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