176 research outputs found
Kasner and Mixmaster behavior in universes with equation of state w \ge 1
We consider cosmological models with a scalar field with equation of state
that contract towards a big crunch singularity, as in recent cyclic
and ekpyrotic scenarios. We show that chaotic mixmaster oscillations due to
anisotropy and curvature are suppressed, and the contraction is described by a
homogeneous and isotropic Friedmann equation if . We generalize the
results to theories where the scalar field couples to p-forms and show that
there exists a finite value of , depending on the p-forms, such that chaotic
oscillations are suppressed. We show that orbifold compactification also
contributes to suppressing chaotic behavior. In particular, chaos is avoided in
contracting heterotic M-theory models if at the crunch.Comment: 25 pages, 2 figures, minor changes, references adde
Adiabatic invariants and Mixmaster catastrophes
We present a rigorous analysis of the role and uses of the adiabatic
invariant in the Mixmaster dynamical system. We propose a new invariant for the
global dynamics which in some respects has an improved behaviour over the
commonly used one. We illustrate its behaviour in a number of numerical
results. We also present a new formulation of the dynamics via Catastrophe
Theory. We find that the change from one era to the next corresponds to a fold
catastrophe, during the Kasner shifts the potential is an Implicit Function
Form whereas, as the anisotropy dissipates, the Mixmaster potential must become
a Morse 0--saddle. We compare and contrast our results to many known works on
the Mixmaster problem and indicate how extensions could be achieved. Further
exploitation of this formulation may lead to a clearer understanding of the
global Mixmaster dynamics.Comment: 24 pages, LaTeX, 5 figures (which can be obtained by sending a
message to the first author), submitted to Phys.Rev.
The Bianchi Ix (MIXMASTER) Cosmological Model is Not Integrable
The perturbation of an exact solution exhibits a movable transcendental
essential singularity, thus proving the nonintegrability. Then, all possible
exact particular solutions which may be written in closed form are isolated
with the perturbative Painlev\'e test; this proves the inexistence of any
vacuum solution other than the three known ones.Comment: 14 pages, no figure
Recommended from our members
Performance of a 500 watt Nd:GGG zigzag slab oscillator
Realization of practical multi-kilowatt Nd:garnet lasers will require the scale-up of crystal dimensions as well as more powerful pumping sources. A high average power zigzag slab crystal amplifier testing facility has been established at LLNL which employs two 100 kW{sub e} vortex stabilized arc lamps, cooled reflectors and a cooled, spectrally filtered, crystal slab mounting fixture. The operational characteristics of the first crystal laser to be tested in this setup, a Nd:GGG zigzag oscillator, are presented. A Nd:GGG crystal of dimensions 18 {times} 7 {times} 0.5 cm{sup 3}, doped at 2 {times} 10{sup 20} cm{sup {minus}3} Nd{sup 3+} atomic density, was pumped by up to 40 kW of filtered argon line emission. A small-signal single pass gain (losses excluded) of 1.09 was measured with a probe laser when the DC input to the lamps was 43 kW{sub e}. Our power supply was then modified to operate in a pulsed mode and provided one to three milliseconds pulses at 120 Hz. An average optical output power of 490 watts was obtained at a lamp input power of 93 kW{sub e} in an unoptimized resonator. The laser output power declined after a few tens of seconds since the slab tips were not properly cooled. A birdhouse specular lamp reflector and a contoured diffuse reflector were tested; in both cases the pump illuminated crystal surface was smaller than the total crystal face area. Fluorescence imaging of the zigzag amplifier's output aperture registered a smoother, more uniform pumping profile when the diffuse reflector was used. Uniformity of pumping results in decreased resonator loss and yields higher laser output power. Thermo-optic distortions observed in these preliminary tests are analyzed with the aid of computer simulations of the thermal fields, stresses, and surface displacements of our crystal slab. 3 refs., 12 figs
Earliest land plants created modern levels of atmospheric oxygen
The progressive oxygenation of the Earth’s atmosphere was pivotal
to the evolution of life, but the puzzle of when and how
atmospheric oxygen (O2) first approached modern levels (~21%)
remains unresolved. Redox proxy data indicate the deep oceans
were oxygenated during 435-392 Ma, and the appearance of
fossil charcoal indicates O2>15-17% by 420-400 Ma. However,
existing models have failed to predict oxygenation at this time.
Here we show that the earliest plants, which colonized the land
surface from ~470 Ma onwards, were responsible for this mid-
Paleozoic oxygenation event, through greatly increasing global
organic carbon burial – the net long-term source of O2. We use
a trait-based ecophysiological model to predict that cryptogamic
vegetation cover could have achieved ~30% of today’s global
terrestrial net primary productivity by~445 Ma. Data from modern
bryophytes suggests this plentiful early plant material had a much
higher molar C:P ratio (~2000) than marine biomass (~100), such
that a given weathering flux of phosphorus could support more
organic carbon burial. Furthermore, recent experiments suggest
that early plants selectively increased the flux of phosphorus (relative
to alkalinity) weathered from rocks. Combining these effects
in a model of long-term biogeochemical cycling, we reproduce a
sustained +2‰ increase in the carbonate carbon isotope (δ13C)
record by ~445 Ma, and predict a corresponding rise in O2 to
present levels by 420-400 Ma, consistent with geochemical data.
