Dynamical Surface Gravity in Spherically Symmetric Black Hole Formation

We study dynamical surface gravity in a general spherically symmetric setting using Painlev\'{e}-Gullstrand (PG) coordinates. Our analysis includes several definitions that have been proposed in the past as well as two new definitions adapted to PG coordinates. Various properties are considered, including general covariance, value at extremality, locality and static limit. We illustrate with specific examples of "dirty" black holes that even for spacetimes possessing a global timelike Killing vector, local definitions of surface gravity can differ substantially from "non-local" ones that require an asymptotic normalization condition. Finally, we present numerical calculations of dynamical surface gravity for black hole formation via spherically symmetric scalar field collapse. Our results highlight the differences between the various definitions in a dynamical setting and provide further insight into the distinction between local and non-local definitions of surface gravity.Comment: Final version to appear in Phys. Rev. D. Slight name change, further improvements to numerics and presentation, 25 pages, 7 figure

Color Fields on the Light-Shell

We study the classical color radiation from very high energy collisions that produce colored particles. In the extreme high energy limit, the classical color fields are confined to a light-shell expanding at $c$ and are associated with a non-linear $\sigma$-model on the 2D light-shell with specific symmetry breaking terms. We argue that the quantum version of this picture exhibits asymptotic freedom and may be a useful starting point for an effective light-shell theory of the structure between the jets at a very high energy collider.Comment: 11 pages, no figure

Polymer quantization, singularity resolution and the 1/r^2 potential

We present a polymer quantization of the -lambda/r^2 potential on the positive real line and compute numerically the bound state eigenenergies in terms of the dimensionless coupling constant lambda. The singularity at the origin is handled in two ways: first, by regularizing the potential and adopting either symmetric or antisymmetric boundary conditions; second, by keeping the potential unregularized but allowing the singularity to be balanced by an antisymmetric boundary condition. The results are compared to the semiclassical limit of the polymer theory and to the conventional Schrodinger quantization on L_2(R_+). The various quantization schemes are in excellent agreement for the highly excited states but differ for the low-lying states, and the polymer spectrum is bounded below even when the Schrodinger spectrum is not. We find as expected that for the antisymmetric boundary condition the regularization of the potential is redundant: the polymer quantum theory is well defined even with the unregularized potential and the regularization of the potential does not significantly affect the spectrum.Comment: 21 pages, LaTeX including 7 figures. v2: analytic bounds improved; references adde

Many-nodes/many-links spinfoam: the homogeneous and isotropic case

I compute the Lorentzian EPRL/FK/KKL spinfoam vertex amplitude for regular graphs, with an arbitrary number of links and nodes, and coherent states peaked on a homogeneous and isotropic geometry. This form of the amplitude can be applied for example to a dipole with an arbitrary number of links or to the 4-simplex given by the compete graph on 5 nodes. All the resulting amplitudes have the same support, independently of the graph used, in the large j (large volume) limit. This implies that they all yield the Friedmann equation: I show this in the presence of the cosmological constant. This result indicates that in the semiclassical limit quantum corrections in spinfoam cosmology do not come from just refining the graph, but rather from relaxing the large j limit.Comment: 8 pages, 4 figure

Spinning Loop Black Holes

In this paper we construct four Kerr-like spacetimes starting from the loop black hole Schwarzschild solutions (LBH) and applying the Newman-Janis transformation. In previous papers the Schwarzschild LBH was obtained replacing the Ashtekar connection with holonomies on a particular graph in a minisuperspace approximation which describes the black hole interior. Starting from this solution, we use a Newman-Janis transformation and we specialize to two different and natural complexifications inspired from the complexifications of the Schwarzschild and Reissner-Nordstrom metrics. We show explicitly that the space-times obtained in this way are singularity free and thus there are no naked singularities. We show that the transformation move, if any, the causality violating regions of the Kerr metric far from r=0. We study the space-time structure with particular attention to the horizons shape. We conclude the paper with a discussion on a regular Reissner-Nordstrom black hole derived from the Schwarzschild LBH and then applying again the Newmann-Janis transformation.Comment: 18 pages, 18 figure

Physical boundary Hilbert space and volume operator in the Lorentzian new spin-foam theory

A covariant spin-foam formulation of quantum gravity has been recently developed, characterized by a kinematics which appears to match well the one of canonical loop quantum gravity. In this paper we reconsider the implementation of the constraints that defines the model. We define in a simple way the boundary Hilbert space of the theory, introducing a slight modification of the embedding of the SU(2) representations into the SL(2,C) ones. We then show directly that all constraints vanish on this space in a weak sense. The vanishing is exact (and not just in the large quantum number limit.) We also generalize the definition of the volume operator in the spinfoam model to the Lorentzian signature, and show that it matches the one of loop quantum gravity, as does in the Euclidean case.Comment: 11 page

