Search Talks

What, exactly, happened around the time of the Big Bang? Exploring new models inspired by superstring theory and supergravity, e.g. ones in which we live on “branes” that collide with a “big bang”. Satellite experiments to test such models.

Physics beyond the standard model: theories of elementary particles with extra space dimensions (large, small, warped and flat); supersymmetry; grand unification; dark matter; inflation and dark energy; as well as relationships between the different subjects.

We present a short review of the local conformal symmetry and its anomalous violation in curved $4d$ space-time. Furthermore we discuss the ambiguities of conformal anomaly and the anomaly-induced effective actions. Despite the conformal symmetry is always broken at quantum level, it is useful for constructing the best known approximations for investigating quantum corrections to the classical action of gravity. These quantum corrections represent an appropriate basis for a number of applications in cosmology and black hole physics.

We use black holes to understand some basic properties of theories of quantum gravity. First, we apply ideas from black hole physics to the physics of accelerated observers to show that the equations of motion of generalized theories of gravity are equivalent to the thermodynamic relation $\delta Q = T \delta S$. Our proof relies on extending previous arguments by using a more general definition of the Noether charge entropy. We have thus completed the implementation of Jacobson's proposal to express Einstein's equations as a thermodynamic equation of state. Additionally, we find that the Noether charge entropy obeys the second law of thermodynamics if the energy momentum tensor obeys the null energy condition. Our results support the idea that gravitation on a macroscopic scale is a manifestation of the thermodynamics of the vacuum. Then, we show that the existence of semiclassical black holes of size as small as a minimal length scale l_{UV} implies a bound on a gravitational analogue of 't-Hooft's coupling $\lambda_G(l)\equiv N(l) G_N/l^2$ at all scales $l \ge l_{UV}$. The proof is valid for any metric theory of gravity that consistently extends Einstein's gravity and is based on two assumptions about semiclassical black holes: i) that they emit as black bodies, and ii) that they are perfect quantum emitters. The examples of higher dimensional gravity and of weakly coupled string theory are used to explicitly check our assumptions and to verify that the proposed bound holds. Finally, we discuss some consequences of the bound for theories of quantum gravity in general and for string theory in particular.

We present a string dual to finite temperature N=4 SYM coupled to Nf massless flavors with abelian symmetry. The solution includes the backreaction of the flavor up to second order in the ratio N_f/N_c times the 't Hooft coupling at the temperature of the dual QGP. The thermodynamics show a departure from conformality as a second order effect, and the energy loss of a quark through the plasma is enhanced by new degrees of freedom.

The screening of electric charge in plasma with Bose condensate of a charged scalar field is calculated. In all previous calculations before 2009 the effects of Bose condensation have not been considered. Due to the condensate the time-time component of the photon polarization tensor in addition to the usual terms k-squared and Debye mass squared, contains infrared singular terms inversely proportional to k and k-squared. Such terms lead to power law oscillation behaviour of the screened potential, which is different form Friedel oscillations known for fermions. An analogue of Friedel oscillations in bosonic case is also considered.

The Standard model of Cosmology consists of a package of ideas that include Cold Dark Matter, Inflation, and the existence of a small Cosmological Constant. While there is no consensus about what lies beyond the Standard Model, there is a leading candidate that also includes a small package of ideas: A Landscape of connected vacua: the idea that the universe started out with a large energy density, and Coleman DeLuccia Tunneling between vacua. An additional idea that comes from string theory and black hole physics is the Holographic Principle. I will explain how the various ingredients for a "post-standard-model" standard model fit together.

By using the AdS/CFT duality, the computation of MSYM scattering amplitudes at strong coupling boils down to the computation of minimal areas on AdS_5 with certain boundary conditions. Unfortunately, this seems to be a hard problem. In this talk we show how one can make progress by restricting to AdS_3.

I review the status of (open covariant) cubic superstring field theories, their successes and their problems. I then propose a new superstring field theory, which avoids previous problems. The picture number is not restricted in this theory and the NS and Ramond sectors are naturally unified. Constructing the BV master action is straightforwards and leads to a theory which is defined in the whole Hilbert space, i.e., including all ghost and picture numbers and all the relevant sectors. When (partially) gauge fixed and restricted to the NS sector, this new theory reduces to the old one. Hence, all the good known properties of the old one are shared by the new theory.

Physics emerged from the twentieth century with two remarkably successful descriptions of nature which stand in striking contrast. Quantum mechanics describes the subatomic realm with intrinsic uncertainties and probabilities. On the other hand, Einstein's general relativity describe gravitational phenomena in an exacting geometric arena. Theoretical physicists have struggled for over fifty years trying to combine these views in a single unified framework. More recently, superstring theory has drawn a huge amount of interest as a leading contender to provide such a unification. Superstring theory is a theory of strings, branes, extra spacetime dimensions and much more. In my lecture, I will try to give a flavour for what superstring theory is all about and why physicists, like myself, continue to be so excited about this, perhaps the final theory.