Theory Seminar

Seminars are at 2pm in Room 179 CP Building unless otherwise indicated.

The information paradox for cosmology

Abstract:

 

The information paradox demands a large departure from semiclassical physics at the black hole horizon. We will develop a similar paradox for cosmology, which will tell us that new physics has to emerge at the scale of the cosmological horizon. For the black hole, the novel effects emerge from the dynamics of vecros: virtual fluctuations of fuzzball-like configurations; the large action for these fluctuations is compensated by the large degeneracy of possible fluctuations. In cosmology, one finds a new variable required to define the spacetime: the vecro distribution function (VDF), which affects dynamics at the scale of the cosmological horizon. We speculate on how the VDF may address the cosmological constant problem, provide a source for inflaton energy and the dark energy today.

 
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Entanglement entropy of energy eigenstates follows a universal scaling function

We consider the entanglement entropies of energy eigenstates in quantum many-

body systems. For the typical models that allow for a field-theoretical

description of the long-range physics, we find that the entanglement entropy of

(almost) all eigenstates is described by a single scaling function. This is

predicated on the validity of the weak or strong eigenstate thermalization

hypothesis (ETH), which then implies that the scaling functions can be deduced

from subsystem entropies of thermal ensembles. The scaling functions describe

the full crossover from the groundstate entanglement regime for low energies

and small subsystem size (area or log-area law) to the extensive volume-law

regime for high energies or large subsystem size. For critical 1d systems, the

scaling function follows from conformal field theory (CFT). We use it to also

deduce the scaling function for Fermi liquids in d>1 dimensions. These

analytical results are complemented by numerics for large non-interacting

systems of fermions in d=1,2,3 and the harmonic lattice model in d=1,2.

Lastly, we demonstrate ETH for entanglement entropies and the validity of the

scaling arguments in integrable and non-integrable interacting spin chains. In

particular, we analyze the XXZ and transverse-field Ising models with and

without next-nearest-neighbor interactions.



References: arXiv:1905.07760, arXiv:1912.10045, arXiv:2010.07265

 

Recording: https://uky.zoom.us/rec/share/GutjuSkIJLS-UsSxmZ398q4iwfWDv9MGu4laXKP0arl0VZafKmv5N4TZDQfZcSJa.FukSni-Fex-uv3lI

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The Return of the String

Two applications of worldsheet string theory illustrate the structure of string theory near the threshold of black hole formation.  In the first, we consider string probes of horizonless microstate geometries and find that strong tidal forces cause infalling strings to become trapped, mimicking the absorptive properties of black holes.  In the second, we re-examine the structure of (near) BPS microstates and find a mechanism whereby the geometrical description breaks down. 

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Long Range Processes in QCD

A rich variety of phenomena in the Standard Model and its extensions manifest in long-range processes involving bound states of quantum chromodynamics (QCD), namely hadrons. These are processes where intermediate hadronic states propagate over a long distance, between electroweak interactions. Examples include virtual Compton scattering and double-beta decay. Such processes are at the cusp of what can be systematically studied given two challenges. First, these reactions involve hadrons, and as a result one must use a non-perturbative tool to access their amplitudes. Currently lattice QCD is the only systematically improvable way we have for doing just this. Second, lattice QCD is defined in a finite, Euclidean spacetime. This introduces its own specific challenges, time in purely imaginary in lattice QCD, and by truncating the space one looses the notion of asymptotic states. In this talk I explain how these issues can all be resolved systematically for a relatively large kinematic region. In presenting the necessary formalism for doing this, I will summarize recent progress in lattice QCD in order to argue that the community is up to the challenge. 
 
 
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Large N Expansion and Strings Out of Equilibrium.

We use the methods of large N expansion in non-equilibrium many-body systems with matrix degrees of freedom, to identify some universal features of the anticipated dual description in terms of non-equilibrium string perturbation theory.  We find that string worldsheets exhibit a triple decomposition, associated with the forward and backward branches of the Schinger-Keldysh time contour, and with the point at the "end of time" where the two branches meet.

Zoom link :https://uky.zoom.us/j/85746922492?pwd=d0hyUndXVEF6NUw0ejA4clVwVHFPQT09

Password: 693452

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Inside the Hologram: The bulk observer's experience

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I will present a holographic framework for reconstructing the experience of bulk observers in AdS/CFT. In particular, I will show how to recover the proper time and energy distribution measured along bulk worldlines, directly in the CFT via a universal, background-independent prescription. For an observer falling into an eternal AdS black hole, the proposal resolves a conceptual puzzle raised by Marolf and Wall. Notably, the prescription does not rely on an external dynamical Hamiltonian or the AdS boundary conditions and is, therefore, outlining a general framework for the emergence of time.

 

Recording: https://uky.zoom.us/rec/share/MNYWXm-2WNwA-aWN4jxOzjfQp5Kf3UkpZXL-awLVccEdkni4Rg3wA5UJR0gPoAJl.sihdRZAuILIOn9rO

 

 

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Quantum Codes and Conformal Field Theories

There is a rich web of connections between classical error-correcting codes, Euclidean lattices, and chiral conformal field theories.  I will explain an analogous relationship between quantum error-correcting codes, Lorentzian lattices, and nonchiral CFTs arising from toroidal compactifications of strings.  Specifically, I will show how quantum stabilizer codes are embedded in the operator algebras of associated Narain CFTs.   With respect to “code” CFTs, the constraints of modular invariance reduce to simple algebraic equations.  
Solving these equations provides many examples of physically distinct CFTs with the same spectrum; we also find many solutions that do not correspond to any code CFT, and likely do not correspond to any CFT at all.  This exposes a crucial limitation of the conformal bootstrap program – that in general, a solution of the bootstrap equations implies neither the existence nor the uniqueness of a corresponding CFT.  
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Machine learning as a discovery tool in hep-th

Machine learning provides a new tool for analyzing Big Data and Small Data in mathematics and theoretical physics. In this talk, I discuss two case studies. The first predicts the volume of the knot complement of hyperbolic knots from the Jones polynomial. The second predicts the masses of baryons such as the proton and neutron from knowledge only of the meson spectrum and distinguishes between different composition hypotheses for exotic QCD resonances. Both investigations point to the existence of new analytic formulae.
 
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Gravity from Averaging CFTs

Abstract:

 

I will explore the idea that certain theories of gravity in Anti-de

Sitter space are dual to an average over an ensemble of quantum

theories, rather than to a specific quantum theory.  I will describe

an average over Narain’s family of two-dimensional conformal field

theories which describe free bosons on a torus, and compute the

partition function using the Siegel-Weil formula.  The result takes

the form of a sum over geometries as one would expect in a theory of

gravity.  But the gravitational theory looks more like a Chern-Simons

theory than like Einstein gravity.

 

Zoom: https://uky.zoom.us/j/94475622713

 

Recording: https://uky.zoom.us/rec/share/QQujHqM5YhGd-8VMLiGw6DUPPftssuHC4vi3h_n-x27Jwbc-wBTtv5TIxvEdpxzg.EVGdktXNhdQIWDgV


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