We study the N = 4 SU(N) super-Yang-Mills stress tensor multiplet four-point function at large N and finite complexified Yang-Mills coupling tau, which is dual to the Type IIB graviton correlator on AdS_5 × S^5 at large string length and finite string coupling. The specific four-point functions we consider are integrated correlators obtained by taking various combinations of four derivatives of m, b, and tau of the sphere free energy deformed by mass m and squashing parameter b, which can be computed using supersymmetric localization. We show that at each order in 1/N, these quantities can be written in terms of modular invariants, such as the well studied non-Holomorphic Eisenstein series as well as some new generalizations thereof. These results reproduce known features of the low-energy expansion of the four-graviton amplitude in type IIB superstring theory in ten-dimensional flat space, which is the first check of AdS/CFT at finite string coupling, and have interesting implications for the structure of the analogous expansion in AdS_5 × S^5. Zoom Join Zoom Meeting https://zoom.us/j/93725965823?pwd=Q2lmeEhjQnJmZUsxMkp2THdVZ1cxUT09 Meeting ID: 937 2596 5823 Passcode: 640955

AdS/CFT provides a consistent non-perturbative definition of quantum gravity in asymptotically AdS spacetimes. Black holes correspond to ensembles of states in the boundary field theory. In the presence of supersymmetry, we rephrase the problem of counting those states in terms of a supersymmetric partition function: the superconformal index. By performing a careful analysis of the index of 4d N=4 SU(N) Super-Yang-Mills theory, with the help of a Bethe Ansatz approach, we are able to exactly reproduce the Bekenstein-Hawking entropy of BPS black holes in AdS5 x S5. The large N limit exhibits many competing contributions, that we are able to identify with complex saddles of the (putative) gravitational path-integral. Along the way we propose a necessary condition for complex saddles to contribute, based on the size of their non-perturbative corrections.

We are faced with an explosion of data in many areas of physics, but very so often, it is not the size but the complexity of the data that makes extracting physics from big datasets challenging. As I will discuss in this talk, data has shape and the shape of data encodes the underlying physics. Persistent homology is a tool in computational topology developed for quantifying the shape of data. I will discuss three applications of topological data analysis: 1) identifying structure of the string landscape, 2) constraining primordial non-Gaussianity from CMB measurements and large scale structures data, and 3) detecting and classifying phases of matter. Persistent homology condenses these datasets into their most relevant (and interpretable) features, so that simple statistical pipelines are sufficient in these contexts. This suggests that TDA can be used in conjunction with machine learning algorithms and improves their architecture. Based on https://arxiv.org/abs/2009.14231, https://arxiv.org/abs/2009.04819, https://arxiv.org/abs/1907.10072, https://arxiv.org/abs/1812.06960, https://arxiv.org/abs/1712.08159. [please email a.held@imperial.ac.uk for zoom link or password]

The most famous slogan of the Swampland program is that not all effective field theories (EFTs) admit ultraviolet completion into quantum gravity. In this talk, we focus on the significance of this claim for EFTs which involve scalar field variations and on the implications for cosmology. To conclude, we examine some concrete string-phenomenological set-ups with warped throats. [please email a.held@imperial.ac.uk for zoom link or password]

``We explore the construction and stability of asymptotically anti-de Sitter Euclidean wormholes in a variety of models. In simple ad hoc low-energy models, it is not hard to construct two-boundary Euclidean wormholes that dominate over disconnected solutions and which are stable (lacking negative modes) in the usual sense of Euclidean quantum gravity. Indeed, the structure of such solutions turns out to strongly resemble that of the Hawking-Page phase transition for AdS-Schwarzschild black holes, in that for boundary sources above some threshold we find both a `large' and a `small' branch of wormhole solutions with the latter being stable and dominating over the disconnected solution for large enough sources. We are also able to construct two-boundary Euclidean wormholes in a variety of string compactifications with a similar that dominate over the disconnected solutions we find and that are stable with respect to field-theoretic perturbations. However, as in classic examples investigated by Maldacena and Maoz, the wormholes in these UV-complete settings always suffer from brane-nucleation instabilities (even when sources that one might hope would stabilize such instabilities are tuned to large values). This indicates the existence of additional disconnected solutions with lower action. We discuss the significance of such results for the factorization problem of AdS/CFT.’' Join Zoom Meeting https://zoom.us/j/98062076339?pwd=aGJNUTBTNjBYeDhqUlZVMzdVWkhGQT09 Meeting ID: 980 6207 6339 Passcode: 913115

