Search Talks

I describe a variety of bubbles of nothing which do not require a Kaluza-Klein circle but instead may be found in asymptotically flat or AdS spaces without any identifications.  There are many such bubbles which expand outwards and threaten to destabilize spacetimes with more than four dimensions.   In the AdS case, one can show there are both bubbles and topologically trivial smooth states which violate all of the energy bounds, both classical and quantum, in the corresponding gauge theory.

Linear confinement in holographic QCD can be obtained with a soft-wall quadratic dilaton background. We present a dynamical five-dimensional model realizing this setup and discuss the implications for the hypothetical string theory dual to QCD.

Searches for neutrinoless double beta decays could determine if neutrinos are Majorana particles and could measure their absolute mass scale. The initial stage of the Enriched Xenon Observatory project, EXO-200, will look for two-neutrino and neutrinoless double-beta decays of Xe-136 in a liquid-xenon time-projection chamber. By combining the ionization signal with detection of the scintillation light collected in Large Area Avalanche Photodiodes (LAAPDs), an energy resolution of about 1.4% at the decay energy can be achieved. All construction materials have been systematically selected to minimize naturally-occurring radioactive impurities. An active muon veto is presently under construction. Using these background reduction techniques and the available 200 kg of isotopically enriched xenon (80% in Xe-136), EXO-200 will be soon able to test present constraints on the effective Majorana-neutrino mass. It will also serve to demonstrate the potential performance of a larger-scale EXO experiment which will further reduce backgrounds by detecting the residual Ba ion produced in the decay. Installation of the EXO-200 detector is now in progress at the Waste Isolation Pilot Plant (WIPP) in New Mexico.

According to Doering and Isham the spectral topos corresponds to any quantum system. The descriptions of the systems become similar to these given by classical theories. Topoi can also modify local smooth spacetime structure. Supposing that a quantum system modifies the local spacetime structure and interacts with a gravitational field via the spectral topos, a natural pattern for non-gravitating quantum zero-point modes of the system, appears. A way how to add gravity into the spectral topos of a system is presented. A theory of gravity and systems should be symmetric with respect to some 2-group derived from the category of systems. Hence, a fundamental symmetry of gravity is rather 2-group of automorphisms of the category of systems. This higher symmetry is responsible for the vanishing of the cotributions to the cosmological constant derived from zero-point modes of energy of quantum systems in spacetime. A connection with strings (via the coefficients of the 2-connection of some 2-bundle, with this 2group as the structure group) is also shown. Institute of Physics, University of Silesia

We systematically explore the parameter space of the state-of-the-art brane-antibrane inflation model (Baumann et al.) which is most rigorously derived from string theory, applying the COBE normalization and constraints on the spectral index. We define an effective volume in parameter space consistent with the constraints, and show that the fine tuning problem is this model is alleviated by four orders of magnitude for the optimal parameter values, relative to a fiducial point which has previously been considered. We also discuss the overshooting problem in this model which restricts the allowed initial conditions on the brane-antibrane separation, showing that the allowed region is expanded (by a factor of 5) when optimal model parameters are chosen. We point out a subtlety for getting correct predictions in the approximation of effective single field inflation, where the Kahler modulus is integrated out.

The Pierre Auger Observatory in Malargue, Argentina, is the world\'s largest detector for the study of the origin of ultrahigh energy cosmic rays. The experiment stretches over 3000 km^2 and measures cosmic rays with energies above 10^18 eV using two complementary detector types: an array of 1600 particle detectors on the ground, and 4 fluorescence detectors overlooking the ground array from the periphery. The Observatory is now nearing completion, but scientific data taking started at the beginning of 2004. The analysis of the data shows first indications that the arrival direction distribution of the highest energy cosmic rays is not isotropic, but might be associated with the positions of nearby extragalactic objects. In this talk, I will review recent results from the first few years of data taking.

Using the general structure of the vacuum polarization tensor at non-zero temperature T and finite magnetic field B, the ring contribution to QED effective potential is determined beyond the static (zero momentum) limit. In the limit of weak magnetic field and at high temperature, the improved ring potential consists of a term proportional to T4®5=2, in ad-dition to the well-known T4®3=2 term. In the limit of strong magnetic field, where QED dynamics is dominated by the lowest Landau level (LLL), the ring potential consists of a novel term proportional to 2¼eB m2 ln ¡2®¼ eB m2 ¢. Using the full effective potential including both the one-loop effective and the improved ring potentials, QED gap equation is determined and the dynamical fermion mass generation is studied in the regime of LLL dominance at non-zero temperature. It is shown that at high temperature limit, where the thermal fluctuations dominate the magnetic catalysis of dynamical chiral symmetry breaking in LLL, a chiral symmetry restoration occurs at certain critical temperature Tc. But, comparing to Tc in the static limit, the critical temperature arising from the improved ring potential is lower.The improved ring contribution is also relevant in studying the electroweak phase transition in the presence of external (strong) magnetic fields [1]. PACS numbers: 11.10.Wx, 11.15.Ex, 12.38.Gc

The validity of the perturbative analysis during inflation imposes bounds on the inflationary parameters. For single field inflation, the current experimental bounds on non-Gaussianity necessarily imply that the physics is weakly coupled at CMB scales. In this talk, I will show that for models with a scale dependent sound speed, the system can become strongly coupled at lower scale. I will also discuss multiple field models which can produce non-Gaussianity at CMB scales. In these scenarios, the extra scalar fields are strongly coupled in a large part of the parameter space.

Many modified gravity schemes predict a non-zero difference (``gravitational slip\'\') between the Newtonian and longitudinal perturbed metric potentials. Such a slip would affect the growth of large scale structure without altering the expansion history of the universe. We quantify the slip with a new parameter varpi, show the effect of non-zero varpi on the growth of cosmic overdensities, and constrain its value using CMB and weak lensing data.

We point out that the strong CP problem can be resolved by the anthropic principle. The key ideas are to allow explicit breaking(s) of the Peccei-Quinn symmetry which connects the problem to the cosmological constant problem, and to conjecture that the probability distribution of the vacuum energy in the landscape is hierarchical. The axion acquires a large mass from the explicit breaking, and does not contribute to the dark matter abundance. The axion may dominate the energy density of the universe after inflation and reheat the universe by the decay, possibly generating the density perturbations. On the other hand, the axion can be integrated out during inflation, if the explicit breaking is strong enough. All the cosmological problems of the (s)axion with a large Peccei-Quinn scale can be solved.