Asymptotic U(1)U(1) charges at spatial infinity

Large gauge symmetries in Minkowski spacetime are often studied in two distinct regimes: either at asymptotic (past or future) times or at spatial infinity. By working in harmonic gauge, we provide a unified description of large gauge symmetries (and their associated charges) that applies to both regimes. At spatial infinity the charges are conserved and interpolate between those defined at the asymptotic past and future. This explains the equality of asymptotic past and future charges, as recently proposed in connection with Weinberg's soft photon theorem.Comment: 23 page

Government Spending and the Real Exchange Rate: a Cross - Country Perspective

In this paper we study, from an empirical point of view, the determinants of the real exchange rate (RER). Relative to the vast previous literature on this topic we aim to distinguish the impact of two important components of government expenditure—public investment and transfers—on the RER, which has usually been neglected. Using panel cointegration techniques, we assess the relevance of those variables in the determination of the RER for a wide set of countries from 1980 to 2009. Our results suggest that changes in either government transfers or public investment have an impact on the RER in emerging economies. On one hand, transfers tend to appreciate the RER because they induce an increase in the relative demand for nontraded goods. On the other, an increase in public investment generates an RER depreciation. This result can be explained by the fact that, in this case, there is an increase in the relative productivity in the nontraded sector of the economy. We also study the effect of countries’ net external assets position on the RER and find that it differs markedly between developed and developing countries: this variable has a significant effect only in the case of developing economies.

Dynamical friction in slab geometries and accretion disks

The evolution of planets, stars and even galaxies is driven, to a large extent, by dynamical friction of gravitational origin. There is now a good understanding of the friction produced by extended media, either collisionless of fluid-like. However, the physics of accretion or protoplanetary disks, for instance, is described by slab-like geometries instead, compact in one spatial direction. Here, we find, for the first time, the gravitational wake due to a massive perturber moving through a slab-like medium, describing e.g. accretion disks with sharp transitions. We show that dynamical friction in such environments can be substantially reduced relatively to spatially extended profiles. Finally, we provide simple and accurate expressions for the gravitational drag force felt by the perturber, in both the subsonic and supersonic regime.Comment: 10 pages, 8 figure

Generalized Paxos Made Byzantine (and Less Complex)

One of the most recent members of the Paxos family of protocols is Generalized Paxos. This variant of Paxos has the characteristic that it departs from the original specification of consensus, allowing for a weaker safety condition where different processes can have a different views on a sequence being agreed upon. However, much like the original Paxos counterpart, Generalized Paxos does not have a simple implementation. Furthermore, with the recent practical adoption of Byzantine fault tolerant protocols, it is timely and important to understand how Generalized Paxos can be implemented in the Byzantine model. In this paper, we make two main contributions. First, we provide a description of Generalized Paxos that is easier to understand, based on a simpler specification and the pseudocode for a solution that can be readily implemented. Second, we extend the protocol to the Byzantine fault model