Quantum Fields and Strings

The search for pragmatic observables of quantum gravity remains at the forefront of fundamental physics research. A large set of ideas collectively known as the gauge-gravity duality have proven fruitful in tackling this problem. While such a duality is believed to universally govern gravitational theories, its nature in theories of gravity that describe our universe to a good degree of approximation is still little understood.

In this talk I will discuss efforts in formulating a holographic correspondence for gravity in four-dimensional asymptotically flat spacetimes. The proposed dual theory lives on a two-dimensional celestial sphere at infinity and is constrained by a wide range of symmetries. I present recent evidence for this proposal by showing that it arises naturally in a flat space limit of AdS/CFT. I will illustrate this construction with two related examples: the propagation of a particle in a shockwave background and the high-energy scattering of 2 particles.

Zoom Link: https://pitp.zoom.us/j/97426266597?pwd=UmhncFR6NExFVzI2SlJwdE1LeUlIQT09

A black hole quantum state (Hartle-Hawking or Boulware)  is usually defined in a fixed background of a classical black hole. In my talk I will discuss the corresponding space-time geometry when the back-reaction is taken into account. The important questions include: does the back-reacted geometry always contain a horizon?; how it depends on the choice of the quantum state?;  and what is the right choice for the quantum state for the non-physical fields such as ghosts? I will answer these and other questions in the context of a two-dimensional dilaton gravity. The talk is based on a joint recent work with D. Sarkar and Y. Potaux, arXiv:2112.03855.

Zoom Link: https://pitp.zoom.us/j/5053597405?pwd=YjU2VVF1L3A1dmR1VWpPMHZFQW9rdz09

Three dimensional conformal field theories(CFTs) describe important critical physics in the real world. In the past few years much progress has been made in 3D CFTs using the conformal bootstrap. In particular, using numerical bootstrap, the critical exponents of the 3D Ising, Super-Ising, O(2), O(3) CFTs have been determined precisely with rigorous error bars, while using analytic bootstrap, the information of the leading twist operators at large spins can be computed analytically. The two bootstrap approaches are sensitive to different regions of the spectrum and complement each other. In this talk, I will first give a general review of the numerical bootstrap and analytic bootstrap. Then I will discuss how to combine the numerical bootstrap with the analytic lightcone bootstrap, i.e. a hybrid bootstrap method. As a result, the bootstrap prediction for the actual CFT can be significantly improved.

Zoom Link: https://pitp.zoom.us/j/94186668943?pwd=ejRRb0M4VXNjeVVCWGdnTHQyaWRyQT09

There are two different standard ways of endowing a physical theory with a symplectic structure: the canonical and the covariant. The former is derived from the well-known symplectic structure of a certain cotangent bundle. The latter is based on the variational calculus. Including a boundary in the canonical formalism poses no problem, however, in the covariant formalism things break apart. In this talk, I will briefly introduce both formalisms without boundary and explain in detail a new framework that allows us to include boundaries in a straightforward way. To show it in action, time permitting, I will apply it to several theories of gravity. Finally, I will briefly show a new result where we proved that, actually, the canonical and covariant formalisms are equivalent in full generality.

Zoom Link: https://pitp.zoom.us/j/97641588140?pwd=dFl0SFdHOG5BYko2djNVWk11UkhSUT09

Holographic complexity proposals are interesting because, on one hand, they express universal properties of black hole interiors and, on the other, they go beyond the area-centric view on quantum gravity. Our best take on complexity in quantum field theory is based on assigning a cost to circuits generated by time-dependent Hamiltonians and its minimization. I will discuss recent progress on understanding how the relevant circuits are realized in holography and what are possible gravity duals to good costs. Based on 2112.12158, 2203.08842 and work in progress.

Zoom Link: https://pitp.zoom.us/j/96487059058?pwd=ZHk1ZHpXMEZoT0UycUZqTEdIb1ZpQT09

Higher dimensional models of black hole evaporation can be studied via double holographic constructions, which can be considered as holographic models of conformal interfaces. In string theory these are realized microscopically by appropriate systems of branes, and in particular limits they give rise to a light graviton mass in the effective gravity theory. For a specific model of D3-D5-NS5 branes in type IIB, I will discuss the properties of the lightest graviton and show how this constrains the existence of islands in Anti de Sitter. I will show a numerical study of islands surfaces for finite temperature black holes, and show how they are affected by a varying dilaton. I will finally comment on the EFT cutoff induced by a light graviton mass, in relation to the islands regime, in function of the microscopic parameters of the string theory construction.

