Search Talks

Several years ago, Nayak and Else argued that Symmetry Protected Topological phases in d dimensions can be classified using non-on-site actions of the symmetry group in d-1 dimensions. Such non-on-site actions can have an “anomaly”, in the sense that the symmetry action cannot be consistently localized. This anomaly is similar but distinct from ’t Hooft anomaly in QFT. Nayak and Else assumed that the symmetry group is finite and the non-on-site action is given by a finite-depth local unitary circuit. I will explain how to generalize the construction of the anomaly index in two directions: to Lie groups as well as to arbitrary actions which preserve locality. For simplicity, I will only discuss the one-dimensional case. One can prove that a nonzero anomaly index prohibits any invariant 1d Hamiltonian from having invariant ground states. This is similar to ’t Hooft anomaly matching in QFT. Lieb-Schultz-Mattis-type theorems arise as a special case where the symmetry group involves translations. This is joint work with Nikita Sopenko.

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Zoom link https://pitp.zoom.us/j/95661517248?pwd=SkMxUFJWTG56SG9hVlNiNS9yeEVrQT09

I will explore subtle aspects of rank-one 4d N=2 supersymmetric QFTs through their low-energy Coulomb-branch physics. The low-energy Lagrangian is famously encoded in "the Seiberg-Witten (SW) curve", which is a one-parameter family of elliptic curves. Here I will explain precisely how "global" aspects of the SQFT such as its spectrum of lines, which cannot be read off from the Lagrangian, are encoded into the SW curve -- more precisely, how they are encoded in its Mordell-Weil group of rational sections. In particular, I will
discuss in detail the difference between the pure SU(2) and the pure SO(3) N=2 SYM theories from this low-energy perspective. I will also comment on the global forms of rank-one 5d SCFTs compactified on a circle.

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Zoom link https://pitp.zoom.us/j/94967350368?pwd=SHZZUnZNL3ZveFc3NElCaVc5YkY3Zz09

We investigate the nature of the Giddings-Strominger wormhole in axion gravity, whose stability remains contested despite previous work in this direction. Unlike what is done in these works, we follow a Lorentzian approach which directly addresses the factorization problem. To probe the thermodynamic stability of wormholes in the gravitational path integral, we look for fixed-area saddle points. However, taking care to choose the appropriate boundary conditions, we find that there are no fixed-area axion wormholes in Lorentzian signature. We then discuss how we could go beyond this analysis, considering off-shell axion wormhole configurations with fixed-length.

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Zoom link https://pitp.zoom.us/j/98119789932?pwd=N1RCanJCdk56eWJ3RHJCRG5KU21lQT09

Spacetime inversion symmetries such as parity and time reversal play a central role in physics, but they are usually treated as global symmetries. In quantum gravity there are no global symmetries, so any spacetime inversion symmetries must be gauge symmetries. In particular this includes CRT symmetry (in even
dimensions usually combined with a rotation to become CPT), which in quantum field theory is always a symmetry and seems likely to be a symmetry of quantum gravity as well. I'll discuss what it means to gauge a spacetime inversion symmetry, and explain some of the more unusual consequences of doing this. In particular I'll argue that the gauging of CRT is automatically implemented by the sum over topologies in the Euclidean gravity path integral, that in a closed universe the Hilbert space of quantum gravity must be a real vector space, and that in Lorentzian signature manifolds which are not time-orientable must be included as valid configurations of the theory.

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Zoom link https://pitp.zoom.us/j/98089579253?pwd=NDBhUjBUTTEwT0R5YmhYVElZenJtZz09

Using a novel version of the gravitational path integral for compact spatial regions at a moment of time symmetry, I argue that a region of space can encode a larger one. In particular, I show that the entanglement entropy of a region of space equals the area of the boundary of the smallest region that contains it. The key insight is to include the effects of the gravitational edge modes associated with the region in the path integral. This result is consistent with a recent conjecture by Bousso and Penington.

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Zoom link https://pitp.zoom.us/j/93301151464?pwd=Z2t5QlpUQ3hoaEkwQlFZS2tITGpEQT09

The cosmic microwave background is a sensitive probe of early-Universe physics, and yet fundamental constants at recombination can differ from their present-day values due to degeneracies in the standard cosmological model. Such scenarios have been invoked to reconcile discrepant measurements of the present-day expansion rate, but even absent such motivation they raise the intriguing possibility of yet-undiscovered physics coupled directly to Standard Model particles. I will discuss theories in which a new scalar field shifts the electron's mass at early times; viable models are already stringently constrained by measurements of quasar absorption lines, the abundances of light elements, and the universality of free fall. I will show that the remaining parameter space is exactly that which allows not only the primary cosmic microwave background but also low-redshift distances to be consistent with observations. After presenting the results of parameter inference I will discuss additional cosmological and laboratory signatures of the model.

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Zoom link https://pitp.zoom.us/j/99705853481?pwd=MTZRWC9hREkvOXpiZkxCM3UvdnRNQT09