101 research outputs found
Quantum inequalities in two dimensional curved spacetimes
We generalize a result of Vollick constraining the possible behaviors of the
renormalized expected stress-energy tensor of a free massless scalar field in
two dimensional spacetimes that are globally conformal to Minkowski spacetime.
Vollick derived a lower bound for the energy density measured by a static
observer in a static spacetime, averaged with respect to the observers proper
time by integrating against a smearing function. Here we extend the result to
arbitrary curves in non-static spacetimes. The proof, like Vollick's proof, is
based on conformal transformations and the use of our earlier optimal bound in
flat Minkowski spacetime. The existence of such a quantum inequality was
previously established by Fewster.Comment: revtex 4, 5 pages, no figures, submitted to Phys. Rev. D. Minor
correction
Can Inflating Braneworlds be Stabilized?
We investigate scalar perturbations from inflation in braneworld cosmologies
with extra dimensions. For this we calculate scalar metric fluctuations around
five dimensional warped geometry with four dimensional de Sitter slices. The
background metric is determined self-consistently by the (arbitrary) bulk
scalar field potential, supplemented by the boundary conditions at both
orbifold branes. Assuming that the inflating branes are stabilized (by the
brane scalar field potentials), we estimate the lowest eigenvalue of the scalar
fluctuations - the radion mass. In the limit of flat branes, we reproduce well
known estimates of the positive radion mass for stabilized branes.
Surprisingly, however, we found that for de Sitter (inflating) branes the
square of the radion mass is typically negative, which leads to a strong
tachyonic instability. Thus, parameters of stabilized inflating braneworlds
must be constrained to avoid this tachyonic instability. Instability of
"stabilized" de Sitter branes is confirmed by the BraneCode numerical
calculations in the accompanying paper hep-th/0309001. If the model's
parameters are such that the radion mass is smaller than the Hubble parameter,
we encounter a new mechanism of generation of primordial scalar fluctuations,
which have a scale free spectrum and acceptable amplitude.Comment: 7 pages, RevTeX 4.
The Sensitivity of Ligo to a Stochastic Background, and its Dependance on the Detector Orientations
We analyze the sensitivity of a network of interferometer gravitational-wave
detectors to the gravitational-wave stochastic background, and derive the
dependence of this sensitivity on the orientations of the detector arms. We
build on and extend the recent work of Christensen, but our conclusion for the
optimal choice of orientations of a pair of detectors differs from his. For a
pair of detectors (such as LIGO) that subtends an angle at the center of the
earth of \,\alt 70^\circ, we find that the optimal configuration is for each
detector to have its arms make an angle of (modulo ) with
the arc of the great circle that joins them. For detectors that are farther
separated, each detector should instead have one arm aligned with this arc. We
also describe in detail the optimal data-analysis algorithm for searching for
the stochastic background with a detector network, which is implicit in earlier
work of Michelson. The LIGO pair of detectors will be separated by . The minimum detectable stochastic energy-density for these
detectors with their currently planned orientations is greater than
what it would be if the orientations were optimal.Comment: 56 pages, 10 figures, Caltech preprint GRP-347, submitted to Phys Rev
D, uses revtex macro
Primordial perturbations in a non singular bouncing universe model
We construct a simple non singular cosmological model in which the currently
observed expansion phase was preceded by a contraction. This is achieved, in
the framework of pure general relativity, by means of a radiation fluid and a
free scalar field having negative energy. We calculate the power spectrum of
the scalar perturbations that are produced in such a bouncing model and find
that, under the assumption of initial vacuum state for the quantum field
associated with the hydrodynamical perturbation, this leads to a spectral index
n=-1. The matching conditions applying to this bouncing model are derived and
shown to be different from those in the case of a sharp transition. We find
that if our bounce transition can be smoothly connected to a slowly contracting
phase, then the resulting power spectrum will be scale invariant.Comment: 11 pages, RevTeX 4, 8 figures, submitted to Phys. Rev.
The Dynamics of Brane-World Cosmological Models
Brane-world cosmology is motivated by recent developments in string/M-theory
and offers a new perspective on the hierarchy problem. In the brane-world
scenario, our Universe is a four-dimensional subspace or {\em brane} embedded
in a higher-dimensional {\em bulk} spacetime. Ordinary matter fields are
confined to the brane while the gravitational field can also propagate in the
bulk, leading to modifications of Einstein's theory of general relativity at
high energies. In particular, the Randall-Sundrum-type models are
self-consistent and simple and allow for an investigation of the essential
non-linear gravitational dynamics. The governing field equations induced on the
brane differ from the general relativistic equations in that there are nonlocal
effects from the free gravitational field in the bulk, transmitted via the
projection of the bulk Weyl tensor, and the local quadratic energy-momentum
corrections, which are significant in the high-energy regime close to the
initial singularity. In this review we discuss the asymptotic dynamical
evolution of spatially homogeneous brane-world cosmological models containing
both a perfect fluid and a scalar field close to the initial singularity. Using
dynamical systems techniques it is found that, for models with a physically
relevant equation of state, an isotropic singularity is a past-attractor in all
orthogonal spatially homogeneous models (including Bianchi type IX models). In
addition, we describe the dynamics in a class of inhomogeneous brane-world
models, and show that these models also have an isotropic initial singularity.
These results provide support for the conjecture that typically the initial
cosmological singularity is isotropic in brane-world cosmology.Comment: Einstein Centennial Review Article: to appear in CJ
Mouse Chromosome 11
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46996/1/335_2004_Article_BF00648429.pd
Whole-genome sequencing reveals host factors underlying critical COVID-19
Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease
Effects of environmental parameters, leaf physiological properties and leaf water relations on leaf water delta18O enrichment in different Eucalyptus species
Stable oxygen isotope ratios (delta18O) have become a valuable tool in the plant and ecosystem sciences. The interpretation of delta18O values in plant material is, however, still complicated owing to the complex interactions among factors that influence leaf water enrichment. This study investigated the interplay among environmental parameters, leaf physiological properties and leaf water relations as drivers of the isotopic enrichment of leaf water across 17 Eucalyptus species growing in a common garden. We observed large differences in maximum daily leaf water delta18O across the 17 species. By fitting different leaf water models to these empirical data, we determined that differences in leaf water delta18O across species are largely explained by variation in the Péclet effect across species. Our analyses also revealed that species-specific differences in transpiration do not explain the observed differences in delta18O while the unconstrained fitting parameter 'effective path length' (L) was highly correlated with delta18O. None of the leaf morphological or leaf water related parameters we quantified in this study correlated with the L values we determined even though L was typically interpreted as a leaf morphological/anatomical property. A sensitivity analysis supported the importance of L for explaining the variability in leaf water delta18O across different species. Our investigation highlighted the importance of future studies to quantify the leaf properties that influence L. Obtaining such information will significantly improve our understanding of what ultimately determines the delta18O values of leaf water across different plant species
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