194 research outputs found
Birkhoff's theorem in the f(T) gravity
Generalized from the so-called teleparallel gravity which is exactly
equivalent to general relativity, the gravity has been proposed as an
alternative gravity model to account for the dark energy phenomena. In this
letter we prove that the external vacuum gravitational field for a spherically
symmetric distribution of source matter in the gravity framework must be
static and the conclusion is independent of the radial distribution and
spherically symmetric motion of the source matter that is, whether it is in
motion or static. As a consequence, the Birkhoff's theorem is valid in the
general theory. We also discuss its application in the de Sitter
space-time evolution phase as preferred to by the nowadays dark energy
observations.Comment: 5p
Extended Birkhoff's Theorem in the f(T) Gravity
The f(T) theory, a generally modified teleparallel gravity, has been proposed
as an alternative gravity model to account for the dark energy phenomena.
Following our previous work [Xin-he Meng and Ying-bin Wang, EPJC(2011),
arXiv:1107.0629v1], we prove that the Birkhoff's theorem holds in a more
general context, specifically with the off diagonal tetrad case, in this
communication letter. Then, we discuss respectively the results of the external
vacuum and internal gravitational field in the f(T) gravity framework, as well
as the extended meaning of this theorem. We also investigate the validity of
the Birkhoff's theorem in the frame of f(T) gravity via conformal
transformation by regarding the Brans-Dicke-like scalar as effective matter,
and study the equivalence between both Einstein frame and Jordan frame.Comment: 7 pages, 1 figure, submitted to EPJ-C. arXiv admin note: substantial
text overlap with arXiv:1107.062
Birkhoff's Theorem in f(T) Gravity up to the Perturbative Order
f(T) gravity, a generally modified teleparallel gravity, has become very
popular in recent times as it is able to reproduce the unification of inflation
and late-time acceleration without the need of a dark energy component or an
inflation field. In this present work, we investigate specifically the range of
validity of Birkhoff's theorem with the general tetrad field via perturbative
approach. At zero order, Birkhoff's theorem is valid and the solution is the
well known Schwarzschild-(A)dS metric. Then considering the special case of the
diagonal tetrad field, we present a new spherically symmetric solution in the
frame of f(T) gravity up to the perturbative order. The results with the
diagonal tetrad field satisfy the physical equivalence between the Jordan and
the so-called Einstein frames, which are realized via conformal transformation,
at least up to the first perturbative order.Comment: 8 pages, no figure. Final version, accepted for publication in EPJ
The role of extracellular matrix on liver stem cell fate: A dynamic relationship in health and disease
The liver stem cell niche is a specialized and dynamic microenvironment with biomechanical and biochemical characteristics that regulate stem cell behavior. This is feasible due to the coordination of a complex network of secreted factors, small molecules, neural, blood inputs and extracellular matrix (ECM) components involved in the regulation of stem cell fate (self-renewal, survival, and differentiation into more mature phenotypes like hepatocytes and cholangiocytes). In this review, we describe and summarize all the major components that play essential roles in the liver stem cell niche, in particular, growth factor signaling and the biomechanical properties of the ECM
QCD ghost f(T)-gravity model
Within the framework of modified teleparallel gravity, we reconstruct a f(T)
model corresponding to the QCD ghost dark energy scenario. For a spatially flat
FRW universe containing only the pressureless matter, we obtain the time
evolution of the torsion scalar T (or the Hubble parameter). Then, we calculate
the effective torsion equation of state parameter of the QCD ghost f(T)-gravity
model as well as the deceleration parameter of the universe. Furthermore, we
fit the model parameters by using the latest observational data including
SNeIa, CMB and BAO data. We also check the viability of our model using a
cosmographic analysis approach. Moreover, we investigate the validity of the
generalized second law (GSL) of gravitational thermodynamics for our model.
