Talks

The Hamiltonian for the quantized gravitational and matter fields contains both positive and negative energy particle contributions, which leads through a positive and negative energy symmetry of the vacuum to a cancellation of the zero-point vacuum energy and a vanishing cosmological constant in the presence of a gravitational field. The positive and negative energy particles interact only weakly through gravity. As in the case of antimatter, the negative energy matter is not found naturally on Earth or in galaxies in the universe. We introduce a graviton momentum cutoff ΛG ≤ 2 x10-3 eV that leads to a gravitational stability of the Minkowski spacetime vacuum with a lifetime greater than the age of the universe. A positive energy spectrum and a consistent unitary field theory for a pseudo- Hermitian Hamiltonian is obtained by demanding that the pseudo-Hermitian Hamiltonian is PT symmetric. The quadratic divergences in the two-point vacuum fluctuations and the self- energy of a scalar field are removed. By adopting a Higgsless model of electroweak theory, we remove the fine-tuning associated with a spontaneous symmetry breaking vacuum density. We also postulate that there are no phase transitions associated with QCD and at higher particle physics energy scales, removing all theorized quark-gluon vacuum density condensates and their fine-tuned vacuum densities and cosmological constant contributions.

Theories of gravity having connection as an independent dynamical variable can have a massive spin -2 particle together with the massless graviton in their spectrum. We study the potentiality of such models as a consistent infrared modification of gravity. It will be shown that these models are free from ghost like Boulware-Deser mode in the background of arbitrary torsionless Einstein manifolds. At least for weak for weak enough curvature the dangerous spin zero mode has a healthy kinetic energy in our backgrounds. vDVZ singularity seems to be a generic feature which may be cured by Vainshtein mechanism as in other infrared modified theories of gravity.

We consider a codimension-2 brane inflation scenario in a warped flux compactification of 6D gauged supergravity. The volume modulus of the model is stabilized by means of potentials localized on the regularized background branes. We discuss the cosmological evolution of the world-volume of a probe codimension-2 brane when it moves along the radial direction of the internal space. In order to have slow-roll inflation, we find that the warping of the internal space is required to be weak, in contrast to the string inflation constructions with strong warping. We discuss the parameter range that the inflation is in agreement with the observationally inferred parameters and which furthermore is consistent with the probe brane approximation. We argue that from a multi-brane solution, the backreaction of the probe brane on the weak warp factor is ignorable

Models with two supersymmetric large extra dimensions (SLED) may provide a way to approach both the cosmological constant problem and dark energy. After reviewing these ideas, I shall discuss warped brane world solutions in 6D supergravity, as a laboratory in which to explore SLED and codimension two branes in general. Solving the linearized perturbations for all the bosonic fields, and some of the fermions, we can observe how the corresponding physics compares with 5D models and standard Kaluza-Klein compactifications. These results should help to better our understanding of the cancellations in the 4D effective vacuum energy with SLED.

Abstract: In this talk a model of inflation is presented where the inflaton fields are non-commutative matrices. The spectrum of adiabatic and iso-curvature perturbations and their implications on CMB are studied. It is argued that our model of matrix inflation can naturally be embedded in string theory.

We estimate the size of loop corrections in various inflationary systems and determine the region of parameter space where the perturbation theory around a quasi de Sitter background is strongly coupled. In some models, we argue that backreaction to the inflatonary background become important before the erturbations become strongly coupled while in others, there seems to exist a legitimate strongly coupled but still inflating regime. We also demonstrate that loop effects could be dominant in the bispectrum while still having a well controlled perturbation theory and we explore the phenomenological implications.

Inflationary cosmology provides a causal mechanism for the generation of super-Hubble cosmological fluctuations. There have been many alternative proposals suggested to accomplish this feat, however these all seem to share the need to violate at least the Null Energy Condition. I will attempt to make this statement more precise, and focusing on the case of string motivated models that contain a gravi-scalar in their spectrum (such as the string theoretic dilaton) we will find a "no-go" theorem. This provides an important challenge for models such as String Gas Cosmology and the Pre-Big Bang, if such models are to become more predictive.