This oxygen rise represents a permanent shift in regulatory regime
to one where fire-mediated negative feedbacks on organic carbon
burial stabilise high O2 levels
Higher-Derivative Quantum Cosmology
The quantum cosmology of a higher-derivative derivative gravity theory
arising from the heterotic string effective action is reviewed. A new type of
Wheeler-DeWitt equation is obtained when the dilaton is coupled to the
quadratic curvature terms. Techniques for solving the Wheeler-DeWitt equation
with appropriate boundary conditions shall be described, and implications for
semiclassical theories of inflationary cosmology will be outlined.Comment: 11 pages TeX. A term has been removed from equation (13
Essential Constants for Spatially Homogeneous Ricci-flat manifolds of dimension 4+1
The present work considers (4+1)-dimensional spatially homogeneous vacuum
cosmological models. Exact solutions -- some already existing in the
literature, and others believed to be new -- are exhibited. Some of them are
the most general for the corresponding Lie group with which each homogeneous
slice is endowed, and some others are quite general. The characterization
``general'' is given based on the counting of the essential constants, the
line-element of each model must contain; indeed, this is the basic contribution
of the work. We give two different ways of calculating the number of essential
constants for the simply transitive spatially homogeneous (4+1)-dimensional
models. The first uses the initial value theorem; the second uses, through
Peano's theorem, the so-called time-dependent automorphism inducing
diffeomorphismsComment: 26 Pages, 2 Tables, latex2
Spacelike Singularities and Hidden Symmetries of Gravity
We review the intimate connection between (super-)gravity close to a
spacelike singularity (the "BKL-limit") and the theory of Lorentzian Kac-Moody
algebras. We show that in this limit the gravitational theory can be
reformulated in terms of billiard motion in a region of hyperbolic space,
revealing that the dynamics is completely determined by a (possibly infinite)
sequence of reflections, which are elements of a Lorentzian Coxeter group. Such
Coxeter groups are the Weyl groups of infinite-dimensional Kac-Moody algebras,
suggesting that these algebras yield symmetries of gravitational theories. Our
presentation is aimed to be a self-contained and comprehensive treatment of the
subject, with all the relevant mathematical background material introduced and
explained in detail. We also review attempts at making the infinite-dimensional
symmetries manifest, through the construction of a geodesic sigma model based
on a Lorentzian Kac-Moody algebra. An explicit example is provided for the case
of the hyperbolic algebra E10, which is conjectured to be an underlying
symmetry of M-theory. Illustrations of this conjecture are also discussed in
the context of cosmological solutions to eleven-dimensional supergravity.Comment: 228 pages. Typos corrected. References added. Subject index added.
Published versio
Differential Effect of Viable Versus Necrotic Neutrophils on Mycobacterium tuberculosis Growth and Cytokine Induction in Whole Blood.
Neutrophils exert both positive and negative influences on the host response to tuberculosis, but the mechanisms by which these differential effects are mediated are unknown. We studied the impact of live and dead neutrophils on the control of Mycobacterium tuberculosis using a whole blood bioluminescence-based assay, and assayed supernatant cytokine concentrations using Luminex™ technology and ELISA. CD15+ granulocyte depletion from blood prior to infection with M. tuberculosis-lux impaired control of mycobacteria by 96 h, with a greater effect than depletion of CD4+, CD8+, or CD14+ cells (p < 0.001). Augmentation of blood with viable granulocytes significantly improved control of mycobacteria by 96 h (p = 0.001), but augmentation with necrotic granulocytes had the opposite effect (p = 0.01). Both augmentations decreased supernatant concentrations of tumor necrosis factor and interleukin (IL)-12 p40/p70, but necrotic granulocyte augmentation also increased concentrations of IL-10, G-CSF, GM-CSF, and CCL2. Necrotic neutrophil augmentation reduced phagocytosis of FITC-labeled M. bovis BCG by all phagocytes, whereas viable neutrophil augmentation specifically reduced early uptake by CD14+ cells. The immunosuppressive effect of dead neutrophils required necrotic debris rather than supernatant. We conclude that viable neutrophils enhance control of M. tuberculosis in blood, but necrotic neutrophils have the opposite effect-the latter associated with induction of IL-10, growth factors, and chemoattractants. Our findings suggest a mechanism by which necrotic neutrophils may exert detrimental effects on the host response in active tuberculosis.Wellcome Trust (087754 to DL,
097684 to NB, 104803 and 203135 to RW, FC00110218 to
Francis Crick Institute); Cancer Research UK (FC00110218
to Francis Crick Institute); Medical Research Council UK
(FC00110218 to Francis Crick Institute); European Union
(FP7-HEALTH-F3-2012-305578 to RW); National Research
Foundation of South Africa (96841 to RW) and Barts Charity
(MGU0292)
- …