The seminar has been cancelled.

We consider graviton scattering in maximal supergravity on Anti-de Sitter space (AdS) in d+1 dimensions for d=3,4,and 6 with no extra compact spacetime factor. Holography suggests that this theory is dual to an exotic maximally supersymmetric conformal field theory (CFT) in d dimensions whose only light single trace operator is the stress tensor. This contrasts with more standard cases like Type IIB string theory on AdS_5x S^5 dual to N=4 Super-Yang-Mills, where the CFT has light single trace operators for each Kaluza-Klein mode on S^5. We compute the 1-loop correction to the pure AdS_{d+1} theory in a small Planck length expansion, which is dual to the large central charge expansion in the CFT. We find that this correction saturates the most general non-perturbative conformal bootstrap bounds on this correlator in the large central charge regime for d=3,4,6. After imposing theory-specific constraints from localization in d=3,4, the bootstrap constraints strengthen and are then saturated by the string/M-theory dual CFT data.

We will review a recent application of Exceptional Field Theory : finding new families of solutions of type IIB supergravity on AdS_4 x S^1 x S^5. To find such solutions, we will compactify type IIB on S^5 x S^1 to obtain a 4d gauged maximal supergravity where new solutions are simply found by extremising a scalar potential. Surprisingly, it is sometime possible to deform our new solutions and break any residual supersymmetry while preserving stability. This is surprising since, from a holographic perspective, these deformations should be dual to non-supersymmetric exactly marginal deformations. We will show that it is a generic behaviour of gravity theories compactified on a circle and we will provide a solution generating technique in terms of a tool called the mapping torus.

In my talk I will review the recent progress in non-local infinite derivative gravity theories. The core of the models under investigation is the infinite derivative generalization of the quadratic gravity theory which was first studied in depth by Stelle in 1977. I will explain why unitarity can only be restored for an infinite tower of derivatives. The rest of the talk will concentrate on the cosmological consequences of this non-local gravity proposal for the Starobinsky inflation.

I will discuss the large N behavior of partition functions of the ABJM theory on compact Euclidean manifolds. I will pay particular attention to the S^3 free energy and the topologically twisted index for which I will present closed form expressions valid to all orders in the large N expansion. These results have important implications for holography and the microscopic entropy counting of AdS_4 black holes which I will discuss. I will also briefly discuss generalizations to other SCFTs arising from M2-branes.

Quantum field theory of higher-spin particles is a formidable subject, where Lorentz-invariant approaches tend to lead to a rich gauge-symmetry structure, which serves to preserve the physical number of degrees of freedom. Introducing consistent interactions in such approaches is a non-trivial task, with most higher-spin Lagrangians specified only up to three points. In this talk, I will discuss a new, chiral description for massive higher-spin particles, which in four spacetime dimensions allows to do away with this kind of gauge symmetry. This greatly facilitates the introduction of consistent interactions. I will concentrate on three theories, in which higher-spin matter is coupled to electrodynamics, non-Abelian gauge theory or gravity. These theories are currently the only examples of consistent interacting field theories with massive higher-spin fields. The presented theories are chiral and have simple Lagrangians, resulting in Feynman rules analogous to those of massive scalars. In particular, I will discuss the resulting tree-level scattering amplitudes with two higher-spin matter particles and any number of positive-helicity photons, gluons or gravitons. These amplitudes were previously computed via on-shell recursion and provided evidence for the existence of such simple massive higher-spin theories.

The fundamental theory of quantum gravity is expected to manifest itself at low energies via a series of higher-derivative corrections to Einstein’s theory. Holography and supersymmetry are of great help to characterize such corrections in controlled scenarios: through holography, quantum gravity in Anti de Sitter (AdS) space has a rigorous definition in terms of a conformal field theory (CFT), while supersymmetry makes it possible to compute exact observables and make quantitative predictions. In this context, we will illustrate how a CFT generating function known as the superconformal index provides a microscopic explanation of the entropy of five-dimensional supersymmetric black holes in AdS. We will show how this match goes beyond the leading Bekenstein-Hawking term and includes higher-derivative corrections.

Matrix theory is a proposed non-perturbative definition of superstring theory in which space is emergent. Recently, it was shown that a 4-dimensional expanding universe can emerge in the IKKT matrix model, with another 6 spatial dimensions stabilized at the string scale. This scenario was also explored in the BFSS model, in which case the emerging phase yields a scale-invariant spectrum of scalar and tensor perturbation. In this talk, we will discuss recent progress in understanding these results. More precisely, we will discuss a possible way of obtaining the metric out of the matrices in the IKKT model, and ways to probe symmetry breaking in the BFSS model.

