## Dec. 4, 2019, 3 p.m.

Casimir Room## Weak field phenomenology of metric f(R) and hybrid metric-Palatini theories of gravity

Fabio Moretti

The determination of the exact number of degrees of freedom pertaining to gravitational waves in metric f(R) theories has been the source of a long debate in the literature. This issue can be addressed by using gauge invariant variables, that allow to analyze the problem without any choice of gauge fixing. The same technique can be implemented to study gravitational waves in more complex extensions of General Relativity, such as a novel class of theories called Hybrid metric-Palatini gravity. A complete analysis of the weak field limit of such extended framework is performed, both from the perspective of gravitational waves and the PPN corrections to the Newtonian potential in the static case.## Dec. 11, 2019, 3 p.m.

Casimir Room## De Sitter Meeting

Guadalupe Canas Herrera + Alessandro Sonnenfeld

## Dec. 13, 2019, noon

Casimir Room## The equation of state after inflation

Kaloian Lozanov

I will talk about our works (arXiv:1608.01213, arXiv:1710.06851, arXiv:1902.06736) in which we calculate the equation of state of the inflaton field after inflation and provide an upper bound on the duration before radiation domination by taking the nonlinear dynamics of the fragmented inflaton field into account. For the models considered, I will discuss how our work significantly reduces the uncertainty in inflationary observables. I will also consider various aspects of the linear and nonlinear dynamics of the inflaton such as parametric self-resonance, the generation of scalar metric perturbations and gravitational waves. I will also review the recent progress in the understanding of the equation of state after inflation in models featuring gauge bosons.## Jan. 15, 2020, 7:30 p.m.

Sitterzaal## The Physics Inside Deep Learning

Max Welling

Physics has produced a number of very elegant elegant theories about how our world is organized. From the partial differential Navier-Stokes equations describing fluid mechanics, via Riemannian differential geometry to describe general relativity, quantum mechanics to describe atoms, to quantum field theories to describe the elementary particles. In a seemingly completely unrelated scientific discipline, machine learning has made great strides in analyzing signals such as speech, image and video. In particular, convolutional neural nets have been extremely successful at processing these raw signals to build abstract representations from which highly accurate predictions can be made. Examples include: classifying skin cancer from smartphone photos, transcribing speech into sentences, translating these sentences to other languages and then synthesizing them back into speech, segmenting and classifying objects in a video stream in real time, and defeating the world champion of GO. What do these two so seemingly different fields have in common? In this talk I will describe our amazing journey of incorporating the principles of physics into these deep learning architectures. From incorporating global symmetries into neural networks (group-CNNs) to deep learning on curved manifold resulting (gauge-CNNs). From using the ideas of quantum mechanics, such as entanglement to describe a new class of "quantum neural networks" (QubeNet) to hybrid systems that combine learned convolutions with PDEs. I will leave the audience with the intriguing question if these synergies are coincidental or whether there is a deeper level to understand this as information processing systems? As John Archibald Wheeler would put it to Claude Shannon: Is it all "it from bit.?## Jan. 29, 2020, 3 p.m.

Casimir Room## Is the standard cosmological model in crisis?

Marco Raveri

The standard cosmological model is today challenged on the theoretical side and by several pieces of experimental evidence. On one hand our cosmological model seems incompatible with the string theory landscape, on the other hand, different observations of early and late times show different pictures of how the universe evolved. After discussing how severe these challenges are I will present different attempts at resolving them by invoking new physical ingredients in the early or late universe. I will then discuss the role of different future cosmological probes in resolving the current crisis.## Feb. 4, 2020, 11:30 a.m.

