Cosmology and Gravitation

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.

Chameleon scalar fields are candidates for the dark energy, the mysterious component causing the observed acceleration of the universe. Their defining property is a mass which depends on the local matter density: they are massive on Earth, where the density is high, but essentially massless in the cosmos, where the density is much lower. All current constraints from tests of general relativity are satisfied. Nevertheless, chameleons lead to striking predictions for tests of gravity in the laboratory and in space. For example, near-future satellite experiments could measure an effective Newton's constant in space different by a factor of order unity from that on Earth, as well as violations of the Equivalence Principle stronger than currently allowed by laboratory experiments. Such signatures raise the exciting possibility of detecting dark energy through local tests of gravity.