6,339 research outputs found
Simple de Sitter Solutions
We present a framework for de Sitter model building in type IIA string
theory, illustrated with specific examples. We find metastable dS minima of the
potential for moduli obtained from a compactification on a product of two Nil
three-manifolds (which have negative scalar curvature) combined with
orientifolds, branes, fractional Chern-Simons forms, and fluxes. As a discrete
quantum number is taken large, the curvature, field strengths, inverse volume,
and four dimensional string coupling become parametrically small, and the de
Sitter Hubble scale can be tuned parametrically smaller than the scales of the
moduli, KK, and winding mode masses. A subtle point in the construction is that
although the curvature remains consistently weak, the circle fibers of the
nilmanifolds become very small in this limit (though this is avoided in
illustrative solutions at modest values of the parameters). In the simplest
version of the construction, the heaviest moduli masses are parametrically of
the same order as the lightest KK and winding masses. However, we provide a
method for separating these marginally overlapping scales, and more generally
the underlying supersymmetry of the model protects against large corrections to
the low-energy moduli potential.Comment: 37 pages, harvmac big, 4 figures. v3: small correction
The GHOSTS survey. II. The diversity of Halo Color and Metallicity Profiles of Massive Disk Galaxies
We study the stellar halo color properties of six nearby massive highly
inclined disk galaxies using Hubble Space Telescope Advanced Camera for Surveys
and Wide Field Camera 3 observations in both F606W and F814W filters from the
GHOSTS survey. The observed fields, placed both along the minor and major axis
of each galaxy, probe the stellar outskirts out to projected distances of ~
50-70 kpc from their galactic centre along the minor axis. The 50% completeness
levels of the color magnitude diagrams are typically at two mag below the tip
of the red giant branch. We find that all galaxies have extended stellar halos
out to ~ 50 kpc and two out to ~ 70 kpc. We determined the halo color
distribution and color profile for each galaxy using the median colors of stars
in the RGB. Within each galaxy we find variations in the median colors as a
function of radius which likely indicates population variations, reflecting
that their outskirts were built from several small accreted objects. We find
that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear
negative color gradient, reflecting a declining metallicity in their halos; the
other have no significant color or population gradient. In addition,
notwithstanding the modest sample size of galaxies, there is no strong
correlation between their halo color/metallicity or gradient with galaxy's
properties such as rotational velocity or stellar mass. The diversity in halo
color profiles observed in the GHOSTS galaxies qualitatively supports the
predicted galaxy-to-galaxy scatter in halo stellar properties; a consequence of
the stochasticity inherent in the assembling history of galaxies.Comment: Accepted by MNRAS, 31 pages, 18 figures. Appendix added and some
editions to match accepted version. Conclusions unchange
A New Handle on de Sitter Compactifications
We construct a large new class of de Sitter (and anti de Sitter) vacua of
critical string theory from flux compactifications on products of Riemann
surfaces. In the construction, the leading effects stabilizing the moduli are
perturbative. We show that these effects self-consistently dominate over
standard estimates for further and quantum corrections, via
tuning available from large flux and brane quantum numbers.Comment: 26 pages, harvmac big. v2: Correction generalizing specific
ingredients required for tunable negative term; conclusions and structure of
potential unchange
Volume Stabilization via Corrections in Type IIB Theory with Fluxes
We consider the Type IIB string theory in the presence of various extra
-brane pairs compactified on a warped Calabi-Yau threefold that
admits a conifold singularity. We demonstrate that the volume modulus can be
stabilized perturbatively at a non-supersymmetric vacuum by the
effective potential that includes the stringy correction
obtained by Becker {\it et al.} together with a combination of positive tension
and anomalous negative tension terms generated by the additional
7-brane-antibrane pairs.Comment: 20 pages, 4 figures, parts of introduction and conclusions are
modifie
Inaccessible Singularities in Toral Cosmology
The familiar Bang/Crunch singularities of classical cosmology have recently
been augmented by new varieties: rips, sudden singularities, and so on. These
tend to be associated with final states. Here we consider an alternative
possibility for the initial state: a singularity which has the novel property
of being inaccessible to physically well-defined probes. These singularities
arise naturally in cosmologies with toral spatial sections.Comment: 10 pages, version to appear in Classical and Quantum Gravit
Testing String Theory with CMB
Future detection/non-detection of tensor modes from inflation in CMB
observations presents a unique way to test certain features of string theory.
Current limit on the ratio of tensor to scalar perturbations, r=T/S, is r <
0.3, future detection may take place for r > 10^{-2}-10^{-3}. At present all
known string theory inflation models predict tensor modes well below the level
of detection. Therefore a possible experimental discovery of tensor modes may
present a challenge to string cosmology.
