Scientific Series

It is shown that inflationary cosmology may be used to test the statistical predictions of quantum theory at very short distances. Hidden-variables theories, such as the pilot-wave theory of de Broglie and Bohm, allow the existence of vacuum states with non-standard field fluctuations (“quantum non-equilibrium”). It is shown that such non-equilibrium vacua lead to statistical anomalies, such as a breaking of scale invariance for the primordial power spectrum. The results depend only weakly on the details of the de Broglie-Bohm dynamics. Recent observations of the cosmic microwave background are used to set limits on violations of quantum theory in the early universe.

The problem of associating beables (hidden variables) to QFT, in the spirit of what Bohm did for nonrelativistic QM, is not trivial. In 1984, John Bell suggested a way of solving the problem, according to which the beables are the positions of fermions, in a discretized version of QFT, and obey a stochastic evolution that simulates all predictions of QFT. In the continuum limit, it will be shown that the Bell model becomes deterministic and that it is related to the choice of the charge density as a beable. Moreover, the charge superselection rule is a consequence of the Bell model. The non-relativistic limit and the derivation of Bohm's first quantized interpretation in this limit are also studied. I will also consider whether the Bell model can be applied to bosons.

From the Quantum Field Theory point of view, matter and gauge fields are generally expected to be localised around branes (topological defects) occurring in extra dimensions. I will discuss a simple scenario where, by starting with a five dimensional SU(3) gauge theory, we end up with several 4-D parallel braneworlds with localised 'chiral' fermions and gauge fields to them. I will show that it is possible to reproduce the electroweak model confined to a single brane, allowing a simple and geometrical approach to the hierarchy problem. Some nice results of this construction are: Gauge and Higgs fields are unified at the 5-D level; and new particles are predicted: a left-handed neutrino (with zero-hypercharge) and a massive vector field coupling together the new neutrino to other leptons.

The information spectrum approach gives general formulae for optimal rates of codes in many areas of information theory. In this talk I shall relate the information spectrum approach to Shannon information theory and explore its relationship to ``entropic'' properties including subadditivity, chain rules, Araki-Lieb inequlities, and monotonicity.

Up to 90% of matter in the Universe could be composed of heavy particles, which were non-relativistic, or 'cold', when they froze-out from the primordial soup. I will review current searches for these hypothetical particles, both via elastic scattering from nuclei in deep underground detectors, and via the observation of their annihilation products in the Sun, galactic halo and galactic center. The emphasis will be on most recent results, and on comparison with reaches of future particle colliders, such as the LHC and ILC.

Cosmic strings are a generic by-product of string theory models of the inflationary epoch. These new cosmic "superstrings," as they are called, are distinct from the grand unified strings once thought to generate large scale structure. I will discuss what limits the WMAP and SDSS data have already placed on the properties of networks of cosmic strings, as well as avenues for their direct detection. I will also introduce cosmic superstrings' distinctive properties: they can bind into a possibly infinite number of higher-tension states, leading to the possibility of network frustration and for a high- string-tension UV-catastrophe. An analytical model constructed by myself and others has shown that superstring networks can evade these catastrophes under certain assumptions for the dynamics of string binding. I will describe ongoing work to verify numerically these binding dynamics. Finally, I will characterize several observational signatures that I and collaborators have identified that could allow us to discriminate between cosmic superstrings and other kinds of cosmic strings.

Space-time measurements and gravitational experiments are made by the mutual relations between objects, fields, particles etc... Any operationally meaningful assertion about spacetime is therefore intrinsic to the degrees of freedom of the matter (i.e. non-gravitational) fields and concepts such as ``locality'' and ``proximity'' should, at least in principle, be definible entirely within the dynamics of the matter fields. We propose to consider the regions of space just as general ``subsystems''. By writing the Hilbert space of the matter fields as a generic tensor product of subsystems we analyse the evolution of a state vector on an information theoretical basis and discuss general principles to recover a posteriori the usual space-time relations. We apply such principles to generic interacting second quantized models with a finite number of fermionic degrees of freedom. Finally, we discuss the possible role of gravity in this framework.

We study the generation of cosmological perturbations during the Hagedorn phase of string gas cosmology. Using tools of string thermodynamics we provide indications that it may be possible to obtain a nearly scale-invariant spectrum of cosmological fluctuations on scales which are of cosmological interest today. In our cosmological scenario, the early Hagedorn phase of string gas cosmology goes over smoothly into the radiation-dominated phase of standard cosmology, without having a period of cosmological inflation. Furthermore, we find that string thermodynamics implies that the fluctuations are Gaussian, and that the spectrum of tensor perturbations will exhibit a scale-invariant spectrum as well. We contrast the predictions of string gas cosmology in the Hagedorn phase with that of scalar field driven inflation, and comment on the possibility of observationally distinguishing between the two scenarios in future experiments.

A definite prediction of string theory is the existence of a scalar field, the dilaton. The presence of the dilaton generally leads to strong violations of the equivalence principle and thus describe a kind of gravitational force radically different from what we experience. String loop corrections, however, may render phenomenologically acceptable the region of the theory characterized by large values of the dilaton field i.e. the region with a strong tree level-coupling. Interestingly, in this framework, violations of the (weak) equivalence principle should be observed in the next satellite-based generation of experiments. A dilaton running towards infinity can also play the role of coupled dark energy and ease the so-called "cosmic coincidence" problem.

I review our recent work on confinement in 2+1 Yang Mills theory using Karabali-Nair variables. I'll discuss our successful prediction of the glueball spectrum, including the manifestations of the QCD string.

The theory of cosmological perturbations provides a bridge between theoretical models of the early universe (often motivated by string theory) and astrophysical observation, e.g of the CMBR. Since extra dimensions are pivotal to string theory, the known lore of perturbation theory needs to be adjusted accordingly. After introducing the needed formalism, I will illustrate its use on an example within the framework of String Gas Cosmology