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

The recent direct detection of gravitational waves (GWs) has opened a new window of observation for physical phenomena in which gravity is the dominant interaction. Collisions of black holes and neutron stars have been observed and a plethora of new events, even involving new physics, are expected to be detected in the next few years. It is natural to explore alternative physical objects that may exist which are different from the standard stars and black holes and that could lead to particular imprints on the GW spectrum. In this talk I will explore the possibility that moduli (gravitationally coupled scalar fields arising in all string compactifications) can compose star-like objects (moduli stars) whose dynamics gives rise to GW production. I will illustrate their formation in the early universe, that has been studied through lattice techniques. After their formation moduli stars can have different behaviours, e.g. they could rapidly collapse to black holes or disperse into scalar radiation, resulting in different phenomenology. I will describe the fate of moduli stars, that has been investigated through numerical relativity techniques. [please email a.held@imperial.ac.uk for zoom link or password]

The double copy in its original form allows us to obtain scattering amplitudes of gravity as the "square" of those of Yang Mills. I will explain how this squaring procedure works and how this relationship has been extended to classical solutions. I will also briefly mentioned how the double copy relationship can be extended to scalar effective field theories. [please email a.held@imperial.ac.uk for zoom link or password]

The integrable deformation of string backgrounds has been one of the significant research directions to construct a variety of examples of gauge/gravity duality preserving integrability and to push forward the confirmation of AdS/CFT correspondence. In this introductory talk, I plan to give a basic review of classical integrability of string sigma models with a focus on Lax pairs, to consider the role of the doubled sigma model. [please email a.held@imperial.ac.uk for zoom link or password]

Integrable models are the cornerstones of theoretical physics, simple enough to be solved exactly and yet rich enough to offer a glimpse into the dynamics of more intricate systems. Also two-dimensional field theories or string theories can be integrable. In this talk I will discuss integrability in field and string theories and present a special class of integrable models that promote the symmetry algebra of the undeformed theory to a quantum group. [please email a.held@imperial.ac.uk for zoom link or password]

I will discuss some aspects of supersymmetric systems from a computational perspective. [please email a.held@imperial.ac.uk for zoom link or password]

I will summarise my research interests and recent work, which is mainly related to the problems of field and gravity theories with extended symmetries. Such systems are the higher-spin and coloured gravity theories, theories of non-abelian chiral p-forms or partially-massless fields (satisfactory interacting theories of which are still missing). [please email a.held@imperial.ac.uk for zoom-link]

Description: Courant algebroids provide a natural framework for working with the low energy limits of string theory. In particular, they are well suited for the study of Poisson-Lie T-duality, which is a generalization of the usual T-duality. We will discuss how to see bosonic fields of type II supergravity as structures on Courant algebroids (generalized metrics, spinors and divergence operators). We will then construct appropriate curvature tensors and use them to prove the compatibility of Poisson-Lie T-duality and the equations of motion of supergravity. The framework also allows us to use algebraic methods to search for new (non-isometric) string backgrounds. This is a joint work with Pavol Å evera. [please email a.held@imperial.ac.uk for zoom-link]

We develop a universal approximation for the Renyi entropies of a pure state at late times in non-integrable systems, which macroscopically resembles an equilibrium density matrix. The resulting expressions are fully determined by properties of the associated equilibrium density matrix, and are hence independent of the details of the initial state, while also being manifestly consistent with unitary time-evolution. For equilibrated pure states in gravity systems, such as those involving black holes, this approximation gives a prescription for calculating entanglement entropies using Euclidean path integrals which is consistent with unitarity and hence can be used to address the information loss paradox of Hawking. Applied to recent models of evaporating black holes and eternal black holes coupled to baths, it provides a derivation of replica wormholes, and elucidates their mathematical and physical origins. In particular, it shows that replica wormholes can arise in a system with a fixed Hamiltonian, without the need for ensemble averages. ----- Follow the usual link or contact the organisers (Antoine Bourget and Edoardo Vescovi).

For the first time, a gravitational calculation was recently shown to yield the Page curve for the entropy of Hawking radiation, consistent with unitary evolution. However, the calculation takes as essential input Hawking's result that the radiation entropy becomes large at late times. We call this apparent contradiction the state paradox. We exhibit its manifestations in standard and doubly-holographic settings, with and without an external bath. We clarify which version(s) of the Ryu-Takayanagi prescription apply in each setting. We show that the two possible homology rules in the presence of a braneworld generate a bulk dual of the state paradox. The paradox is resolved if the gravitational path integral computes averaged quantities in a suitable ensemble of unitary theories, a possibility supported independently by several recent developments. ----- Follow the usual link or contact the organisers (Antoine Bourget and Edoardo Vescovi).

I will highlight the role of wormholes and branes in reconciling semiclassical gravitational (black hole) physics with unitary quantum mechanical evolution. Most of the talk will be based on recent developments in JT gravity. This is a model of quantum gravity in two dimensions which is analytically tractable. I will first explain why we need wormholes in order to capture late time properties of chaotic quantum systems from a geometrical point of view. I then point out that naively the inclusion of wormholes in a gravitational theory comes with an ensemble interpretation of said gravitational theory. Finally I will explain how this conclusion (that gravity is an ensemble) can be avoided by including branes in the bulk geometrical description. These branes can encode the micro structure of a given unitary quantum system in bulk geometry. ----- Follow the usual link or contact the organisers (Antoine Bourget and Edoardo Vescovi).

Defects/interfaces/boundaries are interesting objects to study in QFT, and one powerful way to study them is via the use of holography. In this talk, I will discuss our construction of gravitational solutions that holographically describe two different 4d SCFTs joined together at a co-dimension one, planar RG interface and preserving 3d superconformal symmetry. The RG interface we have constructed joins the 4d N=4 SYM theory on one side with the N=1 Leigh-Strassler SCFT on the other. These solutions in general are associated with spatially dependent mass deformations on the N=4 SYM side, but there is a particularly interesting solution for which these deformations vanish. If time allows, I will also discuss another example of our work involving ABJM theory and two 3d N=1 SCFTs with G_2 symmetry. This talk is based on the work hep-th/2007.07891 with Igal Arav, Jerome Gauntlett, Matt Roberts and Chris Rosen. ----- Follow the usual link or contact the organisers (Antoine Bourget and Edoardo Vescovi).