The strongest bound on r in string inflation follows from the observation
that in most of the models based on the KKLT construction, the value of the
Hubble constant H during inflation must be smaller than the gravitino mass. For
the gravitino mass in the usual range, m_{3/2} < O(1) TeV, this leads to an
extremely strong bound r < 10^{-24}. A discovery of tensor perturbations with r
> 10^{-3} would imply that the gravitinos in this class of models are
superheavy, m_{3/2} > 10^{13} GeV. This would have important implications for
particle phenomenology based on string theory.Comment: 13 pages, 2 figure
Gaugino Condensation and Nonperturbative Superpotentials in Flux Compactifications
There are two known sources of nonperturbative superpotentials for K\"ahler
moduli in type IIB orientifolds, or F-theory compactifications on Calabi-Yau
fourfolds, with flux: Euclidean brane instantons and low-energy dynamics in D7
brane gauge theories. The first class of effects, Euclidean D3 branes which
lift in M-theory to M5 branes wrapping divisors of arithmetic genus 1 in the
fourfold, is relatively well understood. The second class has been less
explored. In this paper, we consider the explicit example of F-theory on with flux. The fluxes lift the D7 brane matter fields, and stabilize
stacks of D7 branes at loci of enhanced gauge symmetry. The resulting theories
exhibit gaugino condensation, and generate a nonperturbative superpotential for
K\"ahler moduli. We describe how the relevant geometries in general contain
cycles of arithmetic genus (and how divisors can
contribute to the superpotential, in the presence of flux). This second class
of effects is likely to be important in finding even larger classes of models
where the KKLT mechanism of moduli stabilization can be realized. We also
address various claims about the situation for IIB models with a single
K\"ahler modulus.Comment: 24 pages, harvmac, no figures, references adde
Excitotoxic Brain Injury Suppresses Striatal High-Affinity Glutamate Uptake in Perinatal Rats
In immature rodent brain, the glutamate receptor agonist N -methyl-D-aspartate (NMDA) is a potent neurotoxin. In postnatal day (PND)-7 rats, intrastriatal injection of 25 nmol of NMDA results in extensive ipsilateral forebrain injury. In this study, we examined alterations in high-affinity [ 3 H]glutamate uptake (HAGU) in NMDA-lesioned striatum. HAGU was assayed in synaptosomes, prepared from lesioned striatum, the corresponding contralateral striatum, or unlesioned controls. Twenty-four hours after NMDA injection (25 nmol), HAGU declined 44 ± 8% in lesioned tissue, compared with the contralateral striatum (mean ± SEM, n = 6 assays, p < 0.006, paired t test). Doses of 5–25 nmol of NMDA resulted in increasing suppression of HAGU (5 nmol, n = 3; 12.5 nmol, n = 3; and 25 nmol, n = 5 assays; p < 0.01, regression analysis). The temporal evolution of HAGU suppression was biphasic. There was an early transient suppression of HAGU (−28 ± 4% at 1 h; p < 0.03, analysis of variance, comparing changes at 0.5, 1, 2, and 3 h after lesioning); 1 or 5 days postinjury there was sustained loss of HAGU (at 5 days, −56 ± 11%, n = 3, p < 0.03, paired t test, lesioned versus contralateral striata). Treatment with the noncompetitive NMDA antagonist MK-801 (1 mg/kg i.p.) attenuated both the early and subsequent irreversible suppression of HAGU (1 h postlesion −28 ± 4%, n = 6 assays versus −12.6 ± 5% with MK-801, n = 4, p = 0.005; 24 h postlesion, −44 ± 8%, n = 5, versus +2.4 ± 6%, n = 3 with MK-801, p = 0.01, Wilcoxon ranked sum tests). In immature brain excitotoxic lesions produce an acute reversible suppression of HAGU, and a delayed long-lasting reduction in HAGU secondary to brain injury. These data suggest that accumulation of endogenous glutamate, as a consequence of the acute disruption of HAGU, could contribute to the pathogenesis of excitotoxic neuronal injury.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65556/1/j.1471-4159.1991.tb02011.x.pd
Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes
Plant genomes contain several hundred defensin-like (DEFL) genes that encode short cysteine-rich proteins resembling defensins, which are well known antimicrobial polypeptides. Little is known about the expression patterns or functions of many DEFLs because most were discovered recently and hence are not well represented on standard microarrays. We designed a custom Affymetrix chip consisting of probe sets for 317 and 684 DEFLs from Arabidopsis thaliana and Medicago truncatula, respectively for cataloging DEFL expression in a variety of plant organs at different developmental stages and during symbiotic and pathogenic associations. The microarray analysis provided evidence for the transcription of 71% and 90% of the DEFLs identified in Arabidopsis and Medicago, respectively, including many of the recently annotated DEFL genes that previously lacked expression information. Both model plants contain a subset of DEFLs specifically expressed in seeds or fruits. A few DEFLs, including some plant defensins, were significantly up-regulated in Arabidopsis leaves inoculated with Alternaria brassicicola or Pseudomonas syringae pathogens. Among these, some were dependent on jasmonic acid signaling or were associated with specific types of immune responses. There were notable differences in DEFL gene expression patterns between Arabidopsis and Medicago, as the majority of Arabidopsis DEFLs were expressed in inflorescences, while only a few exhibited root-enhanced expression. By contrast, Medicago DEFLs were most prominently expressed in nitrogen-fixing root nodules. Thus, our data document salient differences in DEFL temporal and spatial expression between Arabidopsis and Medicago, suggesting distinct signaling routes and distinct roles for these proteins in the two plant species
Competition of ferromagnetic and antiferromagnetic spin ordering in nuclear matter
In the framework of a Fermi liquid theory it is considered the possibility of
ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear
matter with Skyrme effective interaction. The zero temperature dependence of
ferromagnetic and antiferromagnetic spin polarization parameters as functions
of density is found for SkM, SGII effective forces. It is shown that in the
density domain, where both type of solutions of self--consistent equations
exist, ferromagnetic spin state is more preferable than antiferromagnetic one.Comment: 9p., 3 figure
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