Scientific Series

The existence, and enigmatic nature, of 'Dark Energy' is one of the biggest theoretical upsets of recent times. In this seminar we present ideas on alternative theoretical and phenomenological approaches to the Dark Energy problem, in particular the issue of whether dark energy is a matter or gravity-based phenomenon, and the ways in which such approaches can been constrained and guided by observation. We also focus on some of the exciting future approaches that could provide unprecedented insights into the fundamentals of Dark Energy

Using the numerical data of the UBC group simulation, an analysis is done of the properties of the horizon of an evolving black string. The results are consistent with pinch off in infinite affine parameter

The cosmic microwave background (CMB) is our most direct cosmological observable, encoding critical information about the evolution and development of the universe. The Wilkinson Microwave Anisotropy Probe (WMAP) has measured the angular power spectrum of the CMB temperature and temperature-polarization power spectra with unprecedented accuracy from its first year in flight. These recent observations along with developments in supernovae and galaxy surveys are generating critical challenges for theoretical physics, producing fundamental, intriguing questions in particle physics, cosmology and astrophysics that are as yet unresolved. We discuss in this seminar how the theoretical picture of the nature of dark matter and dark energy, and the origins of the universe, is being guided and modified by observations.

Understanding how galaxies form is a major current goal in physical cosmology: although a basic picture is well-accepted, there are outstanding mysteries to be solved. First, what is the origin of the heavy elements seen outside of galaxies? Given that these elements are created only inside galaxies, there must be a process whereby galaxies can expel gas rather than accrete it. Second, galaxy properties are somewhat different from theory predicts, yet extremely regular -- to the extent that it has been seriously argued that modified gravity, rather than dark matter, explains them. I will discuss these mysteries and the possibility that the same culprit -- galactic winds -- may play a key role in solving both.

It was conjectured that the N=4 Yang-Mills perturbation theory in the sector with large R charges corresponds to considering the classical string worldsheets in $AdS_5imes S^5$ as perturbations of the null-surfaces. We discuss this perturbation theory with a special emphasis on a hidden symmetry.

We discuss collective coordinate quantization of the half-BPS geometries of Lin, Lunin and Maldacena (hep-th/0409174). The LLM geometries are parameterized by a single function u on a plane. We treat this function as a collective coordinate. We arrive at the collective coordinate action as well as path integral measure by considering D3 branes in an arbitrary LLM geometry. The resulting functional integral is shown, using known methods (hep-th/9309028), to be the classical limit of a functional integral for free fermions in a harmonic oscillator. The function u gets identified with the classical limit of the Wigner phase space distribution of the fermion theory which satisfies u * u = u. The calculation shows how configuration space of supergravity becomes a phase space (hence noncommutative) in the half-BPS sector. Our method sheds some new light on counting supersymmetric configurations in supergravity.