The volume operator in covariant quantum gravity

A covariant spin-foam formulation of quantum gravity has been recently developed, characterized by a kinematics which appears to match well the one of canonical loop quantum gravity. In particular, the geometrical observable giving the area of a surface has been shown to be the same as the one in loop quantum gravity. Here we discuss the volume observable. We derive the volume operator in the covariant theory, and show that it matches the one of loop quantum gravity, as does the area. We also reconsider the implementation of the constraints that defines the model: we derive in a simple way the boundary Hilbert space of the theory from a suitable form of the classical constraints, and show directly that all constraints vanish weakly on this space.Comment: 10 pages. Version 2: proof extended to gamma > 1

Legumes for Organic Cropping Systems

A field experiment which focuses on the yield performance of both durum wheat and sunflower included in a twoyear rotation between wheat/subterranean clover and sunflower was carried out in an organic farm in Central Italy. Three factors were included in the experiment according to a factorial design: a) two different previous crop sequences (four years of alfalfa, four years of annual crops); b) three fertilization levels (without and with animal manure incorporated at 15 or 40 cm depth); c) three durum wheat genotypes (Appulo, Arcangelo and Daunia cvs). The results obtained show that a previous alfalfa meadow allows the succeeding crops (wheat and sunflower) to achieve an appropriate and stable grain yield (about 4 and 3 t ha-1, respectively) even without organic manure for two consecutive cycles. When the durum wheat follows an annual crop sequence, grain yield appears unstable and the different levels of organic fertilizer determine a negative effect in the first cycle and a positive effect in the second cycle. The reseeding capability of sunflower was reduced by animal manure treatments and, consequently, the subclover biomass and its nitrogen content were generally lower in fertilized plots. Anyway, subclover biomass, which was ploughed in after reseeding as a green manure for sunflower, proved to be effective in stabilizing sunflower grain yield, especially in plots without any organic fertilization

Alternative Cropping Systems with Self Reseeding Annual Legumes in a Mediterranean Environment

Self-reseeding winter annual legumes (subclover and snail medic) have the potential to induce a significant shift towards a less energy-intensive and more environmentally friendly management in the modern and specialized cereal cropping systems. Nevertheless their use is practically unknown in mixed stands with winter cereals, where they can contribute to supply nitrogen and to reduce herbicides input. We conceived an alternative cropping system where an annual legume performs, respectively, as living mulch in a winter cereal (winter wheat), as a cover crop after reseeding and as dry mulch for the succeeding summer cereal (corn). Trials carried out at the Tuscia University (Central Italy) in the period 1995-1997 showed that almost all the tested legumes cultivars were able both to grow sufficiently as a living mulch in the wheat and to provide a good re-establishment and an abundant mulch after the wheat harvest for the next corn. Trifolium yanninicum cv Trikkala and Trifolium subterraneum cvs Karridale and Mount Barker ranked first in reseeding capacity (up to 400 seedlings m-2) and mulch production (up to 5 t ha-1). No difference in grain yield between wheat with a living mulch and wheat in pure stand was found when legumes, such as Trifolium yanninicum cvs Trikkala and Larisa, Trifolium brachycalycinum cv Clare, Trifolium subterraneum cv Nungarin and Medicago scutellata cv Kelson, were grown in binary mixture with wheat. The aboveground biomass production of the succeeding irrigated corn crop was largely dependent on the amount of legume dry mulch left upon the ground

Self- Reseeding Forage Legumes as Green Manures in Mediterranean Cropping Systems

Self-regenerating winter annual legumes, like the Subterranean clover and the Medicago species, are recognised as typical Mediterranean resources able to contribute to more sustainable cropping systems such as cover crops, living mulches and green manures. Experimental evidence is presented on the capacity of several cultivars of subclover (Trifolium subterraneum, L.; Trifolium yanninicum, Katzn. & Morley and Trifolium brachycalycinum, Katzn. & Morley) and annual Medicago species to yield astonishing amounts of aboveground biomass (up to 15 t/ha by the cultivars of Medicago scutellata) with high nitrogen content. The total contribution of nitrogen to soil varies (100-300 kg/ha) with the age of the plant, but it is always possible to find cultivars of Medicago or Trifolium which as green manures are able to meet the nitrogen requirements of both early and late summer crops