Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology

Loop Quantum Cosmology yields two kinds of quantum corrections to the effective equations of motion for cosmological perturbations. Here we focus on the holonomy kind and we study the problem of the closure of the resulting algebra of constraints. Up to now, tensor, vector and scalar perturbations were studied independently, leading to different algebras of constraints. The structures of the related algebras were imposed by the requirement of anomaly freedom. In this article we show that the algebra can be modified by a very simple quantum correction, holding for all types of perturbations. This demonstrates the consistency of the theory and shows that lessons from the study of scalar perturbations should be taken into account when studying tensor modes. The Mukhanov-Sasaki equations of motion are similarly modified by a simple term.Comment: 5 page

Probing Loop Quantum Gravity with Evaporating Black Holes

This letter aims at showing that the observation of evaporating black holes should allow distinguishing between the usual Hawking behavior and Loop Quantum Gravity (LQG) expectations. We present a full Monte-Carlo simulation of the evaporation in LQG and statistical tests that discriminate between competing models. We conclude that contrarily to what was commonly thought, the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal quantum gravity footprints.Comment: 5 pages, 3 figures. Version accpeted by Phys. Rev. Let

Quantum gravity, space-time structure, and cosmology

A set of diverse but mutually consistent results obtained in different settings has spawned a new view of loop quantum gravity and its physical implications, based on the interplay of operator calculations and effective theory: Quantum corrections modify, but do not destroy, space-time and the notion of covariance. Potentially observable effects much more promising than those of higher-curvature effective actions result; loop quantum gravity has turned into a falsifiable framework, with interesting ingredients for new cosmic world views. At Planckian densities, space-time disappears and is replaced by 4-dimensional space without evolution.Comment: 8 pages, 7 figures, Plenary talk at CosGrav12, held at Indian Statistical Institute, Kolkat

A no-singularity scenario in loop quantum gravity

Canonical methods allow the derivation of effective gravitational actions from the behavior of space-time deformations reflecting general covariance. With quantum effects, the deformations and correspondingly the effective actions change, revealing dynamical implications of quantum corrections. A new systematic way of expanding these actions is introduced showing as a first result that inverse-triad corrections of loop quantum gravity simplify the asymptotic dynamics near a spacelike collapse singularity. By generic quantum effects, the singularity is removed.Comment: 10 page

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

Spherically symmetric Einstein-Maxwell theory and loop quantum gravity corrections

Effects of inverse triad corrections and (point) holonomy corrections, occuring in loop quantum gravity, are considered on the properties of Reissner-Nordstr\"om black holes. The version of inverse triad corrections with unmodified constraint algebra reveals the possibility of occurrence of three horizons (over a finite range of mass) and also shows a mass threshold beyond which the inner horizon disappears. For the version with modified constraint algebra, coordinate transformations are no longer a good symmetry. The covariance property of spacetime is regained by using a \emph{quantum} notion of mapping from phase space to spacetime. The resulting quantum effects in both versions of these corrections can be associated with renormalization of either mass, charge or wave function. In neither of the versions, Newton's constant is renormalized. (Point) Holonomy corrections are shown to preclude the undeformed version of constraint algebra as also a static solution, though time-independent solutions exist. A possible reason for difficulty in constructing a covariant metric for these corrections is highlighted. Furthermore, the deformed algebra with holonomy corrections is shown to imply signature change.Comment: 38 pages, 9 figures, matches published versio

Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology

Holonomy corrections to scalar perturbations are investigated in the loop quantum cosmology framework. Due to the effective approach, modifications of the algebra of constraints generically lead to anomalies. In order to remove those anomalies, counter-terms are introduced. We find a way to explicitly fulfill the conditions for anomaly freedom and we give explicit expressions for the counter-terms. Surprisingly, the "new quantization scheme" naturally arises in this procedure. The gauge invariant variables are found and equations of motion for the anomaly-free scalar perturbations are derived. Finally, some cosmological consequences are discussed qualitatively.Comment: 19 pages, 1 figure, v2, new comments and references added, minor correction

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

Dissolution of UK high-level waste glass under simulated hyperalkaline conditions of a colocated geological disposal facility

We report analysis of chemical durability of UK HLW MW+25% simulant glass under model hyperalkaline conditions of a colocated geological disposal facility. Glass powders and monoliths were dissolved for 168 days in saturated Ca(OH). Dissolution in the presence of high concentrations of Ca (>200 mg/L) was an order of magnitude lower than dissolution in water. Dissolution of Si did not occur until a Ca:Si ratio of <2 was achieved. The mechanism of dissolution involved the incorporation of Ca into the hydrated surface (initial, incubation regime), the precipitation of C-S-H phases, including a range of compositions in the C-(N)-(A)-S-H and M-S-H systems (intermediate regime), and the precipitation of C-S-H phases (the residual regime). Thermodynamic analysis and consideration of the CaO-SiO-HO phase diagram suggest that the rate-limiting step of glass dissolution in Ca-rich solutions is Ca-Si equilibrium, involving the precipitation of C-S-H phases, which change in chemical composition as a function of solution chemistry. In low SA/V ratio experiments, the dissolution progressed only to the initial incubation regime, resulting from fewer surface sites for Ca incorporation. Overall, these results suggest that Ca and Si in solution play an important role in the long-term durability of UK HLW in Ca-rich solutions. © 2013 The Authors. International Journal of Applied Glass Science published by John Wiley & Sons Ltd on behalf of The American Ceramic Society and Wiley Periodicals, Inc