Talks

The detection of gravitational waves has opened up new observational windows into the physics of black holes and has the potential to shed light on how imposing unitary evolution modifies the near horizon dynamics. In this talk, I will present how recent developments in holography have provided a way of understanding the physics of black hole thermalization in terms of the spacing statistics of black hole microstates. Based on this, I will suggest that the issue of measuring the quantum aspects of black holes from their ringdown depends on the spectral statistics of their microstates. I will then suggest that certain microstate statistics lend to the possibility of deviations in the ringdown behaviour of black holes in the form of “echoes” which might be interpreted as being due to Planck-scale microstructure near the horizon.

I will discuss work-in-progress for defining a new proposal for the covariant holographic entanglement entropy. The proposal instructs us to find maximal spacelike codimension-2 surfaces on timelike hypersurfaces in the bulk, followed by a minimization among all possible hypersurfaces in the right homology class. We describe and prove various properties of such minimax surfaces, and argue for their equivalence with the more familiar HRT and maximin proposals. Finally, we give compelling reasons to be interested in yet another entanglement entropy proposal: minimax surfaces allow us to prove all higher entropy cone inequalities, showing that the RT and HRT holographic entropy cones are indeed equivalent.

Many non-trivial ideas have been proposed to resolve singularities in quantum gravity. In this talk I argue that singularity resolution can be trivial in gravitational path integrals, because geodesically incomplete singular spacetimes are usually not included in the sum. For theories where this holds, there is no need to develop non-trivial ideas on singularity resolution. Instead, efforts should better be directed to understand tunneling processes and complex-valued spacetimes.

The semi-classical limit of spin foams leads to the Area Regge action. It was long thought that this action leads to flatness and does, in particular, not allow for propagating gravitons. I will present the first systematic studies of the continuum limit of the Area Regge action, using different versions of regular hypercubic lattices. These studies have shown that the Area Regge action does in its continuum limit, lead to leading order to general relativity, and thus to propagating gravitons. The higher order corrections depend on the choice of triangulation for the hypercubic lattice. However, there seems to be a preferred choice, for which the Area Regge action is not singular. In this case the correction term approximates the square of the Weyl curvature tensor, and can be interpreted to arise from an area metric dynamics. We therefore conjecture that the continuum limit of spin foams is described by an area metric theory.

The cosmic radio background (CRB) contains diffuse, discrete and cosmological emission sources from the universe. In this talk, I will discuss  wether an additional contribution coming from low frequency photons is produced by free-free emission during the recombination era of the univers. These photons do not readily thermalise with the electron-proton plasma and therefore might  survive as a small non-thermal relic in the current universe.  Inclusion of such a contribution can alleviate the observed tension between existing CRB models and explain radio source counts between $100 MHz$ to $10 GHz$ to a very high degree of precision.  I will also discuss the challenges and future work we must considere in order to establish wether the signal is created or not.

Zoom Link: https://pitp.zoom.us/j/99032512514?pwd=eldocVA2VHhoUGVudmtFN3F4NHNEQT09

The interplay between strong electronic interaction and non-trivial topology in magic-angle twisted bilayer graphene (tBLG) yields many intriguing phenomena that range from superconductivity to a spontaneous quantum anomalous Hall state with Chern number ±1. WIth the equilibrium phase diagram under much scrutiny, a better theoretical understanding of the excitation spectrum of tBLG is crucial to reveal experimental signatures of competing phases and discern possible pathways of controlling their behavior out of equilibrium. To this end, we study an effective Wannier-orbital model from Kang and Vafek (2019) in the strong coupling limit, which captures the Chern state at three quarters filling as well as its competition with proximal stripe charge order. We compute the density and chiral excitation spectra as well as the charge gap as a function of the overlap of neighboring Wannier orbitals. Our results provide an experimental signature to detect the transition from the stripe phase to the Chern phase and offer insight into steering of the quantum anomalous Hall state via low-frequency driving.

Monitored quantum circuits, composed of local unitary operators and projective measurements, have recently emerged as a rich setting for studying non-equilibrium quantum dynamics. In such systems, sufficient densities of measurements can protect a highly-monitored steady state phase with area law entanglement. Furthermore, it has been shown that such area law phases can host a measurement-protected Ising ferromagnetic order. However, it is not yet known whether such measurement-protected order is a generic phenomenon or whether it relies on the discrete Ising symmetry. To begin answering this question, we introduce a circuit model with continuous symmetry where ferromagnetic order arises in the steady state. Notably, our model requires feedback based on measurement results in order to generate this ferromagnetic order