101 research outputs found
Non-singular Ekpyrotic/Cyclic model in Loop Quantum Cosmology
We study the role of non-perturbative quantum gravity effects in the
Ekpyrotic/Cyclic model using the effective framework of loop quantum cosmology
in the presence of anisotropies. We show that quantum geometric modifications
to the dynamical equations near the Planck scale as understood in the
quantization of Bianchi-I spacetime in loop quantum cosmology lead to the
resolution of classical singularity and result in a non-singular transition of
the universe from the contracting to the expanding branch. In the Planck
regime, the universe undergoes multiple small bounces and the anisotropic shear
remains bounded throughout the evolution. A novel feature, which is absent for
isotropic models, is a natural turn around of the moduli field from the
negative region of the potential leading to a cyclic phenomena as envisioned in
the original paradigm. Our work suggests that incorporation of quantum
gravitational effects in the Ekpyrotic/Cyclic model may lead to a viable
scenario without any violation of the null energy condition.Comment: 24 pages, 11 figures. Additional numerical results discussed to show
robustness of non-singular bounce of the scale factor and turn-around of the
moduli field. References added. To appear in Physical Review
Anomaly-free vector perturbations with holonomy corrections in loop quantum cosmology
We investigate vector perturbations with holonomy corrections in the
framework of loop quantum cosmology. Conditions to achieve anomaly freedom for
these perturbations are found at all orders. This requires the introduction of
counter-terms in the hamiltonian constraint. We also show that anomaly freedom
requires the diffeomorphism constraint to hold its classical form when scalar
matter is added although the issue of a vector matter source, required for full
consistency, remains to be investigated. The gauge-invariant variable and the
corresponding equation of motion are derived. The propagation of vector modes
through the bounce is finally discussed.Comment: 16 pages, 1 figure. Matches version published in Class. Quantum Gra
Loop quantum gravity: the first twenty five years
This is a review paper invited by the journal "Classical ad Quantum Gravity"
for a "Cluster Issue" on approaches to quantum gravity. I give a synthetic
presentation of loop gravity. I spell-out the aims of the theory and compare
the results obtained with the initial hopes that motivated the early interest
in this research direction. I give my own perspective on the status of the
program and attempt of a critical evaluation of its successes and limits.Comment: 24 pages, 3 figure
Self-healing capacity of nuclear glass observed by NMR spectroscopy
Safe management of high level nuclear waste is a worldwide significant issue for which vitrification has been selected by many countries. There exists a crucial need for improving our understanding of the ageing of the glass under irradiation. While external irradiation by ions provides a rapid simulation of damage induced by alpha decays, short lived actinide doping is more representative of the reality. Here, we report radiological NMR experiments to compare the damage in International Simplified Glass (ISG) when irradiated by these two methods. In the 0.1 mole percent 244Cm doped glass, accumulation of high alpha decay only shows small modifications of the local structure, in sharp contrast to heavy ion irradiation. These results reveal the ability of the alpha particle to partially repair the damage generated by the heavy recoil nuclei highlighting the radiation resistance of nuclear glass and the difficulty to accurately simulate its behaviour by single ion beam irradiations
Singular fate of the universe in modified theories of gravity
In this paper we study the final fate of the universe in modified theories of
gravity. As compared with general relativistic formulations, in these scenarios
the Friedmann equation has additional terms which are relevant for low density
epochs. We analyze the sort of future singularities to be found under the usual
assumption the expanding Universe is solely filled with a pressureless
component. We report our results using two schemes: one concerned with the
behavior of curvature scalars, and a more refined one linked to observers. Some
examples with a very solid theoretical motivation and some others with a more
phenomenological nature are used for illustration.Comment: 6 pages, final version, accepted for publication in Physics Letters
B, references added, revte
Corrosion of the International Simple Glass under acidic to hyperalkaline conditions
Assessment of glass dissolution kinetics, under disposal relevant temperature and pH environments, is required to credibly estimate radionuclide release rates from vitrified radioactive waste. Leaching of the International Simple Glass (ISG) under acidic to hyperalkaline conditions was examined. Forward rate measurements have been obtained using the dynamic leaching SPFT protocol and rate parameters for B, Na and Si in the basic regime; errors in rates predicted using these parameters at high pH and temperature are significant because the fitting uses logarithmic data. Longer term behaviour under hyperalkaline conditions, representative of some disposal environments, was investigated using the PCT and MCC-1 static leaching protocols with Ca(OH)2 solutions for up to 120 days (PCT) and 720 days (MCC-1). In hyperalkaline conditions dissolution was incongruent for all elements and the presence of alternating zirconia-rich and zirconia-poor alteration layers was observed on all leached monoliths, indicating the occurrence of a self-organisation phenomenon during leaching
Relationship between cell cycle and diosgenin-induced megakaryocytic differentiation of HEL cells using sedimentation field-flow fractionation
International audienc
Laminin-binding integrins induce Dll4 expression and Notch signaling in endothelial cells.
Rationale: Integrins play a crucial role in controlling endothelial cell proliferation and migration during angiogenesis. The Delta-like 4 (Dll4)/Notch pathway establishes an adequate ratio between stalk and tip cell populations by restricting tip cell formation through "lateral inhibition" in response to a vascular endothelial growth factor gradient. Because angiogenesis requires a tight coordination of these cellular processes, we hypothesized that adhesion, vascular endothelial growth factor, and Notch signaling pathways are interconnected. Objective: This study was aimed at characterizing the cross-talk between integrin and Notch signaling in endothelial cells. Methods and results: Adhesion of primary human endothelial cells to laminin-111 triggers Dll4 expression, leading to subsequent Notch pathway activation. SiRNA-mediated knockdown of α2β1 and α6β1 integrins abolishes Dll4 induction, which discloses a selective integrin signaling acting upstream of Notch pathway. The increase in Foxc2 transcription, triggered by α2β1 binding to laminin, is required but not sufficient per se for Dll4 expression. Furthermore, vascular endothelial growth factor stimulates laminin γ1 deposition, which leads to integrin signaling and Dll4 induction. Interestingly, loss of integrins α2 or α6 mimics the effects of Dll4 silencing and induces excessive network branching in an in vitro sprouting angiogenesis assay on three-dimensional matrigel. Conclusions: We show that, in endothelial cells, ligation of α2β1 and α6β1 integrins induces the Notch pathway, and we disclose a novel role of basement membrane proteins in the processes controlling tip vs stalk cell selection
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