Relativistic gravitational collapse in comoving coordinates: The post-quasistatic approximation

A general iterative method proposed some years ago for the description of relativistic collapse, is presented here in comoving coordinates. For doing that we redefine the basic concepts required for the implementation of the method for comoving coordinates. In particular the definition of the post-quasistatic approximation in comoving coordinates is given. We write the field equations, the boundary conditions and a set of ordinary differential equations (the surface equations) which play a fundamental role in the algorithm. As an illustration of the method, we show how to build up a model inspired in the well known Schwarzschild interior solution. Both, the adiabatic and non adiabatic, cases are considered.Comment: 14 pages, 11 figures; updated version to appear in Int. J. Modern Phys.

The Post-Quasistatic Approximation as a test bed for Numerical Relativity

It is shown that observers in the standard ADM 3+1 treatment of matter are the same as the observers used in the matter treatment of Bondi: they are comoving and local Minkowskian. Bondi's observers are the basis of the post--quasitatic approximation (PQSA) to study a contracting distribution of matter. This correspondence suggests the possibility of using the PQSA as a test bed for Numerical Relativity. The treatment of matter by the PQSA and its connection with the ADM 3+1 treatment are presented, for its practical use as a calibration tool and as a test bed for numerical relativistic hydrodynamic codes.Comment: 4 pages; to appear as a Brief Report in Physical Review

Assessment of Alternative Hydrogen Pathways: Natural Gas and Biomass

Achieving large-scale changes to develop a sustained hydrogen economy requires a large amount of planning and cooperation at national and international levels alike. ECS developed a long-term hydrogen-based scenario (B1-H2) of the global energy system to examine the future perspectives of fuel cells (Barreto et al., 2002). That earlier study, done with the collaboration and support of the Tokyo Electric Power Company (TEPCO), illustrated the key role of hydrogen towards a clean and sustainable energy future. In an affluent, low-population-growth, equity and sustainability-oriented world, hydrogen technologies experience substantial but plausible performance and costs improvements and diffuse extensively. Fuel cells and other hydrogen-using technologies play a major role in a transformation towards a more flexible, less vulnerable, distributed energy system that meets energy needs in a cleaner, more efficient and cost-effective way. This profound structural transformation of the global energy system brings substantial improvements in energy intensity and an accelerated decarbonizaton of the energy mix, resulting in relatively low climate impacts. In order to understand the future potential of hydrogen, in this report we compare the two main hydrogen production alternatives from natural gas and biomass as identified in the above-mentioned (B1-H2) scenario in more detail. The first alternative, steam reforming of natural gas, is a well-established technology and the most common and current method to produce hydrogen (Ogden, 1999a). The second technology, biomass gasification, is still in its infancy. A small number of demonstration facilities are in place. Many issues still have to be addressed before the technology can be expected to reach an adequate technical performance and hence become economically competitive (Milne et al., 2002). Nevertheless, biomass-based systems are a very promising option for ensuring the sustainability of a future hydrogen-supply system. The report includes a comparative analysis of both systems and their potential for carbon mitigation via CO2 capture and sequestration. Estimates of the hydrogen costs for alternative production chains are presented, and the competitiveness of the systems under alternative CO2 taxes are analyzed. Both technologies appear as economically attractive and environmentally compatible options for shaping a sustainable hydrogen economy and contributing to the mitigation of greenhouse gas emissions in the long term

A Framework for Efficient Adaptively Secure Composable Oblivious Transfer in the ROM

Oblivious Transfer (OT) is a fundamental cryptographic protocol that finds a number of applications, in particular, as an essential building block for two-party and multi-party computation. We construct a round-optimal (2 rounds) universally composable (UC) protocol for oblivious transfer secure against active adaptive adversaries from any OW-CPA secure public-key encryption scheme with certain properties in the random oracle model (ROM). In terms of computation, our protocol only requires the generation of a public/secret-key pair, two encryption operations and one decryption operation, apart from a few calls to the random oracle. In~terms of communication, our protocol only requires the transfer of one public-key, two ciphertexts, and three binary strings of roughly the same size as the message. Next, we show how to instantiate our construction under the low noise LPN, McEliece, QC-MDPC, LWE, and CDH assumptions. Our instantiations based on the low noise LPN, McEliece, and QC-MDPC assumptions are the first UC-secure OT protocols based on coding assumptions to achieve: 1) adaptive security, 2) optimal round complexity, 3) low communication and computational complexities. Previous results in this setting only achieved static security and used costly cut-and-choose techniques.Our instantiation based on CDH achieves adaptive security at the small cost of communicating only two more group elements as compared to the gap-DH based Simplest OT protocol of Chou and Orlandi (Latincrypt 15), which only achieves static security in the ROM