Second order thermal phase transitions are driven my an order parameter which comes with an amplitude. Fluctuations of this amplitude lead to a slowly decaying mode whose gap closes to zero at the critical point. I will use holographic techniques to discuss in detail how this gapped mode determines the linear response of scalar operators close to the phase transition and give a geometric expression for the dissipative coefficient that fixes the relevant Green’s functions.

In recent years, it has been understood how local operators do not paint the entire picture of a quantum field theory, but we need to introduce extended operators to understand finer details about these theories. Motivated by this, we investigate a subset of these extended operators in particular in the context of 4d N=2 superconformal field theories. I will start by introducing the setup that we use to describe the possible configurations of these extended operators. I will also compare and contrast the operators that appear in our framework with the more familiar (Wilson and 't Hooft) line operators. Time permitting, I will then introduce twists of these theories by a choice of an appropriate nilpotent supercharge. The restriction to (extended) operators living in the cohomology of this supercharge gives rise to interesting algebraic structures, that are analogous to, or rather an extension of, the 2d vertex operator algebras that have now become familiar familiar in the context of 4d N=2 theories.

In this talk we will introduce CosmoLattice, a modern code for simulating the non-linear dynamics of interactive scalar-gauge theories in an expanding universe. As a demonstration of its power we will solve three very different problems of early Universe cosmology: i) the generation and use of gravitational waves as a probe of particle couplings, ii) the dynamics of non-minimally coupled scalar fields in the Jordan frame, and iii) the non-linear dynamics of helical gauge field production during the last efoldings of axion-inflation

Massive fields oscillating around their minima produce specific oscillatory patterns in the density perturbations that record the time dependence of the scale factor in the primordial Universe (hence the name 'Primordial Standard Clocks'). In this talk, I present recent developments on the model building of inflationary classical primordial standard clocks (CPSC) and emphasize their observable consequences. I then present constraints on these models from the latest Planck temperature and polarization data and show how CPSC can address anomalies at different multipoles. Finally, I discuss the prospects for detecting CPSCs with upcoming CMB missions and to distinguish them from other types of features.

The string landscape, together with the mechanism of eternal inflation for populating vacua, leads to the seemingly inescapable conclusion that we are part of a vast multiverse. As an inhabitant of the multiverse, how should we reason probabilistically about the expected physical properties of our observable universe? Probabilities in eternal inflation are usually defined in terms of frequencies, but this approach has failed to yield a unique answer. In this talk, I will present a different approach to the problem, based on Bayesian reasoning. By adopting the least informative priors on the model parameters, we will be led to well-defined, time-reparametrization invariant (and non-anthropic) probabilities for occupying different vacua. Remarkably, these probabilities favor vacua whose surrounding landscape topography is that of a deep valley or funnel, akin to folding funnels of naturally-occurring proteins. Furthermore, by modeling the landscape of vacua as a random network, I will show that our probabilities favor regions that are close to the directed percolation phase transition. As usual, the predictive power of criticality lies in scale invariant observables characterized by critical exponents. As an example, I will show that the probability distribution for the cosmological constant is power-law, with a particular critical exponent, and favors a naturally small and positive vacuum energy. Tantalizingly, this hints at a deep connection between non-equilibrium critical phenomena on the landscape and the near-criticality of our universe.

I will discuss different theoretical and phenomenological challenges of quintessence model building in string theory in order to be able to give an informed answer to the question in the title.

I can formulate the purpose of my research as an attempt to look to the quantum gravity through both the theoretical and observational windows. My previous research was mainly dedicated to obtaining both observational predictions and theoretical constraints for the models of inflation which can be stated as 'minimal'. They contain only the fields which are proved to exist: Higgs field and gravity. As the results of this work, we proved the self-consistency and investigated the reheating mechanism in a model describing inflation driven by the interplay of Higgs and gravity. My recent research is more focused on the theoretical window to the nature of quantum gravity based on the scattering amplitudes and dispersive relations. The scattering amplitudes through the graviton exchange contain the IR singularities in forward limit. The divergences at $t\rightarrow 0$ can be cancelled in the dispersive relation only if the UV limit of the amplitude is tuned in a specific way which establishes the non-trivial connection between UV and IR forms of the amplitude. We show that this connection can be expressed in terms of the Laplace transform and it can give an information about the UV amplitude in the limit $t \log{s}\rightarrow 0$. We discuss the implications of this approach for QED with gravity.

