Lords of Finance

I recently read "Lords of Finance: The Bankers Who Broke the World" by Liaquat Ahamed. It was about the central bankers who controlled international finance in the period leading up to the Great Depression. Well-written and (perhaps surprisingly) witty and entertaining, given that this might seem to be a rather dry subject.

Very High Energy Cosmic Rays

A Faraway Quasar in the Direction of the Highest Energy Auger Event

The highest energy cosmic ray event reported by the Auger Observatory has an energy of 148 EeV. It does not correlate with any nearby (z$<$0.024) object capable of originating such a high energy event. Intrigued by the fact that the highest energy event ever recorded (by the Fly's Eye collaboration) points to a faraway quasar with very high radio luminosity and large Faraday rotation measurement, we have searched for a similar source for the Auger event. We find that the Auger highest energy event points to a quasar with similar characteristics to the one correlated to the Fly's Eye event. We also find the same kind of correlation for one of the highest energy AGASA events. We conclude that so far these types of quasars are the best source candidates for both Auger and Fly's Eye highest energy events. We discuss a few exotic candidates that could reach us from gigaparsec distances.

Note: an EeV is 10^18 electronvolts or 0.1602 Joule!

Phase change memory

Phase change memory technology

We survey the current state of phase change memory (PCM), a non-volatile solid-state memory technology built around the large electrical contrast between the highly-resistive amorphous and highly-conductive crystalline states in so-called phase change materials. PCM technology has made rapid progress in a short time, having passed older technologies in terms of both sophisticated demonstrations of scaling to small device dimensions, as well as integrated large-array demonstrators with impressive retention, endurance, performance and yield characteristics.
We introduce the physics behind PCM technology, assess how its characteristics match up with various potential applications across the memory-storage hierarchy, and discuss its strengths including scalability and rapid switching speed. We then address challenges for the technology, including the design of PCM cells for low RESET current, the need to control device-to-device variability, and undesirable changes in the phase change material that can be induced by the fabrication procedure. We then turn to issues related to operation of PCM devices, including retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. Several factors that can be expected to enhance PCM in the future are addressed, including Multi-Level Cell technology for PCM (which offers higher density through the use of intermediate resistance states), the role of coding, and possible routes to an ultra-high density PCM technology.

Anomalous X-ray Pulsar

The spectacular X-ray echo of a magnetar burst

The Anomalous X-ray Pulsar (AXP) 1E 1547.0-5408 reactivated in 2009 January with the emission of dozens of short bursts. Follow-up observations with Swift/XRT and XMM-Newton showed the presence of multiple expanding rings around the position of the AXP. These rings are due to scattering, by different layers of interstellar dust, of a very high fluence burst emitted by 1E 1547.0-5408 on 2009 January 22. Thanks to the exceptional brightness of the X-ray rings, we could carry out a detailed study of their spatial and spectral time evolution until 2009 February 4. This analysis gives the possibility to estimate the distance of 1E 1547.0-5408. We also derived constraints on the properties of the dust and of the burst responsible for this rare phenomenon.

Neutron Stars

Neutron Stars, the Most Exotic Nuclear Lab in the Universe

In this lecture, we give a first introduction to neutron stars, based on fundamental physical principles. After outlining their amazing macroscopic properties, as obtained from observations, we infer the extreme conditions of matter in their interiors. We then describe two crucial physical phenomena which characterize compact stars, gravitational stability of strongly degenerate matter and neutronization of nuclear matter with increasing density, and explain how the formation and properties of neutron stars are a consequence of the extreme compression of matter under gravity. Finally, we describe how astronomical observations of various external macroscopic features can give invaluable information about the exotic microscopic scenario inside: neutrons stars represent a unique probe to study super-dense, isospin-asymmetric, superfluid, bulk hadronic matter.