Zoom Link: https://pitp.zoom.us/j/97993595112?pwd=dE1xRm42MmY3bEVYZ2c4VURtaWV4dz09

Based on 2203.09537. In the context of toy models of holography arising from 3d Chern-Simons theory, I will describe an approach in which, rather than summing over bulk geometries, one gauges a one-form global symmetry of the bulk theory. This ensures that the bulk theory has no global symmetries, and it makes the partition function on spacetimes with boundaries coincide with that of a modular-invariant 2d CFT on the boundary. In particular, on wormhole geometries one finds a factorized answer for the partition function.

Zoom Link: https://pitp.zoom.us/j/91694973383?pwd=VWFCZWxtdy9GczUrN1JFc2xMVUhmdz09

I will discuss the large-N limit of two-dimensional symmetric product orbifolds. The goal is to single out which symmetric product orbifold theory could lead to a strongly coupled theory, whose dual could be a semi-classical theory of AdS_3 gravity, by quantifying the large-N limit of the BPS spectrum and the behaviour under exactly marginal deformations. From this analysis, I will propose an infinite family of new holographic CFTs.

Zoom Link: https://pitp.zoom.us/j/99948208493?pwd=RDRYclc2a05kMmNZL3ZhMWxMdWdNUT09

We suggest a framework for cosmology based on gravitational effective field theories with a negative fundamental cosmological constant, which may exhibit accelerated expansion due to the positive potential energy of rolling scalar fields. The framework postulates an exact time-reversal symmetry of the quantum state (with a time-symmetric big bang / big crunch background cosmology) and an analyticity property that relates cosmological observables to observables in a Euclidean gravitational theory defined with a pair of asymptotically AdS planar boundaries. The latter can be given a microscopic definition using holography, so the model is UV complete. While it is not yet clear whether the framework can give realistic predictions, it has the potential to resolve various naturalness puzzles without the need for inflation.

Zoom Link: https://pitp.zoom.us/j/95099004342?pwd=OWZoanJJMHNzMXF4S3VYM2Vxcklodz09

The volume of the interior of a two-sided eternal black hole classically grows forever. I will derive a microscopic formula for this volume in JT gravity by summing the non-perturbative contribution of higher topologies. The non-perturbative corrections lead to the saturation of the volume of the interior at times exponential in the entropy of the black hole. I will connect the microscopic formula for the volume with properties of a "thermo-averaged" density matrix, in particular with its second Renyi entropy, which I argue to measure the number of nearly orthogonal states visited by time evolution. I will discuss various problems with this interpretation.

Zoom Link: https://pitp.zoom.us/j/99880539557?pwd=VWZiMFI5Rk5pdnNZRE5kalRvUk1Zdz09

JT gravity in AdS was shown by Saad, Shenker and Stanford to be described by a matrix ensemble of random hamiltonians. We couple JT to a bulk scalar field and extend the matrix ensemble to include a second matrix, dual to the scalar field. We therefore consider a 2-matrix model that can be thought of as a (better defined) generalization of Eigenstate Thermalization Hypotheses: it is a coupled matrix model of a random hamiltonian and a random operator. The 2-matrix model has an interesting integrability structure: correlation functions are expressed via SL(2,R) 6j-symbols that obey unlacing rules and Yang-Baxter equations. We compute the two-sided 2-point function on the double-trumpet geometry from the matrix model and find agreement with the matter loop contribution in the bulk. Based on work in progress with Jafferis, Kolchmeyer and Sonner.

Zoom Link: https://pitp.zoom.us/j/92268935307?pwd=dytCdTJlWVc3QXkweWxzcy83Sk9PZz09

I will describe 4pt conformal blocks for scalar operators in diverse dimensions by using a single unified formalism, and explain a property of the conformal blocks known as stability. This stability implies that when writing conformal blocks as certain multivariate series, the coefficients of this expansion only depend on Young diagrams. In particular, we can bosonise the conformal computation and simply focus on compact groups. In this framework the blocks are polynomials of the BC root system after we apply complementation. I will then explain the connection with mathematics and show how to formulate a superconformal Cauchy identity which yields the CPW of any free theory diagram in any dimension. For discussion, I will finally mention q-deformations results.

Zoom Link: https://pitp.zoom.us/j/96912692269?pwd=TE8vT01mam9lQjhVV3VSYUtWR2d5Zz09