Search Talks

Fracton order is a new kind of phase of matter which is similar to topological order, except its excitations have mobility constraints. The excitations are bound to various n-dimensional surfaces with exotic fusion rules that determine how excitations on intersecting surfaces can combine.

This talk will focus on one of the simplest fracton models: the X-cube model. I will explain how this fracton model can be thought of as starting from a 3D toric code and stacks of 2D toric code layers, and then condensing certain excitations on the 2D layers. This is the first TQFT picture of the X-cube model as a 3D order with non-invertible 2D "defects". When we zoom out so that the layers appear close together, I'll show that the model can be described by what I call a foliated field theory. I define a foliated field theory to be a field theory that couples to a foliation field, which describes the geometry of the foliating layers (in analogy to a metic which describes the geometry of spacetime). In this case, the foliated field theory takes the form of a 3+1D BF theory which is coupled to stacks of 2+1 BF theories, with stacking structure described by the foliation field.

This talk is based on arXiv:1812.01613 and another forthcoming work.

New physics in the neutrino sector might be necessary to address anomalies between different neutrino oscillation experiments. Intriguingly, it also offers a possible solution to the discrepant cosmological measurements of H_0. 
We show here that delaying the onset of neutrino free-streaming until close to the epoch of matter-radiation equality can naturally accommodate a larger value for the Hubble constant, while not degrading the fit to the cosmic microwave background (CMB) damping tail. We achieve this by introducing neutrino self-interactions in the presence of a non-vanishing sum of neutrino masses. This "strongly interacting" neutrino cosmology prefers a 3+1 neutrino scenario, which has interesting implications for particle model-building and neutrino oscillation anomalies. Due to their impact on the evolution of the gravitational potential at early times, self-interacting neutrinos and their subsequent decoupling leave a tell-tale structure on the matter power spectrum. 
Our analysis shows that it is possible to find radically different cosmological models that nonetheless provide excellent fits to the data, hence providing an impetus to thoroughly explore alternate cosmological scenarios.

Modified gravity theories typically feature numerous additional parameters and functions as compared to general relativity, which are unmotivated by observations and challenging to meaningfully constrain. We instead propose a new theory of gravity with the startling property of having *fewer* degrees of freedom than general relativity with a cosmological constant, by invoking a duality property within a first-order formulation that supports torsion. In this theory the cosmological constant becomes a space-time variable without introduction of kinetic terms, but its behaviour is tied to that of matter. I will describe the main properties of the theory, including its implications for cosmology. While there are strong indications that the simplest incarnation of the theory is not observationally viable, the theory can be used as a starting point for various extensions which appear more promising.

Based on arXiv:1905.10380 and arXiv:1905.10382, with Alexander, Cortês, Magueijo, Sims, and Smolin.

I shall  analyze three specific general-relativistic  problems in which gravitomagnetism plays important role: the dragging of  magnetic fields  around  rotating black holes, dragging inside a collapsing slowly rotating spherical shell of dust, compared with the dragging by rotating gravitational waves (CQG 34, 205006 (2017), Phys. Rev. D 85 124003, (2012) etc). I shall also briefly show how "instantaneous Machian gauges“ can be useful in the cosmological perturbation theory (Phys. Rev. D 76, 063501 (2007)).

We derive an effective Hamiltonian constraint for the Schwarzschild geometry starting from the full loop quantum gravity Hamiltonian constraint and computing its expectation value on coherent states sharply peaked around a spherically symmetric geometry. We use this effective Hamiltonian to study the interior region of a Schwarzschild black hole, where a homogeneous foliation is available. Descending from the full theory, our effective Hamiltonian preserves all relevant information about the graph structure of quantum space and encapsulates all dominant quantum gravity corrections to spatially homogeneous geometries at the effective level. It carries significant differences from the effective Hamiltonian postulated in the context of minisuperspace loop quantization models in the previous literature. We show how, for several geometrically and physically well motivated choices of coherent states, the classical black hole singularity is replaced by a homogeneous expanding Universe. The resultant geometries have no significant deviations from the classical Schwarzschild geometry in the pre-bounce sub-Planckian curvature regime, evidencing the fact that large quantum effects are avoided in these models. In all cases, we find no evidence of a white hole horizon formation. However, various aspects of the post-bounce effective geometry depend on the choice of quantum states. Finally, we show how a de Sitter Universe extending the classical spacetime past the singularity can be recovered by means of the simplicity constraint.

Following the advent of LIGO measurements, it has been recently observed that QFT amplitudes can be used to derive observables appearing in the scattering of two black holes, to very high orders in perturbation theory. Such framework easily fits into the Post-Newtonian and Post-Minkowskian expansions appearing in the treatment of the binary inspiral. In this talk we will review recent progress in this direction for the case of spinning black holes, focusing on radiation and the multipole expansion. From the QFT point of view these are in close relation to long-studied Soft Theorems.

Computing has had many fundamental platform shifts in its history, and each came shrouded with mystery, hype, and dazzling potential: Alan Turing's universal machines, Doug Engelbart's Dynamic Knowledge Repository, J.C.R. Licklider's Intergalactic Network, the development of the internet, and all the waves of personal computers. More recently, Web 1.0, Web 2.0, and now Web 3.0 have all been heralded with barely-working demos and baffling hype, only to quietly install and broadly distribute fundamental improvements to our everyday life, to our work, and to our society. Each time the smoke cleared, our civilization had been transformed.

Right now, there are fundamental improvements being designed, built, and deployed in the web 3.0 landscape. These improvements and the applications they enable have the potential to transform our lives, our societies, and our civilization yet again. Some of those changes have started to happen, but the vast majority loom in the horizon. To understand the potential changes to our future, we must first understand what the technologies are, what properties they have, and what applications and actions they enable. After looking at the pieces concretely, both in theory and in practice, we can then put the puzzle of the future back together.

This colloquium will explore:

- What web 3.0 is, and its key technologies
- Decentralized Web systems, and their applications
- Blockchain systems, as a next generation platform for computing
- Cryptocurrencies, and the systems they enable
- Smart contracts and autonomous programs
- Cryptoeconomics and incentive structure engineering
- Open Services -- open source internet-wide utilities
- and a set of Open Problems in the field.

The need for a time-shift invariant formulation of quantum theory arises from fundamental symmetry principles as well as heuristic cosmological considerations. Such a description then leaves open the question of how to reconcile global invariance with the perception of change, locally. By introducing relative time observables, we are able to make rigorous the Page-Wootters conditional probability formalism to show how local Heisenberg evolution is compatible with global invariance.

Using Monte Carlo methods we explore how well does the recent proposal for computing conformal dimensions, using a large charge expansion, work. We focus on the O(2) and the O(4) Wilson-Fisher fixed points as test cases. Since the traditional Monte Carlo approach suffers from a severe signal-to-noise ratio problem in the large charge sectors, we use worldline formulations that eliminate such problems. In particular we argue that the O(4) model can be simplified drastically by studying what we refer to as a "qubit" formulation. Such simpler  formulations of quantum field theories have become interesting recently from the perspective of quantum computing. Using our studies we confirm that the conformal dimensions of both conformal field theories with O(2) and O(4) symmetries obey a simple formula predicted by the large charge expansion. We also compute the two leading universal low energy constants in both cases , that play an important role in the large charge expansion.