U(N) tools for Loop Quantum Gravity: The Return of the Spinor

We explore the classical setting for the U(N) framework for SU(2) intertwiners for loop quantum gravity (LQG) and describe the corresponding phase space in terms of spinors with appropriate constraints. We show how its quantization leads back to the standard Hilbert space of intertwiner states defined as holomorphic functionals. We then explain how to glue these intertwiners states in order to construct spin network states as wave-functions on the spinor phase space. In particular, we translate the usual loop gravity holonomy observables to our classical framework. Finally, we propose how to derive our phase space structure from an action principle which induces non-trivial dynamics for the spin network states. We conclude by applying explicitly our framework to states living on the simple 2-vertex graph and discuss the properties of the resulting Hamiltonian.Comment: 23 page

Sonic analog of gravitational black holes in Bose-Einstein condensates

It is shown that, in dilute-gas Bose-Einstein condensates, there exist both dynamically stable and unstable configurations which, in the hydrodynamic limit, exhibit a behavior resembling that of gravitational black holes. The dynamical instabilities involve creation of quasiparticle pairs in positive and negative energy states, as in the well-known suggested mechanism for black hole evaporation. We propose a scheme to generate a stable sonic black hole in a ring trap.Comment: RevTeX 3.1, 1 figure, 4 page

A parsec-scale flow associated with the IRAS 16547-4247 radio jet

IRAS 16547-4247 is the most luminous (6.2 x 10^4 Lsun) embedded young stellar object known to harbor a thermal radio jet. We report the discovery using VLT-ISAAC of a chain of H_2 2.12 um emission knots that trace a collimated flow extending over 1.5 pc. The alignment of the H_2 flow and the central location of the radio jet implies that these phenomena are intimately linked. We have also detected using TIMMI2 an isolated, unresolved 12 um infrared source towards the radio jet . Our findings affirm that IRAS 16547-4247 is excited by a single O-type star that is driving a collimated jet. We argue that the accretion mechanism which produces jets in low-mass star formation also operates in the higher mass regime.Comment: Accepted for publication in ApJL, 10 pages, 2 figure

New tools for Loop Quantum Gravity with applications to a simple model

Loop Quantum Gravity is now a well established approach to quantum gravity. One of the main challenges still faced by the theory is constructing a consistent dynamics which would lead back to the standard dynamics of the gravitational field at large scales. Here we will review the recent U(N) framework for Loop Quantum Gravity and the new spinor representation (that provides a classical setting for the U(N) framework). Then, we will apply these techniques to a simple model in order to propose a dynamics for a symmetry reduced sector of the theory. Furthermore, we will explore certain analogies of this model with Loop Quantum Cosmology.Comment: 4 pages, to appear in Proceedings of Spanish Relativity Meeting 2011 (ERE 2011) held in Madrid, Spai

Monotonic Prefix Consistency in Distributed Systems

We study the issue of data consistency in distributed systems. Specifically, we consider a distributed system that replicates its data at multiple sites, which is prone to partitions, and which is assumed to be available (in the sense that queries are always eventually answered). In such a setting, strong consistency, where all replicas of the system apply synchronously every operation, is not possible to implement. However, many weaker consistency criteria that allow a greater number of behaviors than strong consistency, are implementable in available distributed systems. We focus on determining the strongest consistency criterion that can be implemented in a convergent and available distributed system that tolerates partitions. We focus on objects where the set of operations can be split into updates and queries. We show that no criterion stronger than Monotonic Prefix Consistency (MPC) can be implemented.Comment: Submitted pape

Updatable Blockchains

Software updates for blockchain systems become a real challenge when they impact the underlying consensus mechanism. The activation of such changes might jeopardize the integrity of the blockchain by resulting in chain splits. Moreover, the software update process should be handed over to the community and this means that the blockchain should support updates without relying on a trusted party. In this paper, we introduce the notion of updatable blockchains and show how to construct blockchains that satisfy this definition. Informally, an updatable blockchain is a secure blockchain and in addition it allows to update its protocol preserving the history of the chain. In this work, we focus only on the processes that allow securely switching from one blockchain protocol to another assuming that the blockchain protocols are correct. That is, we do not aim at providing a mechanism that allows reaching consensus on what is the code of the new blockchain protocol. We just assume that such a mechanism exists (like the one proposed in NDSS 2019 by Zhang et. al), and show how to securely go from the old protocol to the new one. The contribution of this paper can be summarized as follows. We provide the first formal definition of updatable ledgers and propose the description of two compilers. These compilers take a blockchain and turn it into an updatable blockchain. The first compiler requires the structure of the current and the updated blockchain to be very similar (only the structure of the blocks can be different) but it allows for an update process more simple, efficient. The second compiler that we propose is very generic (i.e., makes few assumptions on the similarities between the structure of the current blockchain and the update blockchain). The drawback of this compiler is that it requires the new blockchain to be resilient against a specific adversarial behaviour and requires all the honest parties to be online during the update process. However, we show how to get rid of the latest requirement (the honest parties being online during the update) in the case of proof-of-work and proof-of-stake ledgers

Submillimeter H2O masers in water-fountain nebulae

We report the first detection of submillimeter water maser emission toward water-fountain nebulae, which are post-AGB stars that exhibit high-velocity water masers. Using APEX we found emission in the ortho-H2O (10_29-9_36) transition at 321.226 GHz toward three sources: IRAS 15445-5449, IRAS 18043-2116 and IRAS 18286-0959. Similarly to the 22 GHz masers, the submillimeter water masers are expanding with a velocity larger than that of the OH masers, suggesting that these masers also originate in fast bipolar outflows. In IRAS 18043-2116 and IRAS 18286-0959, which figure among the sources with the fastest water masers, the velocity range of the 321 GHz masers coincides with that of the 22 GHz masers, indicating that they likely coexist. Towards IRAS 15445-5449 the submillimeter masers appear in a different velocity range, indicating that they are tracing different regions. The intensity of the submillimeter masers is comparable to that of the 22 GHz masers, implying that the kinetic temperature of the region where the masers originate should be Tk > 1000 K. We propose that the passage of two shocks through the same gas can create the conditions necessary to explain the presence of strong high-velocity 321 GHz masers coexisting with the 22 GHz masers in the same region.Comment: 4 pages, 1 figure. Accepted for publication in A&A Letter

High Angular Resolution Observations of the Collimated Jet Source Associated with a Massive Protostar in IRAS 16547-4247

A triple radio source recently detected in association with the luminous infrared source IRAS 16547-4247 has been studied with high angular resolution and high sensitivity with the Very Large Array at 3.6 and 2 cm. Our observations confirm the interpretation that the central object is a thermal radio jet, while the two outer lobes are most probably heavily obscured HH objects. The thermal radio jet is resolved angularly for the first time and found to align closely with the outer lobes. The opening angle of the thermal jet is estimated to be $\sim 25^\circ$, confirming that collimated outflows can also be present in massive protostars. The proper motions of the outer lobes should be measurable over timescales of a few years. Several fainter sources detected in the region are most probably associated with other stars in a young cluster.Comment: 9 pages, 2 figure