Finally, we point out the growth rate of matter density perturbation. We
conclude that in QCD ghost f(T)-gravity model, the universe begins a matter
dominated phase and approaches a de Sitter regime at late times, as expected.
Also this model is consistent with current data, passes the cosmographic test,
satisfies the GSL and fits the data of the growth factor well as the LCDM
model.Comment: 19 pages, 9 figures, 2 tables. arXiv admin note: substantial text
overlap with arXiv:1111.726
Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel
Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure. <br /
The AgriRover : a reinvented mechatronic platform from space robotics for precision farming
This paper presents an investigation of a novel development of a multi-functional mobile platform for agriculture applications. This is achieved through a reinven-tion process of a mechatronic design by spinning off space robotic technologies in terrestrial applications in the AgriRover project. The AgriRover prototype is the first of its kind in exploiting and applying space robotic technologies in precision farming. To optimize energy consumption of the mobile platform, a new dynamic total cost of transport algorithm is proposed and validated. An autonomous navi-gation system has been developed to enable the AgriRover to operate safely in unstructured farming environments. An object recognition algorithm specific to agriculture- has been investigated and implemented. A novel soil sample collect-ing mechanism has been designed and prototyped for on-board and in-situ soil quality measurement. The design of the whole system has benefited from the use of a mechatronic design process known as the Tiv model through which a plane-tary exploration rover is reinvented into the AgriRover for agricultural applica-tions. The AgriRover system has gone through three sets of field trials in the UK and some of these results are reported
Loss of the SxxSS Motif in a Human T-Cell Factor-4 Isoform Confers Hypoxia Resistance to Liver Cancer: An Oncogenic Switch in Wnt Signaling
PURPOSE: Aberrantly activated Wnt/β-catenin signaling is important in hepatocellular carcinoma (HCC) development. Downstream gene expressions involving the Wnt/β-catenin cascade occur through T-cell factor (TCF) proteins. Here, we show the oncogenic potential of human TCF-4 isoforms based on the expression of a single conserved SxxSS motif. METHODS: We investigated the TCF-4J and K isoform pair characterized by the presence (K) or absence (J) of the SxxSS motif. The mRNA expression profiles were examined in 47 pairs of human HCCs and adjacent non-cancerous liver tissues by RT-PCR. Proliferation, sphere assays and immunoblot analysis were performed under normoxia and hypoxia conditions. The ability of HCC cells overexpressing TCF-4J (J cells) and K (K cells) to grow as solid tumors in nude mice was explored. RESULTS: TCF-4J expression was significantly upregulated in HCC tumors compared to corresponding peritumor and normal liver and was preferentially expressed in poorly differentiated HCCs. In contrast, TCF-4K was downregulated in those same HCC tumors. TCF-4J-overexpressing HCC cells (J cells) revealed a survival advantage under hypoxic conditions, high proliferation rate and formation of aggregates/spheres compared to overexpression of TCF-4K (K cells). The hypoxic J cells had high expression levels of HIF-2α and EGFR as possible mechanisms to promote tumorigenesis. Increased stability of HIF-2α under hypoxia in J cells was associated with a decreased level of von Hippel-Lindau (VHL) protein, a known E3 ligase for HIF-αs. In a xenograft model, the J cells rapidly developed tumors compared to K cells. Tumor tissues derived from J cells exhibited high expression levels of HIF-2α and EGFR compared to the slow developing and small K cell derived tumors. CONCLUSIONS: Our results suggest that the specific TCF-4J isoform, which lacks a regulatory SxxSS motif, has robust tumor-initiating potential under hypoxic conditions
Cosmic acceleration and phantom crossing in -gravity
In this paper, we propose two new models in gravity to realize
universe acceleration and phantom crossing due to dark torsion in the
formalism. The model parameters are constrained and the observational test are
discussed. The best fit results favors an accelerating universe with possible
phantom crossing in the near past or future followed respectively by matter and
radiation dominated era.Comment: 20 pages, 18 figures, Will appear in Astrophys Space Sc
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