Talks

In his classic essay, “The Usefulness of Useless Knowledge,” Abraham Flexner, the founding director of the Institute for Advanced Study in Princeton and the man who helped bring Albert Einstein to the United States, describes a great paradox of scientific research. The search for answers to deep questions, motivated solely by curiosity and without concern for applications, often leads not only to the greatest scientific discoveries  but also the most revolutionary technological breakthroughs. In short, no quantum mechanics, no computer chips. Robbert Dijkgraaf, the Institute’s current director, explains how Flexner’s defense of the value of “the unobstructed pursuit of useless knowledge” may be even more relevant today than it was in the early twentieth century. Dijkgraaf describes how basic research has led to major transformations in the past century and explains why it is an essential precondition of innovation and the first step in social and cultural change.

The Standard Model of particle physics and its implications for cosmology leave several fundamental questions unanswered, including the strong CP problem and the origins of neutrino masses, dark matter, and dark energy. Previous directions of model building beyond the Standard Model have usually focused on new high-energy physics. As an alternative direction, we have developed a class of low-energy neutrino mass and axion models at a new infrared gravitational scale, which is numerically coincident with the scale of dark energy. In this seminar, I will present this novel class of models and expand on the aspect of gravitational neutrino mass generation. My talk will also cover the wide-ranging cosmological and phenomenological model predictions, in particular the invalidity of the cosmological neutrino mass bound, enhanced neutrino decays, and soft topological defects. 

In both Causal Set Quantum Gravity as well as in the String Landscape, we face the challenging tasks of sifting through large state spaces and searching for the set of solutions which best model our physical universe. I demonstrate in this talk how efficient parallel algorithms can give us access to areas of physics previously unstudied due to computational barriers. I first present new methods to accelerate the evolution of causal set Markov chains, which enables us to look for the spontaneous emergence of manifoldlike structure. Then, I use a similar graph theoretic approach to show how a dynamic vacuum selection mechanism naturally emerges in F-Theory and Type IIB String Theory in the context of eternal inflation. We will discuss in detail relevant numerical aspects, including practical limitations and algorithms from graph theory, and how machine learning can be used in future work.

Many researchers have been studying the time evolution of entanglement entropy in the sudden quenches where a characteristic mass scale suddenly changes. It is well-know that in these quenches, the change of entanglement entropy become thermal entropy which is proportional to a subsystem size in the late time. However, we do not know which quenches thermalize a subsystem. In our works, we have been studied the time evolution of quantum entanglement in the global quenches with finite quench rate (smooth quenches). Thus, we found that diabaticity plays an important role, so that quenches thermalize the subsystem.

If a component of the dark matter has dissipative interactions, it could collapse to form a thin dark disk in our Galaxy coincident with the baryonic disk. It has been suggested that dark disks could explain a variety of observed phenomena, including periodic comet impacts. Using the first data release from the Gaia mission, we search for a dark disk via its effect on stellar kinematics in the Milky Way. I will present new limits on the presence of a thin dark matter disk, as well as measurements on the matter density in the solar neighborhood.

After more than 12 years of continuous data taking, the Pierre Auger Observatory has collected the largest dataset of ultra-high energy cosmic rays (UHECR) to date. The results obtained in the last years include, for example, precise and accurate measurements of the UHECR flux across a few decades in energy, revealing distinctive spectral features that can bring valuable information on different astrophysical processes like: the transition from galactic to extragalactic fluxes; the different energy loss processes to which ultra-relativistic charged particles are subject during their propagation; the energetics of the production and acceleration of particles at the candidate sources. In this talk, I should however focus on a particular observational probe, that is, the small levels of anisotropy in the flux of UHECR at different angular scales: from the small and intermediate ones, important for the identification of possible point sources, to the large angular scales, usually used to search for signs of the galactic to extragalactic transition. In particular, special attention will be devoted to the first observation of a large scale anisotropy signal at energies above 8x10^18 eV recently reported by the collaboration.

Quantum Monte Carlo methods, when applicable, offer reliable ways to extract the nonperturbative physics of strongly-correlated many-body systems. However, there are some bottlenecks to the applicability of these methods including the sign problem and algorithmic update inefficiencies. Using the t-V model Hamiltonian as the example, I demonstrate how the Fermion Bag Approach--originally developed in the context of lattice field theories--has aided in solving the sign problem for this model as well as aided in developing a more efficient algorithm to study the model. Finally, I discuss some other potential uses for the new algorithm, including for a broader class of models known to be sign-problem-free due to fermion bag ideas.

Atiyah and Segal's axiomatic approach to topological and conformal quantum field theories provided a beautiful link between the geometry of "spacetimes" (cobordisms) and algebraic structures. Combining this with the physical notion of "locality" led to the introduction of the language of higher categories into the topic.
Natural targets for extended topological field theories are higher Morita categories: generalizations of the bicategory of algebras, bimodules, and homomorphisms.

After giving an introduction to topological field theories, I will explain how one can use geometric arguments to obtain results on dualizablity in a ``factorization version’’ of the Morita category and using this, examples of low-dimensional field theories “relative” to their observables. An example will be given by polynomial differential operators, i.e. the Weyl algebra, in positive characteristic and its center. This is joint work with Owen Gwilliam.

In this talk, I will discuss how to assign geometries, such as metric tensors, to certain tensor networks using quantum entanglement and tensor Radon transform. In addition, we show that behaviour similar to linearized gravity can naturally emerge in said tensor networks, provided a modified version of Jacobson's entanglement equilibrium is satisfied. Since the aforementioned properties can be reached without relying on AdS/CFT, the approach also shows promise towards constructing tensor network models for cosmological spacetimes. 

TBA In the framework set by the AdS/MERA conjecture, we investigate a generalisation of the Tensor Network description of bulk geometry in the language of Group Field Theories, a promising convergence of insights and results from Matrix Models, Loop Quantum Gravity and simplicial approaches. We establish a first dictionary between Group Field Theory and Tensor Network states. With such a dictionary at hand, we target the calculation of the Ryu-Takayanagi formula recently derived for Random Tensor Networks in the quantum gravity formalism.

A great deal of progress has been made toward a classification of bosonic topological orders whose microscopic constituents are bosons. Much less is known about the classification of their fermionic counterparts. In this talk I will describe a systematic way of producing fermionic topological orders using the technique of fermion condensation. Roughly, this can be understood as binding a physical fermion to an emergent fermion and condensing the pair. I will discuss the `super pivotal categories' that describe universal properties of these phases and use them to construct exactly solvable string-net models. These string-net models feature conventional anyons and two flavours of vortices. I will show that one of the vortex types is similar to a vortex in a p+ip superconductor binding a Majorana zero mode, and will mention some possible applications.

In this talk I will discuss issues and possibilities to outline Quantum Gravity Phenomenology using cosmological and astrophysical data. After a brief review on some formal aspect of the problem I will focus on the analysis of in-vacuo dispersion features for GRB (gamma-ray-burst) neutrinos of energy in the range of 100 TeV, and for GRB photons with energy in the range of 10 GeV. I will introduce a strategy of data analysis which has the advantage of being applicable to several alternative possibilities for the laws of propagation of neutrinos and other particles in a quantum spacetime. Far from being conclusive, the results to be shown should nonetheless motivate a vigorous program of investigation following the methodology of multi-messenger astronomy.