80 research outputs found
Inflation as a probe of new physics
In this paper we consider inflation as a probe of new physics near the string
or Planck scale. We discuss how new physics can be captured by the choice of
vacuum, and how this leads to modifications of the primordial spectrum as well
as the way in which the universe expands during inflation. Provided there is a
large number of fields contributing to the vacuum energy -- as typically is
expected in string theory -- we will argue that both types of effects can be
present simultaneously and be of observational relevance. Our conclusion is
that the ambiguity in choice of vacuum is an interesting new parameter in
serious model building.Comment: 14 page
Classical and Quantum Solutions and the Problem of Time in Cosmology
We have studied various classical solutions in cosmology. Especially we
have obtained general classical solutions in pure \ cosmology. Even in the
quantum theory, we can solve the Wheeler-DeWitt equation in pure \
cosmology exactly. Comparing these classical and quantum solutions in \
cosmology, we have studied the problem of time in general relativity.Comment: 17 pages, latex, no figure, one reference is correcte
Moduli Stabilization in Brane Gas Cosmology with Superpotentials
In the context of brane gas cosmology in superstring theory, we show why it
is impossible to simultaneously stabilize the dilaton and the radion with a
general gas of strings (including massless modes) and D-branes. Although this
requires invoking a different mechanism to stabilize these moduli fields, we
find that the brane gas can still play a crucial role in the early universe in
assisting moduli stabilization. We show that a modest energy density of
specific types of brane gas can solve the overshoot problem that typically
afflicts potentials arising from gaugino condensation.Comment: minor changes to match the journal versio
Interactions of Eight-branes in String Theory and M(atrix) Theory
We consider eight-brane configurations in M(atrix) theory and compute their
interaction potentials with gravitons, membranes, and four-branes. We compare
these results with the interactions of D8-branes with D0-branes, D2-branes, and
D4-branes in IIA string theory. We find agreement between the two approaches
for eight-brane interactions with two-branes and four-branes. A discrepancy is
noted in the case with zero-branes.Comment: 17 pages, harvmac. Minor changes, reference adde
(Twisted) Toroidal Compactification of pp-Waves
The maximally supersymmetric type IIB pp-wave is compactified on spatial
circles, with and without an auxiliary rotational twist. All spatial circles of
constant radius are identified. Without the twist, an S compactification
can preserve 24, 20 or 16 supercharges. compactifications can preserve
20, 18 or 16 supercharges; compactifications can preserve 18 or 16
supercharges and higher compactifications preserve 16 supercharges. The
worldsheet theory of this background is discussed. The T-dual and
decompactified type IIA and M-theoretic solutions which preserve 24
supercharges are given. Some comments are made regarding the AdS parent and the
CFT description.Comment: 22 pages REVTeX 4 and AMSLaTeX. v3: References and a paragraph on
nine dimensional Killing spinors were added. v4: A few typos corrected and a
footnote was modifie
Optical geometry for gravitational collapse and Hawking radiation
The notion of optical geometry, introduced more than twenty years ago as a
formal tool in quantum field theory on a static background, has recently found
several applications to the study of physical processes around compact objects.
In this paper we define optical geometry for spherically symmetric
gravitational collapse, with the purpose of extending the current formalism to
physically interesting spacetimes which are not conformally static. The
treatment is fully general but, as an example, we also discuss the special case
of the Oppenheimer-Snyder model. The analysis of the late time behaviour shows
a close correspondence between the structure of optical spacetime for
gravitational collapse and that of flat spacetime with an accelerating
boundary. Thus, optical geometry provides a natural physical interpretation for
derivations of the Hawking effect based on the ``moving mirror analogy.''
