Search Talks

The history of human knowledge is often highlighted by our efforts to explore beyond our apparent horizon. In this talk, I will describe how this challenge has now evolved into our quest to understand the physics at/beyond the cosmological horizon, some twenty orders of magnitude above Columbus’s original goal. I also argue why inflationary paradigm predicts the existence of non-trivial physics beyond the cosmological horizon, and how we can use the Integrated Sachs-Wolfe effect in the Cosmic Microwave Background to probe this physics, including the nature of gravity and primordial non-gaussianity on the horizon scale.

We consider a consistent construction of the supersymmetric action for a codimension-two brane in six-dimensional Salam-Sezgin supergravity. When the brane carries a tension, we supersymmetrize the brane tension action by introducing a localized Fayet-Iliopoulos term on the brane and modifying the bulk SUSY transformations. As a result, we find that among the axisymmetric vacua of the system, the unwarped background with football-shaped extra dimensions respects N=1 supersymmetry. We extend the analysis to include the brane multiplets with the couplings to the bulk fields.

I shall review the potential relevance of antisymmetric tensor fields in physics, perhaps the most intriguing being a massive antisymmetric tensor as dark matter. Next, based on the most general quadratic action for the antisymmetric tensor field, I shall discuss what are possible extensions of Einstein\\\'s theory which include antisymmetric tensor field and thus torsion in a dynamical fashion.

After almost a century of observations, the ultra-high energy sky has finally displayed an anisotropic distribution. A significant correlation between the arrival directions of ultra-high cosmic rays measured by the Pierre Auger Observatory and the distribution of nearby active galactic nuclei signals the dawn of particle astronomy. These historic results have important implications to both astrophysics and particle physics.

With the discovery of many new satellite galaxies, in recent years our understanding of the Milky Way environment has undergone a dramatic transformation. I will discuss what these discoveries are telling us about galaxy formation and the nature of dark matter itself. Issues I will focus on include: identifying the least luminous dark matter halo in the Universe, distinguishing between warm and cold dark matter, and indirect dark matter detection.

We highlight the unexpected impact of nucleosynthesis on the detectability of tracking quintessence dynamics at late times, showing that dynamics may be invisible until Stage-IV dark energy experiments (DUNE, JDEM, LSST, SKA). Nucleosynthesis forces |wŒ(0)| <0.2 for the models we consider and strongly limits potential deviations from _LCDM . Surprisingly, the standard CPL parametrisation, w(z) = w0 +waz/(1+z), cannot match the nucleosynthesis bound for minimally coupled tracking scalar fields. Given that such models are arguably the best-motivated alternatives to a cosmological constant these results may significantly impact future cosmological survey design and imply that dark energy may be dynamical even if we do not detect any dynamics in the next decade.

The Cosmic Microwave Background (CMB) consists of a bath of photons emitted when the universe was 380,000 years old. Carrying the imprint of primordial fluctuations that seeded the formation of structure in the universe, the CMB is one of the most valuable known tools for studying the early universe. In our modern, post WMAP era, the utility of studying temperature anisotropies in the CMB is clear and much of the work has been done. I will describe two exciting new directions in which the field is currently heading: small-scale secondary CMB anisotropy and CMB polarization anisotropy. In this context, I will briefly discuss preliminary results from our small-scale secondary anisotropy experiment, the Sunyaev-Zel\'dovich Array (SZA), and will describe our two upcoming CMB polarization experiments, the Q U Imaging ExperimenT (QUIET) and the E B EXperiment (EBEX).

We have two strong reasons to argue that Einstein\'s theory of general relativity may be incomplete. First, given that it cannot be expressed within a consistent quantum field theory there is reason to expect higher energy corrections. Second, the observation that we are undergoing a current epoch of accelerated expansion might indicate that our understanding of gravity breaks down at the largest scales. A generic result of modified gravity is the creation of a new degree of freedom within the gravitational sector. This new degree of freedom then generically connects local physics to cosmological dynamics. I will present the results of studying two modified theories of gravity emphasizing how they bridge the gap between local and cosmological physics. First I will discuss work I have done on f(R) modified gravity theories, delineating under what conditions these theories deviate strongly from general relativity. Using these results I will talk about some recent work on attempting to detect a characteristic signature of these theories from gravitational lensing. Second I will discuss recent results on ways we may test Chern-Simons gravity (a result of the low energy effective string action) in the Solar System. Chern-Simons gravity has been identified as a candidate for leptogenesis as well as a source for circularly polarized gravitational-waves from inflation. As I will discuss, constraints to Chern-Simons gravity may improve in the near future with further observations of double pulsar systems.

The Universe offers environments with extreme physical conditions that cannot be realized in laboratories on Earth. These environments provide unprecedented tests for extensions of the Standard Model. I will describe three such \"astrophysical laboratories\", which are likely to represent new frontiers in cosmology and astrophysics over the next decade. One provides a novel probe of the initial conditions from inflation and the nature of the dark matter, based on 3D mapping of the distribution of cosmic hydrogen through its resonant 21cm line. The second allows to constrain the metric around supermassive black holes based on direct imaging or the detection of gravitational waves. The third involves the acceleration of high-energy particles in cosmological shock waves. I will describe past and future observations of these environments and some related theoretical work.