The search for extensions to GR, has prompted the developments of many potential options which, to varying degrees, have been studied and confronted with observations in special regimes --typically linear ones with respect to rather 'simple' scenarios. Gravitational wave astronomy provides the opportunity to confront them in a much richer and complex regime which unearths significant roadblocks that obstruct the definition and/or study of consequences of many of such extensions. This talk will review some of these problems, illustrate consequences and discuss a potential way through for studying the potential role such extensions might have in the strongly gravitating/non-linear regime. [please email a.held@imperial.ac.uk for zoom link or password]

Cosmic strings arise as remnants of phase transitions in the early Universe, often related to theories of grand unification (GUTs). If such a phase transitions occurs at high energies, the resulting cosmic string network generates a sizable amount of gravitational waves. Most work so far has focused on the gravitational wave signal from topologically stable cosmic strings. In this talk I will introduce metastable cosmic strings, which are a generic consequence of many GUTs. I will discuss how this idea can be probed in various ongoing and upcoming gravitational wave experiments, from pulsar timing arrays to space- and ground-based interferometers. In the final part of my talk I will discuss a recent proposal on using the radio telescopes to probe this and other sources of ultra high frequency gravitational waves. [please email a.held@imperial.ac.uk for zoom link or password]

I will perform a consistent reduction of 6d matter-coupled F(4) gauged supergravity on a compact Riemann surface of genus g, at the level of the bosonic fields. The result is an N=2 gauged supergravity coupled to two vector multiplets and a single hypermultiplet. The four-dimensional model is holographically dual to the 3d superconformal field theories of class F, describing different brane systems in massive type IIA and IIB wrapped on the Riemann Surface. I will then use the results to find the first examples of dyonic static black holes of class F and perform a microscopic counting of their entropy via the 5d topologically twisted index. I will also find new asymptotically AdS solutions, including rotating black holes, in the subtruncations of the 4d model, providing predictions for the squashed S^3 partition functions and the superconformal and refined twisted indices of class F field theories. Join Zoom Meeting https://zoom.us/j/92805841030?pwd=dTVGQ2h5ZUw4cndKd3JUYTNYVTMrdz09 Meeting ID: 928 0584 1030 Passcode: 902526

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The n-point tachyon amplitude is a classic and well-known result in bosonic string theory. Rather than studying tachyon amplitudes, we will compute scattering amplitudes involving highly excited string states. Along the way we review and elaborate on the DDF construction of string vertex operators, and describe properties of a generic excited string. Based on work in progress with D. Gross.

Recently, theories with higher rank gauge symmetries are drawing attention in condensed matter physics. Such theories are thought to be related to the so-called "fractons," quasiparticles with limited mobility that appear in some lattice models. We show that one version of such higher ranked symmetry can be "nonlinearlized" into volume-preserving diffeomorphism. We demonstrate that several condensed matter systems, including the fractional quantum Hall effect and ferromagnetism, secretly possess this symmetry, which sheds another light to some peculiar features of the quasiparticles in these systems. [please email a.held@imperial.ac.uk for zoom link or password]

Long-lived, spatially-localized and coherently oscillating field configurations (pseudo-solitons, for example, axion stars and oscillons) can form naturally in cosmologically relevant axion-like fields constituting dark matter today or in fields at the end of inflation. When these clumps collide and merge, they can give rise to bursts of electromagnetic and gravitational waves. In this talk, I will first discuss earlier work on soliton formation mechanisms and their interactions, as well as gravitational wave production. Then, for the main part of the talk, I will focus on the production of potentially detectable electromagnetic bursts resulting from the merger of dense scalar field oscillons/axion stars in the current universe [this is based on our recent paper, https://arxiv.org/abs/2009.11337]. Time permitting, I will also discuss possible connections to Fast Radio Bursts. [please email a.held@imperial.ac.uk for zoom link or password]

