21,833 research outputs found
Quasi-Rip: A New Type of Rip Model without Cosmic Doomsday
The fate of our universe is an unceasing topic of cosmology and the human
being. The discovery of the current accelerated expansion of the universe
significantly changed our view of the fate of the universe. Recently, some
interesting scenarios concerning the fate of the universe attracted much
attention in the community, namely the so-called "Little Rip" and "Pseudo-Rip".
It is worth noting that all the Big Rip, Little Rip and Pseudo-Rip arise from
the assumption that the dark energy density is monotonically
increasing. In the present work, we are interested to investigate what will
happen if this assumption is broken, and then propose a so-called "Quasi-Rip"
scenario, which is driven by a type of quintom dark energy. In this work, we
consider an explicit model of Quasi-Rip in detail. We show that Quasi-Rip has
an unique feature different from Big Rip, Little Rip and Pseudo-Rip. Our
universe has a chance to be rebuilt from the ashes after the terrible rip. This
might be the last hope in the "hopeless" rip.Comment: 9 pages, 2 figures, 1 table, revtex4; v2: discussions added, Phys.
Rev. D in press; v3: published versio
Made-to-measure galaxy models - III Modelling with Milky Way observations
We demonstrate how the Syer & Tremaine made-to-measure method of stellar
dynamical modelling can be adapted to model a rotating galactic bar. We
validate our made-to-measure changes using observations constructed from the
existing Shen et al. (2010) N-body model of the Milky Way bar, together with
kinematic observations of the Milky Way bulge and bar taken by the Bulge Radial
Velocity Assay (BRAVA). Our results for a combined determination of the bar
angle and bar pattern speed (~30 degrees and ~40 km/s/kpc) are consistent with
those determined by the N-body model. Whilst the made-to-measure techniques we
have developed are applied using a particular N-body model and observational
data set, they are in fact general and could be applied to other Milky Way
modelling scenarios utilising different N-body models and data sets.
Additionally, we use the exercise as a vehicle for illustrating how N-body and
made-to-measure methods might be combined into a more effective method.Comment: Accepted for publication, 10 pages, 7 figure
Maximum Estrada Index of Bicyclic Graphs
Let be a simple graph of order , let
be the eigenvalues of the
adjacency matrix of . The Esrada index of is defined as
. In this paper we determine the unique
graph with maximum Estrada index among bicyclic graphs with fixed order
Decay constants of the pseudoscalar mesons in the framework of the coupled Schwinger-Dyson equation and Bethe-Salpeter equation
In this article, we investigate the structures of the pseudoscalar mesons
(, , , , and ) in the framework of the coupled rainbow
Schwinger-Dyson equation and ladder
Bethe-Salpeter equation with the confining effective potential (infrared
modified flat bottom potential).
The Schwinger-Dyson functions for the , and quarks are greatly
renormalized at small momentum region and the curves are steep at about
which indicates an explicitly dynamical symmetry breaking. The
Euclidean time fourier transformed quark propagators have no mass poles in the
time-like region which naturally implements confinement. As for the and
quarks, the current masses are very large, the renormalization are more tender,
however, mass poles in the time-like region are also absent. The Bethe-Salpeter
wavefunctions for those mesons have the same type (Gaussian type) momentum
dependence and center around small momentum which indicate that the bound
states exist in the infrared region. The decay constants for those pseudoscalar
mesons are compatible with the values of experimental extractions and
theoretical calculations, such as lattice simulations and QCD sum rules.Comment: 10 pages, 4 figures, Revised Versio
Photosynthesis across African cassava germplasm is limited by Rubisco and mesophyll conductance at steady-state, but by stomatal conductance in fluctuating light
SubâSaharan Africa is projected to see a 55% increase in food demand by 2035, where cassava (Manihot esculenta) is the most planted crop and a major calorie source. Cassava yield has not increased significantly for 13 years. Improvement of genetic yield potential, the basis of the first Green Revolution, could be increased by improving photosynthetic efficiency. First, the factors limiting photosynthesis and their genetic variability within extant germplasm must be understood. Biochemical and diffusive limitations to leaf photosynthetic CO2 uptake under steadyâstate and fluctuating light in thirteen farmâpreferred and highâyielding African cultivars were analyzed. A cassava leaf metabolic model was developed to quantify the value of overcoming limitations to leaf photosynthesis. At steadyâstate, in vivo Rubisco activity and mesophyll conductance accounted for 84% of the limitation whereas under nonâsteadyâstate conditions of shade to sun transition stomatal conductance was the major limitation contributing resulting in an estimated 13% and 5% losses in CO2 uptake and water use efficiency, across a diurnal period. Triose phosphate utilization, while sufficient to support observed rates, would limit improvement in leaf photosynthesis to 33%, unless improved itself. The variation of carbon assimilation among cultivars were three times greater under nonâsteadyâstate compared to steadyâstate, pinpointing important overlooked breeding targets for improved photosynthetic efficiency in cassava
Pseudo-supersymmetry, Consistent Sphere Reduction and Killing Spinors for the Bosonic String
Certain supergravity theories admit a remarkable consistent dimensional
reduction in which the internal space is a sphere. Examples include type IIB
supergravity reduced on S^5, and eleven-dimensional supergravity reduced on S^4
or S^7. Consistency means that any solution of the dimensionally-reduced theory
lifts to give a solution in the higher dimension. Although supersymmetry seems
to play a role in the consistency of these reductions, it cannot be the whole
story since consistent sphere reductions of non-supersymmetric theories are
also known, such as the reduction of the effective action of the bosonic string
in any dimension D on either a 3-sphere or a (D-3)-sphere, retaining the gauge
bosons of SO(4) or SO(D-2) respectively. We show that although there is no
supersymmetry, there is nevertheless a natural Killing spinor equation for the
D-dimensional bosonic string. A projection of the full integrability condition
for these Killing spinors gives rise to the bosonic equations of motion (just
as happens in the supergravity examples). Thus it appears that by extending the
notion of supersymmetry to "pseudo-supersymmetry" in this way, one may be able
to obtain a broader understanding of a relation between Killing spinors and
consistent sphere reductions.Comment: Latex, 15 page
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