scales. Why is the spacetime on large scales modeled to a good approximation by
the Friedmann equations? Are we sure that small-scale non-linearities do not
induce a large backreaction? Related to this, what is the effective theory that
describes the universe on large scales? In this paper we make progress in
addressing these questions. We show that the effective theory for the
long-wavelength universe behaves as a viscous fluid coupled to gravity:
integrating out short-wavelength perturbations renormalizes the homogeneous
background and introduces dissipative dynamics into the evolution of
long-wavelength perturbations. The effective fluid has small perturbations and
is characterized by a few parameters like an equation of state, a sound speed
and a viscosity parameter. These parameters can be matched to numerical
simulations or fitted from observations. We find that the backreaction of
small-scale non-linearities is very small, being suppressed by the large
hierarchy between the scale of non-linearities and the horizon scale. The
effective pressure of the fluid is always positive and much too small to
significantly affect the background evolution. Moreover, we prove that
virialized scales decouple completely from the large-scale dynamics, at all
orders in the post-Newtonian expansion. We propose that our effective theory be
used to formulate a well-defined and controlled alternative to conventional
perturbation theory, and we discuss possible observational applications.
Finally, our way of reformulating results in second-order perturbation theory
in terms of a long-wavelength effective fluid provides the opportunity to
understand non-linear effects in a simple and physically intuitive way.Comment: 84 pages, 3 figure
The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang
We propose a cosmological scenario in which the hot big bang universe is
produced by the collision of a brane in the bulk space with a bounding orbifold
plane, beginning from an otherwise cold, vacuous, static universe. The model
addresses the cosmological horizon, flatness and monopole problems and
generates a nearly scale-invariant spectrum of density perturbations without
invoking superluminal expansion (inflation). The scenario relies, instead, on
physical phenomena that arise naturally in theories based on extra dimensions
and branes. As an example, we present our scenario predominantly within the
context of heterotic M-theory. A prediction that distinguishes this scenario
from standard inflationary cosmology is a strongly blue gravitational wave
spectrum, which has consequences for microwave background polarization
experiments and gravitational wave detectors.Comment: 67 pages, 4 figures. v2,v3: minor corrections, references adde
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Pyrotechnic Universe
One of the central points of the ekpyrotic cosmological scenario based on
Horava-Witten theory is that we live on a negative tension brane. However, the
tension of the visible brane is positive in the usual HW phenomenology with
stronger coupling on the hidden brane, both for standard and non-standard
embedding. To make ekpyrotic scenario realistic one must solve the problem of
the negative cosmological constant on the visible brane and fine-tune the bulk
brane potential with an accuracy of . In terms of a canonically
normalized scalar field describing the position of the brane, this
potential must take a very unusual form . We describe the problems which appear when one attempts to obtain this
potential in string theory. The mechanism for the generation of density
perturbations in this scenario is not brane-specific; it is a particular
limiting case of the mechanism of tachyonic preheating. Unlike inflation, this
mechanism exponentially amplifies not only quantum fluctuations, but also
initial inhomogeneities. As a result, to solve the homogeneity problem in this
scenario, one would need the branes to be parallel to each other with an
accuracy better than on a scale times greater than the
distance between the branes. Thus, at present, inflation remains the only
robust mechanism that produces density perturbations with a flat spectrum and
simultaneously solves all major cosmological problems.Comment: 35 pages, 2 figs. The issue of the brane tension is clarified and a
new Appendix explaining constraints on the parameters is adde