1,900 research outputs found
Effective Actions for Heterotic M-Theory
We discuss the moduli space approximation for heterotic M-theory, both for
the minimal case of two boundary branes only, and when a bulk brane is
included. The resulting effective actions may be used to describe the
cosmological dynamics in the regime where the branes are moving slowly, away
from singularities. We make use of the recently derived colliding branes
solution to determine the global structure of moduli space, finding a boundary
at which the trajectories undergo a hard wall reflection. This has important
consequences for the allowed moduli space trajectories, and for the behaviour
of cosmological perturbations in the model.Comment: 21 pages, 4 figures. References added and some discussions clarifie
The Holographic Universe
We present a holographic description of four-dimensional single-scalar
inflationary universes in terms of a three-dimensional quantum field theory.
The holographic description correctly reproduces standard inflationary
predictions in their regime of applicability. In the opposite case, wherein
gravity is strongly coupled at early times, we propose a holographic
description in terms of perturbative QFT and present models capable of
satisfying the current observational constraints while exhibiting a
phenomenology distinct from standard inflation. This provides a qualitatively
new method for generating a nearly scale-invariant spectrum of primordial
cosmological perturbations.Comment: 20 pages, 5 figs; extended version of arXiv:0907.5542 including
background material and detailed derivations. To appear in Proceedings of 1st
Mediterranean Conference on Classical and Quantum Gravit
Universality of Cluster Dynamics
We have studied the kinetics of cluster formation for dynamical systems of
dimensions up to interacting through elastic collisions or coalescence.
These systems could serve as possible models for gas kinetics, polymerization
and self-assembly. In the case of elastic collisions, we found that the cluster
size probability distribution undergoes a phase transition at a critical time
which can be predicted from the average time between collisions. This enables
forecasting of rare events based on limited statistical sampling of the
collision dynamics over short time windows. The analysis was extended to
L-normed spaces () to allow for some amount of
interpenetration or volume exclusion. The results for the elastic collisions
are consistent with previously published low-dimensional results in that a
power law is observed for the empirical cluster size distribution at the
critical time. We found that the same power law also exists for all dimensions
, 2D L norms, and even for coalescing collisions in 2D. This
broad universality in behavior may be indicative of a more fundamental process
governing the growth of clusters
PII: S0966-842X(97)01185-2
Microsporidia are obligate intracellular parasites that infect a wide range of eukaryotes, causing severe diseases in immunocompromised humans and losses to apiaries, fisheries and silk farms. They have often been considered to be primitive eukaryotes; however, more recent evidence suggests they are more closely related to fungi
Colliding Branes in Heterotic M-theory
We study the collision of two flat, parallel end-of-the-world branes in
heterotic M-theory. By insisting that there is no divergence in the Riemann
curvature as the collision approaches, we are able to single out a unique
solution possessing the local geometry of (2d compactified Milne)/Z_2 x R_3,
times a finite-volume Calabi-Yau manifold in the vicinity of the collision. At
a finite time before and after the collision, a second type of singularity
appears momentarily on the negative-tension brane, representing its bouncing
off a zero of the bulk warp factor. We find this singularity to be remarkably
mild and easily regularised. The various different cosmological solutions to
heterotic M-theory previously found by other authors are shown to merely
represent different portions of a unique flat cosmological solution to
heterotic M-theory.Comment: 29 pp, 4 figs; Appendix B revised to include junction condition for
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Calcium negatively regulates secretion from dense granules in Toxoplasma gondii.
Apicomplexan parasites including Toxoplasma gondii and Plasmodium spp. manufacture a complex arsenal of secreted proteins used to interact with and manipulate their host environment. These proteins are organised into three principle exocytotic compartment types according to their functions: micronemes for extracellular attachment and motility, rhoptries for host cell penetration, and dense granules for subsequent manipulation of the host intracellular environment. The order and timing of these events during the parasite's invasion cycle dictates when exocytosis from each compartment occurs. Tight control of compartment secretion is, therefore, an integral part of apicomplexan biology. Control of microneme exocytosis is best understood, where cytosolic intermediate molecular messengers cGMP and Ca2+ act as positive signals. The mechanisms for controlling secretion from rhoptries and dense granules, however, are virtually unknown. Here, we present evidence that dense granule exocytosis is negatively regulated by cytosolic Ca2+ , and we show that this Ca2+ -mediated response is contingent on the function of calcium-dependent protein kinases TgCDPK1 and TgCDPK3. Reciprocal control of micronemes and dense granules provides an elegant solution to the mutually exclusive functions of these exocytotic compartments in parasite invasion cycles and further demonstrates the central role that Ca2+ signalling plays in the invasion biology of apicomplexan parasites.Medical Research Council
Australian Research Counci
Differential gene transfers and gene duplications in primary and secondary endosymbioses
BACKGROUND: Most genes introduced into phototrophic eukaryotes during the process of endosymbiosis are either lost or relocated into the host nuclear genome. In contrast, groEL homologues are found in different genome compartments among phototrophic eukaryotes. Comparative sequence analyses of recently available genome data, have allowed us to reconstruct the evolutionary history of these genes and propose a hypothesis that explains the unusual genome distribution of groEL homologues. RESULTS: Our analyses indicate that while two distinct groEL genes were introduced into eukaryotes by a progenitor of plastids, these particular homologues have not been maintained in all evolutionary lineages. This is of significant interest, because two chaperone proteins always co-occur in oxygenic photosynthetic organisms. We infer strikingly different lineage specific processes of evolution involving deletion, duplication and targeting of groEL proteins. CONCLUSION: The requirement of two groEL homologues for chaperon function in phototrophs has provided a constraint that has shaped convergent evolutionary scenarios in divergent evolutionary lineages. GroEL provides a general evolutionary model for studying gene transfers and convergent evolutionary processes among eukaryotic lineages
Many-worlds interpretation of quantum theory and mesoscopic anthropic principle
We suggest to combine the Anthropic Principle with Many-Worlds Interpretation
of Quantum Theory. Realizing the multiplicity of worlds it provides an
opportunity of explanation of some important events which are assumed to be
extremely improbable. The Mesoscopic Anthropic Principle suggested here is
aimed to explain appearance of such events which are necessary for emergence of
Life and Mind. It is complementary to Cosmological Anthropic Principle
explaining the fine tuning of fundamental constants. We briefly discuss various
possible applications of Mesoscopic Anthropic Principle including the Solar
Eclipses and assembling of complex molecules. Besides, we address the problem
of Time's Arrow in the framework of Many-World Interpretation. We suggest the
recipe for disentangling of quantities defined by fundamental physical laws and
by an anthropic selection.Comment: 11 page
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