Finally, we briefly discuss the issue of back-reaction in black hole
evaporation and the information paradox from the perspective of optical
geometry.Comment: 13 pages, 10 figures, aps, revtex, To be published in PR
The Two Faces of Anomaly Mediation
Anomaly mediation is a ubiquitous source of supersymmetry (SUSY) breaking
which appears in almost every theory of supergravity. In this paper, we show
that anomaly mediation really consists of two physically distinct phenomena,
which we dub "gravitino mediation" and "Kahler mediation". Gravitino mediation
arises from minimally uplifting SUSY anti-de Sitter (AdS) space to Minkowski
space, generating soft masses proportional to the gravitino mass. Kahler
mediation arises when visible sector fields have linear couplings to SUSY
breaking in the Kahler potential, generating soft masses proportional to beta
function coefficients. In the literature, these two phenomena are lumped
together under the name "anomaly mediation", but here we demonstrate that they
can be physically disentangled by measuring associated couplings to the
goldstino. In particular, we use the example of gaugino soft masses to show
that gravitino mediation generates soft masses without corresponding goldstino
couplings. This result naively violates the goldstino equivalence theorem but
is in fact necessary for supercurrent conservation in AdS space. Since
gravitino mediation persists even when the visible sector is sequestered from
SUSY breaking, we can use the absence of goldstino couplings as an unambiguous
definition of sequestering.Comment: 21 pages, 1 table; v2, references added, extended discussion in
introduction and appendix; v3, JHEP versio
Effects of dog ownership in early childhood on immune development and atopic diseases
Summary Background Exposure to pets in childhood has been associated with a reduced risk of wheezing and atopy. Objective Our objective was to determine whether the effects of pet exposure on immune development and atopy in early childhood can be explained by alterations in exposure to innate immune stimuli in settled dust. Methods Two hundred and seventy-five children at increased risk of developing allergic diseases were evaluated to age 3 years for pet ownership, blood cell cytokine responses, and atopy. Can f 1, Fel d 1, endotoxin, ergosterol, and muramic acid were measured in settled dust from 101 homes. Results Dog exposure at birth was associated with decreased atopic dermatitis (AD) (12% vs. 27%; P = 0.004) and wheezing (19% vs. 36%; P = 0.005) in year 3. The rates of AD (23%) and wheezing (42%) in year 3 were relatively high in children who acquired dogs after birth. The prevalence of dog sensitization (10-12%) did not vary according to dog exposure. Can f 1 levels in bedroom dust were positively associated with IL-10 (r = 0.26; P = 0.01), IL-5 (r = 0.34, P o 0.001), and IL-13 (r = 0.28; P = 0.004) responses at age 1, and IL-5 (r = 0.24; P = 0.022) and IL-13 (r = 0.25; P = 0.015) responses at age 3. In contrast, endotoxin was associated with IFN-g (r = 0.31; P = 0.002) and IL-13 (r = 0.27; P = 0.01) responses at age 3 but not at age 1, and similar relationships were present for muramic acid. Adjustment for levels of innate immune stimuli in house dust did not significantly affect the relationships between Can f 1 and cytokine responses. Conclusions Exposure to dogs in infancy, and especially around the time of birth, is associated with changes in immune development and reductions in wheezing and atopy. These findings are not explained by exposure to endotoxin, ergosterol, or muramic acid
Brane-World Gravity
The observable universe could be a 1+3-surface (the "brane") embedded in a
1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model
particles and fields trapped on the brane while gravity is free to access the
bulk. At least one of the \textit{d} extra spatial dimensions could be very
large relative to the Planck scale, which lowers the fundamental gravity scale,
possibly even down to the electroweak ( TeV) level. This revolutionary
picture arises in the framework of recent developments in M theory. The
1+10-dimensional M theory encompasses the known 1+9-dimensional superstring
theories, and is widely considered to be a promising potential route to quantum
gravity. At low energies, gravity is localized at the brane and general
relativity is recovered, but at high energies gravity "leaks" into the bulk,
behaving in a truly higher-dimensional way. This introduces significant changes
to gravitational dynamics and perturbations, with interesting and potentially
testable implications for high-energy astrophysics, black holes, and cosmology.
Brane-world models offer a phenomenological way to test some of the novel
predictions and corrections to general relativity that are implied by M theory.
This review analyzes the geometry, dynamics and perturbations of simple
brane-world models for cosmology and astrophysics, mainly focusing on warped
5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover
the simplest brane-world models in which 4-dimensional gravity on the brane is
modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati
models. Then we discuss co-dimension two branes in 6-dimensional models.Comment: A major update of Living Reviews in Relativity 7:7 (2004)
"Brane-World Gravity", 119 pages, 28 figures, the update contains new
material on RS perturbations, including full numerical solutions of
gravitational waves and scalar perturbations, on DGP models, and also on 6D
models. A published version in Living Reviews in Relativit
M(atrix) Theory: Matrix Quantum Mechanics as a Fundamental Theory
A self-contained review is given of the matrix model of M-theory. The
introductory part of the review is intended to be accessible to the general
reader. M-theory is an eleven-dimensional quantum theory of gravity which is
believed to underlie all superstring theories. This is the only candidate at
present for a theory of fundamental physics which reconciles gravity and
quantum field theory in a potentially realistic fashion. Evidence for the
existence of M-theory is still only circumstantial---no complete
background-independent formulation of the theory yet exists. Matrix theory was
first developed as a regularized theory of a supersymmetric quantum membrane.
More recently, the theory appeared in a different guise as the discrete
light-cone quantization of M-theory in flat space. These two approaches to
matrix theory are described in detail and compared. It is shown that matrix
theory is a well-defined quantum theory which reduces to a supersymmetric
theory of gravity at low energies. Although the fundamental degrees of freedom
of matrix theory are essentially pointlike, it is shown that higher-dimensional
fluctuating objects (branes) arise through the nonabelian structure of the
matrix degrees of freedom. The problem of formulating matrix theory in a
general space-time background is discussed, and the connections between matrix
theory and other related models are reviewed.Comment: 56 pages, 3 figures, LaTeX, revtex style; v2: references adde
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