Talks

A 2-group is a categorical generalization of a group: it's a category with a multiplication operation which satisfies the usual group axioms only up to coherent isomorphisms. The isomorphism classes of its objects form an ordinary group, G. Given a 2-group G with underlying group G, we can similarly define a categorical generalization of the notion of principal bundles over a manifold (or stack) X, and obtain a bicategory Bun_G(X), living over the category Bun_G(X) of ordinary G-bundles on X. For G finite and X a Riemann surface, we prove that this gives a categorification of the Freed--Quinn line bundle, a mapping-class group equivariant line bundle on Bun_G(X) which plays an important role in Dijkgraaf--Witten theory (i.e. Chern--Simons theory for the finite group G). This talk is based on joint work with Daniel Berwick-Evans, Laura Murray, Apurva Nakade, and Emma Phillips.

I will not assume previous knowledge of 2-groups: I will provide a quick overview in the main talk, as well as a more detailed discussion during a pre-talk on Tuesday. 

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We define a map from an arbitrary quantum circuit to a local Hamiltonian whose ground state encodes the quantum computation. All previous maps relied on the Feynman-Kitaev construction, which introduces an ancillary ‘clock register’ to track the computational steps. Our construction, on the other hand, relies on injective tensor networks with associated parent Hamiltonians, avoiding the introduction of a clock register. This comes at the cost of the ground state containing only a noisy version of the quantum computation, with independent stochastic noise. We can remedy this - making our construction robust - by using quantum fault tolerance. In addition to the stochastic noise, we show that any state with energy density exponentially small in the circuit depth encodes a noisy version of the quantum computation with adversarial noise. We also show that any ‘combinatorial state’ with energy density polynomially small in depth encodes the quantum computation with adversarial noise. This serves as evidence that any state with energy density polynomially small in depth has a similar property. As an application, we give a new proof of the QMA-completeness of the local Hamiltonian problem (with logarithmic locality) and show that contracting injective tensor networks to additive error is BQP- hard. We also discuss the implication of our construction to the quantum PCP conjecture, combining with an observation that QMA verification can be done in logarithmic depth. Based on joint work with Anurag Anshu and Nikolas P. Breuckmann. (https://arxiv.org/abs/2309.16475)

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It is well known that the study and observation of core collapse Supernovae (SN) provide powerful tools to probe possible scenarios of physics beyond the Standard Model (SM). After reviewing the basics beyond the mechanism of a SN explosion and how these observations can constraint models of new physics, I will focus on two novel ideas to exploit the already available data from past SN and the observation of a future, long due, galactic one. Firstly. I will discuss constraints, from the cooling on the SN, on the interactions of SM particles with an hypothetical dark sector, leading to bounds on the mediators of such interactions competitive with collider ones. Then, I will turn on the constrains on the emission of axion-like particles (ALPs) that can convert into photons in an external magnetic field, leading to a gamma-ray signal. I will discuss the possibility that ALPs can convert to gamma-rays in the stellar magnetic fields of the progenitor stars. Applying this concept to gamma-ray data from SN1987A leads to the strongest constraints on axion-like particles for masses within a few orders of magnitude of 10^-5 eV. The implications for a future galactic blue supergiant supernova will be discussed.

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In this talk, I will present a novel framework for modeling the weak lensing source galaxy redshift distribution and galaxy intrinsic alignment via a shared luminosity function. In the context of LSST Year 1 and Year 10 cosmic shear analysis, I demonstrate the significant impact of the luminosity function on source galaxy redshift distributions and intrinsic alignment contamination. I establish the influence of Schechter luminosity function parameters on the redshift distribution of a magnitude-limited sample and show the effects of marginalizing over these parameters in intrinsic alignment modeling. I forecast how this joint modeling approach affects cosmological parameter constraints, finding that it yields constraints comparable to standard analyses while mitigating potential biases from incorrectly fixed luminosity function parameters. I highlight the specific impact of the luminosity function shape on the cosmic shear data vector and discuss the method's potential for modeling generic selection functions and extending to a 3x2pt analysis with galaxy bias incorporation. While focused on LSST cosmic shear, this framework is broadly applicable to weak lensing surveys.

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Presented as part of the SciComm Collider 2 workshop.
All PI Residents are invited. 

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A colloquium featuring 2 perspectives on science communication.

Tatiana Erukhimova - Making Physics Fun and Approachable
Matthew Strassler - The Challenge of Communicating Physics Accessibly, Clearly and Accurately

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The correlation functions of large-scale fluctuations are crucial observables in modern cosmology. These boundary correlators contain rich information about the bulk dynamics and can be used to probe physics at the inflation scale. There have been considerable efforts in the analytical study of inflation correlators in recent years. In this talk, I will introduce the basic structure of inflation correlators and several analytical methods we have developed for their computation. In particular, I will introduce the method of partial Mellin-Barnes (PMB) representation. With this method, we can largely solve the massive tree correlators analytically. At the loop level, we use PMB to prove a factorization theorem for the nonanalytical part of inflation correlators, which produces a characteristic nonlocal signal. Finally, we can recover the full correlator starting from this nonanalytical part using a dispersion integral.

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I will review the "gyroscopic gravitational memory”, the permanent effect of gravitational waves on freely-falling gyroscopes far from the source of gravitational radiation. Then, I will compute the effect created by binary systems in the post-Newtonian approximation. The discussion naturally involves the helicity of gravitational waves and gravitational electric-magnetic duality.

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Presented as part of the SciComm Collider 2 workshop.
All PI Residents are invited. 

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While scientists and journalists have many overlapping interests and skills, they often differ in their goals, resources, and expertise. This talk covers strategies, tips, and approaches researchers can use to make their interactions with writers and journalists rewarding and enjoyable. It will also cover ideas about pitching and writing for popular science and mainstream media publications. This session will be as interactive as possible, so feel free to bring your questions, concerns, and ideas.

Speaker Bio:
Patchen Barss is a Toronto-based science journalist and author. He has designed and run many workshops and training sessions for scientists and other researchers engaging in media relations and public engagement.

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We will discuss work, joint with Victor Ginzburg, on the quantization (non-commutative deformation) of the Ngô morphism, a morphism of group schemes which plays a key role in Ngô’s proof of the fundamental lemma in the Langlands program. We will also discuss how the tools used to construct this morphism can be used to prove conjectures of Ben-Zvi—Gunningham, which predict that this morphism gives “spectral decomposition” of DG categories with an action of a reductive group over the coarse quotient of a maximal Cartan subalgebra by the affine Weyl group.

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