Scientific Series

To study the continuum limit of a microscopic model of gravity we need microscopic observables that have a clear interpretation in terms of continuum geometry. In general the construction of such observables is notoriously difficult. In the model of causal dynamical triangulations (CDT) it is clear what the microscopic observables are, but at present the only known well-behaved observables with a continuum interpretation are spatial volumes. In this talk I will demonstrate what it takes to go beyond these by introducing the moduli as observables for CDT in 2+1 dimensions with spatial topology of the torus. Measurements of these observables using computer simulations provide valuable clues concerning the effective action describing CDT in the continuum. In particular I will present numerical evidence indicating that the effective kinetic term is well described by a modified Wheeler-De Witt metric like the one appearing in Horava-Lifshitz gravity.

Paul Dirac has been called ‘the first truly modern theoretical physicist’. In the latter part of his life, he was obsessed by the idea that the fundamental laws of nature must have mathematical beauty. This was ‘almost a religion to him’, he said. In this talk, I shall trace the origins of his fascination with this idea (going back to his school education) and question the account he gave of his contribution to quantum mechanics and field theory, which he often said emerged from his aesthetic perspective. I shall also give some insights into the extraordinary character of this man, whom Niels Bohr called ‘the strangest man’ who ever visited his Institute in Copenhagen.

The description of non-Fermi liquid metals is one of the central problems in the theory of correlated electron systems. I present a holographic theory which builds on general features of the thermal entropy density and the entanglement entropy. Remarkable connections emerge between the holographic approach, and the postulated strong-coupling behavior of the field-theoretic approach.

We show that a baryon asymmetry can be generated by dissipative effects during warm inflation via a supersymmetric two-stage mechanism, where the inflaton is coupled to heavy mediator fields that then decay into light species through B- and CP-violating interactions. In contrast with thermal GUT baryogenesis models, the temperature during inflation is always below the heavy mass threshold, simultaneously suppressing thermal and quantum corrections to the inflaton potential and the production of dangerous GUT relics. This naturally gives a small baryon asymmetry close to the observed value, although parametrically larger values may be diluted after inflation along with any gravitino overabundance. Furthermore, this process yields baryon isocurvature perturbations within the range of future experiments, making this an attractive and testable model of GUT baryogenesis.

I will discuss some work on the collider phenomenology and cosmology of light gravitino dark matter, and will touch on some related issues concerning infrared divergences in charged-particle decay at finite temperature.
Light gravitinos, with mass in the eV to MeV range, are well-motivated in particle physics, but their status as dark-matter candidates is muddled by early-Universe uncertainties.

Both Tevatron experiments have recently reported an anomalous forward-backward asymmetry in top-antitop production. Their inclusive results are roughly 3 standard deviations larger than the standard model prediction and may be evidence of new physics that couples to the top quark. In this talk, I will present a weakly-coupled light axigluon model (< Mtop) with flavor universal couplings as a possible explanation. Surprisingly,a particle with these properties can generate a large ttbar asymmetry with model-parameters safe from dijet resonance searches, flavor changing neutral currents, and constraints from the Z pole.

Although the typical physical system achieves an ordered state at low temperatures, spin liquids stay disordered even in their ground state. In addition to an increasing number of experimental candidates for spin liquids, recent numerical work from Meng, et. al and Yan, et. al. has supplied strong numerical evidence for natural Hamiltonians having spin liquid ground states. Their featureless nature, though, makes  learning about these states particularly difficult. In this talk, we explore what variational ansatz can teach us about them. Additionally, we examine whether the canonical theoretical framework makes sense in the context of the best wave-functions. Finally, we look at what other tools we have to make sense of spin liquids.

The entanglement spectrum denotes the eigenvalues of the reduced density matrix of a region in the ground state of a many-body system. Given these eigenvalues, one can compute the entanglement entropy of the region, but the full spectrum contains much more information. I will review geometric methods to extract this spectrum for special subregions in lorentz and conformally invariant field theories (and any theory whose universal low energy physics is captured by such a field theory). Using this technology, I give a new proof that the entanglement spectrum in quantum hall states is the same as that of a physical edge. I will discuss a number of other applications of the formalism, including, if there is time, a more complicated calculation of universal terms in the entanglement entropy at certain deconfined quantum critical points. The ultimate messages are that geometry is intimately bound up with entanglement and that the entanglement cut should be viewed as a physical cut.

In massive gravity the so-far-found black hole solutions on Minkowski space happen to convert horizons into a certain type of singularities. I will discuss whether these singularities can be avoided if space-time is not asymptotically Minkowskian. As an illustration, I will present an exact analytic black hole solution  which evades  the above problem by a transition at large scales to self-induced de Sitter space-time. The solution demonstrates that in massive GR, in the Schwarzschild coordinate system, a BH  metric has to be accompanied by the St\"uckelberg fields with nontrivial backgrounds to  prevent the horizons to convert into the singularities.