Cosmology and Gravitation

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.

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.

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.

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

Highest energy cosmic rays reach {\it macroscopic} energies $> 10^{20}$ eV ($\sim 10$ joules; corresponding linear momentum in one proton is similar to a slapshot hockey puck's). Such protons can either be accelerated by nearby astrophysical sources or be by-products of decay of unknown superheavy fundamental particles. After reviewing phenomenology of cosmic rays, I will discuss a novel {\it non-stochastic} acceleration mechanism in jets of powerful active galactic nuclei. The mystery of ultra high energy cosmic rays is likely soon to be resolved by Pierre Auger observatory.

We show that the entropy resulting from the counting of microstates of non extremal black holes using field theory duals of string theories can be interpreted as arising from entanglement. The conditions for making such an interpretation consistent are discussed. First, we interpret the entropy (and thermodynamics) of spacetimes with non degenerate, bifurcating Killing horizons as arising from entanglement. We use a path integral method to define the Hartle-Hawking vacuum state in such spacetimes and discuss explicitly its entangled nature and its relation to the geometry. If string theory on such spacetimes has a field theory dual, then, in the low-energy, weak coupling limit, the field theory state that is dual to the Hartle-Hawking state is a thermofield double state. This allows the comparison of the entanglement entropy with the entropy of the field theory dual, and thus, with the Bekenstein-Hawking entropy of the black hole. As an example, we discuss in detail the case of the five dimensional anti-de Sitter, black hole spacetime.