Gravitational-wave (GW) cosmology provides a new way to measure the expansion history of the Universe and test General Relativity (GR) at cosmological scales in the tensor sector, based on the fact that GWs are direct distance tracers. Obtaining the redshift (whose knowledge is essential to test cosmology) is instead the challenge of GW cosmology. In absence of a direct electromagnetic counterpart to the GW event, the source goes under the name of ``dark siren'' and statistical techniques are used. This talk aims at giving an overview of the state-of-the-art for these techniques as well as discussing perspectives for the future. After introducing GW cosmology and statistical methods, I will present the latest measurements of the Hubble parameter and of the phenomenon of ``modified GW propagation'' (that takes place whenever GR is modified at cosmological scales), obtained from the latest Gravitational Wave Transient Catalog 3 with new, independent, open-source codes. In particular, the two techniques applied to real data so far consist in using the statistical correlation with galaxy catalogues and information from the mass distribution of Binary Black Holes. I will discuss methodological aspects, relevant sources of systematics, the interplay with population studies, current challenges and possible ways forward. I will finally present some not-yet-applied ideas for statistical dark siren techniques, in particular for third generation (3G) ground-based GW detectors.

Gravitational Waves (GWs) have become one of the most powerful tools to explore our universe from its early epoch until nowadays, thanks to their freely propagating nature. After the GW detections from resolved sources by the LIGO/Virgo collaboration the next target of present and future ground and space-based interferometers is the detection of the stochastic background of GW (SGWB), both astrophysical or cosmological. General Relativity provides us with an extremely powerful and for free tool to extract astrophysical and cosmological information from the SGWB: the cross-correlation with other cosmological tracers, since their anisotropies share a common origin and the same perturbed geodesics. In this talk I will present some recent results about the study of the cross-correlation of the cosmological and astrophysical SGWBs with Cosmic Microwave Background (CMB) anisotropies, showing that future GW detectors, such as LISA or BBO, have the ability to measure such cross-correlation signals. I will also present a new tool in this context which can be used to reconstruct the expected SGWB maps starting from high resolution real Planck CMB maps.

In this talk I will review some of the recent developments in the S-matrix Bootstrap focussing on applications to effective field theories. As an example, I will apply the bootstrap methods to the supergravity effective field theory in ten dimensions. I will show the improved numerical bootstrap bounds on the first correction to the universal graviton scattering and compare the result with the String Theory predictions. In the last part, I will comment on some new numerical ideas that will boost the explorations in different dimensions and for higher dimensional operators.

Scattering amplitudes of elementary particles exhibit a fascinating simplicity, which is entirely obscured in textbook Feynman-diagram computations. While these quantities find their primary application to collider physics, describing the dynamics of the tiniest particles in the universe, they also characterise the interactions among some of its heaviest objects, such as black holes. Violent collisions among black holes occur where tremendous amounts of energy are emitted, in the form of gravitational waves. 100 years after having been predicted by Einstein, their extraordinary direct detection in 2015 opened a fascinating window of observation of our universe at extreme energies never probed before, and it is now crucial to develop novel efficient methods for highly needed high-precision predictions. Thanks to their inherent simplicity, amplitudes are ideally suited to this task. I will begin by reviewing the computation of a very familiar quantity Newton's potential, from scattering amplitudes and unitarity. I will then explain how to compute directly observable quantities such as the scattering angle for light or for gravitons passing by a heavy mass such as a black hole. These computations are further simplified thanks to a remarkable, yet still mysterious connection between scattering amplitudes of gluons (in Yang-Mills theory) and those of gravitons (in Einstein's General relativity), known as the "double copy", whereby the latter amplitudes can be expressed, schematically, as sums of squares of the former -- a property that cannot be possibly guessed by simply staring at the Lagrangians of the two theories. I will conclude by discussing the prospects of performing computations in Einstein gravity to higher orders in Newton's constant using a new, gauge-invariant version of the double copy, and as an example I will briefly discuss the computation of the scattering angle for classical black hole scattering to third post-Minkowskian order (or O(G^3) in Newton's constant G).

I will argue that Double Field Theory (DFT) can describe some, but not all, alpha'-corrections to the tree-level string effective action. In particular, I will discuss how the first and second alpha'-corrections to the bosonic and heterotic string can be derived from DFT.