Recent developments on the black hole information paradox have shown that Euclidean wormholes — so called “replica wormholes’’ can dominate the von Neumann entropy as computed by a gravitational path integral, and that inclusion of these wormholes results in a unitary Page curve. This development raises some puzzles from the perspective of factorization, and has raised questions regarding what the gravitational path integral is computing. In this talk, I will focus on understanding the relationship between the gravitational path integral and the partition function via the gravitational free energy (more generally the generating functional). A proper computation of the free energy requires a replica trick distinct from the usual one used to compute the entropy. I will show that in JT gravity there is a regime where the free energy computed without replica wormholes is pathological. Interestingly, the inclusion of replica wormholes is not quite sufficient to resolve the pathology: an alternative analytic continuation is required. I will discuss the implications of this for various interpretations of the gravitational path integral (e.g. as computing an ensemble average) and also mention some parallels with spin glasses. [please email a.held@imperial.ac.uk for zoom link or password]

Higher-Spin/CFT dualities are a promising playground to unravel the nuts and bolts of the AdS/CFT correspondence away from supersymmetry, as the bulk and boundary descriptions are simultaneously perturbative. In this talk we demonstrate how the free CFT three-point correlator involving two scalars and one conserved current of arbitrary spin organizes itself into the dual AdS amplitude. Techniques from the embedding space formalism for AdS and CFT, as well as an insightful variation of Schwinger parametrization, proposed in hep-th/0308184 play an important role. The talk will be based on arXiv:2009.10110. Join Zoom Meeting https://zoom.us/j/3515880979?pwd=MWU0V1ZOUzJ1eUozL1Vkb1RtbTZhZz09 Meeting ID: 351 588 0979 Passcode: the T word in AdS/CFT

In this talk I will sketch how we can test gravity with unprecedented precision by combining new gravitational wave measurements, observational bounds from cosmology, and novel particle theory inspired insights, in particular allowing us to zoom in on the nature of dark energy. In doing so, I will highlight the interplay between gravitational wave constraints on gravity post-GW170817, data constraints from cosmic microwave background and galaxy clustering measurements, and novel theoretical priors on such theories (ranging from purely classical stability requirements to radiative stability- and positivity-related priors). [please email a.held@imperial.ac.uk for zoom link or password]

This is the live session included as part of the LonTI lecture on Branes and the moduli space of instantons. Please register at https://lonti.weebly.com/registration.html to receive joining instructions for this live session which will be held via Zoom. Instantons, or solutions to the self dual Yang Mills (SDYM) equations are well known solutions, introduced in the mid 70s and played a role in a host of applications in QFT and String Theory. This talk will show a simple brane construction which allows the computation of the moduli space of solutions to the SDYM equations, introduce the student to the world of quivers, and demonstrate simple computations which allow evaluations of these moduli spaces. ​

I will show that metastable higher spin particles, free or interacting, cannot couple to gravity while preserving causality unless there exist higher spin states in the gravitational sector much below the Planck scale. Causality imposes an upper bound on the mass of the lightest higher spin particle in the gravity sector in terms of quantities in the non-gravitational sector. I will argue that any weakly coupled UV completion of such a theory must have a gravity sector containing infinite towers of asymptotically parallel, equispaced, and linear Regge trajectories. This implies that the gravity sector has a stringy structure with an upper bound on the string scale. Another consequence of this bound is that all metastable higher spin particles in 4d with masses below the string scale must satisfy a weak gravity condition. Moreover, these bounds also have surprising implications for large N QCD coupled to gravity and cosmology. [please email a.held@imperial.ac.uk for zoom link or password]

I will give a (biased) review on the main current ideas to explain the fundamental nature of Dark Energy using string theory. The possibility of a de Sitter vacuum in string theory, corresponding to a cosmological constant with w=-1, has proven difficult to achieve, leading to the conjecture that such vacua might lie in the String Theory Swampland. Other interesting string candidates include axion or runaway quintessence. I will in particular discuss the Thermal Dark Energy proposal. Here finite temperature effects in a light hidden sector hold a hidden scalar away from the minimum of its zero-temperature potential, leading to an effective cosmological constant, consistently with the Swampland Conjectures and with potentially observable consequences. [please email a.held@imperial.ac.uk for zoom link or password]

Jackiw–Teitelboim (JT) gravity and supergravity theories are exciting solvable models that teach us about low temperature black hole dynamics, and aspects of quantum chaotic behaviour. They have been shown (by Saad, Shenker and Stanford, and by Stanford and Witten) to have double scaled random matrix model descriptions, capturing the (spacetime) topological perturbative expansion of the partition function. I will describe an alternative method for building the matrix model description, using techniques from minimal string theory. This method is particularly useful for supplying non-perturbative definitions of the physics. I show how this allows for the computation of key aspects of the physics at low energy and temperature. For the zoom link, please email antoine.brgt@gmail.com