Search Talks

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.

Wilson loops are important observables in gauge theory. In this talk, we study half-BPS Wilson loops of a large class of five dimensional supersymmetric quiver gauge theories with 8 supercharges. The Wilson loops are codimension 4 defects of the quiver gauge theory, and their interaction with self-dual instantons is captured by a 1d ADHM quantum mechanics. We compute the partition function as its Witten index. It turns out that we can understand the 5d physics in 3d gauge theory terms. This comes about from so-called gauge/vortex duality; namely, we study the vortices on the Higgs branch of the 5d theory, and reinterpret the physics from the point of view of the vortices. This perspective has an advantage: it has a dual description in terms of "deformed" Toda Theory on a cylinder, in the Coulomb gas formalism. We show that the gauge theory partition function is equal to a (chiral) correlator of the deformed Toda Theory, with stress tensor and higher spin operator insertions. We derive all the above results from type IIB string theory, compactified on a resolved ADE singularity X times a cylinder with punctures. The 5d quiver gauge theory arises as the low energy limit of a system of D5 branes wrapping various two-cycles of X, the Wilson loops are D1 branes, and the duality to Toda theory emerges after introducing additional D3 branes.

Embezzlement of entanglement is the (impossible) task of producing an entangled state from a product state via a local change of basis, when a suitable catalytic entangled state is available.  The possibility to approximate this task was first observed by van Dam and Hayden in 2002.  Since then, the phenomenon is found to play crucial roles in many aspects of quantum information theory.  In this colloquium, we will explain various methods to embezzlement entanglement and explore applications (such as an extension to approximately violate other conservation laws, a Bell inequality that cannot be violated maximally with finite amount of entanglement, consequences for resource theories, and the quantum reverse Shannon theorem).

Trisections were introduced by Gay and Kirby in 2013 as a way to study 4-manifolds. They are similar in spirit to a common tool in a lower dimension: Heegaard splittings of 3-manifolds. In both cases, one understands a manifold by examining the ways that standard building blocks can be put together. They both also have the advantage of changing problems about manifolds into problems about diagrams of curves on surfaces. This talk will be a relaxed introduction to these decompositions.

We discover an infinite hierarchical web of the products of supersymmetric generators sustained by the superconformal Virasoro algebra. This hierarchy structure forms the mathematical foundation underpinning the explicit derivation of the character for the self-symmetric Ramond highest weight $c/24$. To consistently fit these exact results into the modular-invariant torus partition function, we advocate a necessary augmentation of the representation theory in the original Friedan--Qiu--Shenker construction via symmetrizing the ground-state manifold associated with the $c/24$ Verma module. Under the newly-proposed scheme, we invoke a quantum-interference mechanism between the two independent Ishibashi states to construct the boundary Cardy states for the whole family of the $\mathcal{N}=1$ superconformal minimal series, based on which the extra fusion channels are unveiled through the obtained Verlinde formula. Our work thus provides the first complete solution to this thirty-year-old question.

Black hole (more generally, horizon) thermodynamics is a window into quantum gravity. Can horizon thermodynamics---and ultimately quantum gravity---be quasi-localized? A special case is the static patch of de Sitter spacetime, known since the work of Gibbons and Hawking to admit a thermodynamic equilibrium interpretation. It turns out this interpretation requires that a negative temperature is assigned to the state. I'll discuss this example, and its generalization to all causal diamonds in maximally symmetric spacetimes. This story includes a Smarr formula and first law of causal diamonds, analogous to those of black hole mechanics. I’ll connect this first law to the statement that generalized entropy in a small diamond is maximized in the vacuum at fixed volume.