Pair-Instability Supernovas

Supernova 2007bi as a pair-instability explosion

Stars with initial masses 10 M_{solar} < M_{initial} < 100 M_{solar} fuse progressively heavier elements in their centres, up to inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion -- an iron-core-collapse supernova (SN). In contrast, extremely massive stars (M_{initial} > 140 M_{solar}), if such exist, have oxygen cores which exceed M_{core} = 50 M_{solar}. There, high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron-positron pairs occurs prior to oxygen ignition, and leads to a violent contraction that triggers a catastrophic nuclear explosion. Tremendous energies (>~ 10^{52} erg) are released, completely unbinding the star in a pair-instability SN (PISN), with no compact remnant. Transitional objects with 100 M_{solar} < M_{initial} < 140 M_{solar}, which end up as iron-core-collapse supernovae following violent mass ejections, perhaps due to short instances of the pair instability, may have been identified. However, genuine PISNe, perhaps common in the early Universe, have not been observed to date. Here, we present our discovery of SN 2007bi, a luminous, slowly evolving supernova located within a dwarf galaxy (~1% the size of the Milky Way). We measure the exploding core mass to be likely ~100 M_{solar}, in which case theory unambiguously predicts a PISN outcome. We show that >3 M_{solar} of radioactive 56Ni were synthesized, and that our observations are well fit by PISN models. A PISN explosion in the local Universe indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic limit, perhaps resulting from star formation processes similar to those that created the first stars in the Universe.

Simulated view of a black hole

Cosmic Acceleration

Gif Lectures on Cosmic Acceleration

These lecture notes cover some of the theoretical topics associated with cosmic acceleration. Plausible explanations to cosmic acceleration include dark energy, modified gravity and a violation of the Copernican principle. Each of these possibilities are briefly described.

Coordinate Systems for Global Positioning

Relativistic versus Newtonian frames: emission coordinates

Only a causal class among the 199 Lorentzian ones, which do not exists in the Newtonian spacetime, is privileged to construct a generic, gravity free and immediate (non retarded) relativistic positioning system. This is the causal class of the null emission coordinates. Emission coordinates are defined and generated by four emitters broadcasting their proper times. The emission coordinates are covariant (frame independent) and hence valid for any user. Any observer can obtain the values of his(her) null emission coordinates from the emitters which provide him his(her) trajectory.

A null frame for spacetime positioning by means of pulsating sources

We introduce an operational approach to the use of pulsating sources, located at spatial infinity, for defining a relativistic positioning and navigation system, based on the use of four-dimensional bases of null four-vectors, in flat spacetime. As a prototypical case, we show how pulsars can be used to define such a positioning system. The reception of the pulses for a set of different sources whose positions in the sky and periods are assumed to be known allows the determination of the user's coordinates and spacetime trajectory, in the reference frame where the sources are at rest. In doing so, the phases of the received pulses play the role of coordinates in the null frame. We describe our approach in flat Minkowski spacetime, and discuss the valididty of this and other approximations considered

Introduction to Loop Quantum Gravity

Introduction to Loop Quantum Gravity

The questions I have been asked during the 5th International School on Field Theory and Gravitation, have compelled me to give an account of the premises that I consider important for a beginner's approach to Loop Quantum Gravity. After a description of some general arguments and an introduction to the canonical theory of gravity, I review the background independent approach to quantum gravity, giving only a brief survey of Loop Quantum Gravity.

The Planck scale

Six easy roads to the Planck scale

We give six arguments that the Planck scale should be viewed as a fundamental minimum or boundary for the classical concept of spacetime, beyond which quantum effects cannot be neglected and the basic nature of spacetime must be reconsidered. The arguments are elementary, heuristic, and plausible, and as much as possible rely on only general principles of quantum theory and gravity theory. The paper is primarily pedagogical, and its main goal is to give physics students, non-specialists, engineers etc. an awareness and appreciation of the Planck scale and the role it should play in present and future theories of quantum spacetime and quantum gravity