Talks

We present analytically solvable nonlinear models of structure formation in a Universe with only dust, using LTB solutions. We show that the luminosity distance-redshift relation has significant corrections at low redshift (Doppler effect). We discuss different possibilities that could further enhance this effect and mimick Dark Energy. We find negligible integrated effect, suppressed by $(L/R_{H})^3$ (where $L$ is the size of the structure, and $R_{H}$ is the Hubble radius) and we make contact with cosmological perturbation theory.

We consider the effect that dynamical selection principles could have on the string landscape and for determining the value of the cosmological constant. The underlying symmetries of string theory, along with the dynamics of moduli in the low energy effective field theory, seem to suggest that not all vacua are created equal. However, in some simple models many vacua are alike and this degeneracy may suggest a *non-anthropic* approach to understanding the observed value of the cosmological constant. The approach may also lead to a viable model of inflation, without the need of fine-tuned potentials.

Brane worlds may provide insight into the cosmological constant problem because a large vacuum energy on the brane can curve the extra dimensions rather than the local 4D spacetime. Moreover, such models with supersymmetric large extra dimensions reveal a tantalizing numerology, in which the size of the two extra dimensions can lead not only to the electroweak hierarchy but also to the observed dark energy scale. I will review this proposal, its promises and problems, and then describe some of the novel physics that can arise in 6D brane worlds. The dynamical stability of models in 6D supergravity constrains the matter content of the theory, but surprisingly this can be relaxed with negative tension branes. Meanwhile, the Kaluza-Klein mass gap can remain finite even in the infinite volume limit.

One of the possible explanations for the current acceleration of the universe comes from a coupling between the Dark Energy and the Neutrino sectors. This coupling causes the neutrino mass to vary with cosmic time, what opens a new window to constrain this dark energy candidate. In this work, we analyze the mass-varying neutrino scenario in a model independent way, focusing on its effects for the Cosmic Microwave Background and Large Scale Structure.

A modification of Gravity in the low-curvature regime may account for the late time acceleration of our universe, and is therefore an interesting alternative to Dark Energy.In such models, the modified Einstein equations admit self-accelerated solutions in the presence of negligible matter. At the level of perturbation theory,the modified equations give rise to new dynamics for the perturbations of the metric and matter. I will consider scalar perturbations, presenting in some details the dynamics of linear perturbations for two specific models, f(R) Gravity and Modified Source Gravity. I will conclude by demonstrating how some characteristic features of these models are likely to be common to general models of Modified Gravity and how Large Scale Structure formation and the Integrated Sachs Wolfe effect might be useful probes to distinguish between Dark Energy and Modified Gravity.

I will present the embedding of runaway quintessence models in supergravity coupled to observable matter and hidden supersymmetry breaking. Serious obstructions appear either in the gravitational sector of the theory or cosmologically. Alternatives will be discussed.

We explore the hypothesis that a dynamical dark energy is related to a time-variation of the fundamental mass scale. A dilatation anomaly induced by quantum fluctuations could explain the small value of the present dark energy. Reformulated as a scalar field theory this would predict a quintessence potential which asymptotically relaxes to zero rather than to a nonvanishing constant. An observable consequence of such a scenario results in "early dark energy " contributing a few percent to the energy density of the Universe even at high redshift. Quintessence would be related to a new "fundamental" macroscopic force and induce a small time variation of fundamental constants.

In nearly every quantum algorithm which exponentially outperforms the best classical algorithm the quantum Fourier transform plays a central role. Recently, however, cracks in the quantum Fourier transform paradigm have begun to emerge. In this talk I will discuss one such development which arises in a new efficient quantum algorithm for the Heisenberg hidden subgroup problem. In particular I will show how considerations of symmetry for this hidden subgroup problem lead naturally to a different transform than the quantum Fourier transform, the Clebsch-Gordan transform over the Heisenberg group. Clebsch-Gordan transforms over finite groups thus appear to be an important new tool for those attempting to find new quantum algorithms. [Part of this work was done in collaboration with Andrew Childs (Caltech) and Wim van Dam (UCSB)]