Scientific Series

The discovery of cosmic acceleration has generated tremendous excitement among researchers in fundamental physics and cosmology. Most experts agree that nothing short of a revolution will be required to fully integrate the observed cosmic acceleration (which many attribute to a mysterious "dark energy") with established physics. Currently this discovery is driving very exciting research in both the theoretical and observational domain. I will present two of these topics that particularly interest me: 1) Dark Energy and Cosmic Equilibrium: How a cosmological constant could make the universe look like a box of gas (and what this could mean for cosmology). 2) Probes of Dark Energy: A host of new probes promise to tell us more about dark energy, but what do we really want to know?

Strongly correlated many-body systems are often formulated as gauge theories where gauge field plays a role of Lagrangian multiplier and fundamental matter field represents a fractional degree of freedom which carries only a fractional quantum number of microscopic particle. Although the fractional particles are prone to be confined at high energy owing to an infinite bare gauge coupling, they can emerge as deconfined degrees of freedom at low/intermediate energy scales as the gauge coupling is renormalized to a finite value by fluctuating matter fields. The resulting dynamics of the gauge field crucially depends on the number and the dynamics of the matter fields. In this talk, I will discuss how a change in the dynamics of matter fields affects the dynamics of gauge field. I will consider a field theory where a 2+1D compact U(1) gauge field is coupled to a large number of fundamental matter fields with an infinite bare gauge coupling and the matter fields are, in turn, subject to an additional strong interaction. This field theory can be realized as a low energy theory of a D-brane configuration and nonperturbative effects of the strong interaction can be studied from the AdS/CFT correspondence. Using the dual gravity theory, I will discuss how the strong interaction between matter fields can drastically modify the dynamics of the U(1) gauge field. Our result suggests that even an unstable brane configuration can define a consistent field theory once tachyonic modes are frozen.

Quantum information methods have been recently used for studying the properties of ground state entanglement in several many body and field theory systems. We will discuss a thought experiment wherein entanglement can be extracted from the vacuum of a relativistic field theory into a pair of arbitrarily spatially separated atoms. In order to simulate the detection process, we will consider the ground state of a linear chain of cooled trapped ions, and discuss a scheme for detecting the entanglement between the ion's motional degrees of freedom.

Kilometer-scale neutrino detectors such as IceCube are discovery instruments covering nuclear and particle physics, cosmology and astronomy. Examples of their multidisciplinary missions include the search for the particle nature of dark matter and for additional small dimensions of space. In the end, their conceptual design is very much anchored to the observational fact that Nature produces protons and photons with energies in excess of 1020 and1013 electronvolts, respectively. The cosmic ray connection sets the scale of cosmic neutrino fluxes. The problem has been to develop a robust and affordable technology to build the kilometer-scale neutrino detectors required to do the science. The AMANDA telescope using clear deep Antarctic ice as a Cherenkov detector of muons and showers initiated by neutrinos of all 3 flavors, has met this challenge. We review the results obtained with more than 10,000 well-reconstructed neutrinos in the 50 GeV~500 TeV energy range collected during its first 5 years of operation. More importantly, we will show that AMANDA represents a proof of concept for the ultimate kilometer-scale neutrino observatory, IceCube, now under construction.

Collapse models are one of the most promising attempts to overcome the measurement problem of quanum mechanics: they descibe, within one single framework, both the quantum properties of microscopic systems and the classical properties of macroscopic objects, and in particular they explain why measurements always have definite outcomes, distributed according to the Born probability rule. We will discuss some recent developments in this field: i) we will show how it is possible to formulate collapse models in such a way that the mean energy of physical system does non increse indefinitely, a typical feature of the models first proposed in the literature; ii) we will discuss recent experiments aiming at testing the validity of the superposition principle, thus of collapse models, at the mesoscopic level.

The TeV energy range is a privileged part of the EM spectrum for astrophysical observations, allowing a view of some of the most energetic processes in the Universe, in objects as diverse as supernova remnants and black-hole driven Active Galactic Nuclei. Driven by new instruments, TeV gamma-rays astrophysics has made enormous strides in recent years with the discovery of many new sources, including new classes of sources such as galactic micro-quasars. This talk will give an overview of the state of TeV gamma-ray astrophysics, including the air Cherenkov technique used by ground-based TeV gamma ray detectors, the new instruments in operation or coming on line soon, and some of the results already obtained.

We will postulate a novel notion of probability; this will involve introducing an extra axiom of probability that seems natural from a Bayesian perspective. We will then provide an analogue of Gleason's theorem for these probabilities. We will also discuss why this approach may be useful for generalizations of quantum theory such as quantum gravity theories; this will involve discussing an analogy between Bayesian approaches and relational approaches.