Casimir room## Spontaneous symmetry breaking in finite extended Dicke model

Sergei Mukhin

We study a quantum phase transition in the system of N spins 1/2 coupled to photon mode via guage field entering the photon oscillator term. The Hamiltonian of the system maps on the extended Dicke Hamiltonian with infinitely coordinated antiferromagnetic (frustrating) interaction between the spins. The spontaneous symmetry breaking occurs in the infinite coupling limit for finite N and is caused by exponentially decreasing energy gap between the eigenstates with different parity. The non-commutativity of the limits of infinitesimal parity breaking superradiant condensate and infinite coupling strength is demonstrated.This type of Dicke Hamiltonian arises for a system of N low-capacitance Josephson junctions capacitively coupled to a single photon mode in a resonant microwave cavity, that was considered by us previously. In this work we found quantum phase transition on the line in the phase diagram of the system that was previously considered as the boundary between the super- and subradiant phases.## Feb. 5, 2020, 7:30 p.m.

Sitterzaal## ** CANCELLED ** Magic Angle Graphene: a New Platform for Strongly Correlated Physics

Pablo Jarillo-Herrero

The understanding of strongly-correlated quantum matter has challenged physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. In this talk I will present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, I will show that when two graphene sheets are twisted by an angle close to the theoretically predicted 'magic angle', the resulting flat band structure near the Dirac point gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a correlated insulator arising from electrons localized in the moiré superlattice. Moreover, upon doping, we find electrically tunable superconductivity in this system, with many characteristics similar to high-temperature cuprates superconductivity. These unique properties of magic-angle twisted bilayer graphene open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as quantum spin liquids or correlated topological insulators.## Feb. 13, 2020, 11:30 a.m.

Casimir room## Wormholes and Entropy

Mihailo Cubrovic

## March 4, 2020, 7:30 p.m.

Sitterzaal## Universality of Laws of Information and Unitarity

Gregory Dvali

We discuss how the diverse systems such as solitons, baryons and instantons on one hand and black holes and cosmological universes on the other store and process the quantum information in ways that are strikingly similar. This similarity reveals a deep underlying connection between unitarity and entropy. Among other things, this sheds a very different light at the microscopic structure of non-perturbative objects in quantum field theory and gravity. We discuss some observational consequences for particle physics and cosmology.- March 4, 2020, 7:30 p.m. CE Gregory Dvali (Lorentz Professor, NYU and LMU Munich) Universality of Laws of Information and Unitarity
## March 18, 2020, 2 p.m.

de Sitterzaal## Axion, anomalies and gravity

Gia Dvali

After describing the standard formulation of strong-CP problem and its Peccei-Quinn solution, we reformulate the model in the language of a gauge three-form theory and its Higgs phase. We give analogous description for generation of the mass of eta-prime meson in QCD. Thanks to the power of gauge redundancy and anomalies, this formulation makes several things physically very transparent. For example, it shows that in the limit of vanishing Yukawa coupling for one of the quarks, the eta-prime meson becomes an axion. It also allows to understand under what conditions gravity interferes with the axion solution of the strong-CP problem and how this interference can be avoided in the Standard Model and its extensions. We discuss implications for neutrino physics such as the generation of neutrino masses from the gravitational anomaly.- March 18, 2020, 2 p.m. Gia Dvali (Lorentz Professor (lecture 1)) Axion, anomalies and gravity
## March 25, 2020, 2 p.m.

de Sitterzaal## The Hierarchy Problem and Naturalness

Gia Dvali

We discuss the essence of the Hierarchy Problem and the crucial role of gravity in it. We then explain the concept of the ``vacuum-entropy naturalness" and how its application eliminates the problem without the need of new low-energy physics for stabilizing the Higgs mass against quantum corrections. We discuss how this concept of naturanless differs from the standard one by 't Hooft. Here the observed value of the Higgs mass corresponds to a vacuum of infinite degeneracy and infinite entropy. Therefore, it represents and attractor point of cosmic inflationary evolution. This information is unavailable for a low energy observer living in one of such vacua. By not seeing any stabilizing physics at LHC such an observer is puzzled and creates an artificial problem of naturalness which in reality does not exist. We explain why this solution is fully compatible with the concept of Wilsonian decoupling.- March 25, 2020, 2 p.m. Gia Dvali (Lorentz Professor (lecture 2)) The Hierarchy Problem and Naturalness
## April 1, 2020, 2 p.m.

de Sitterzaal## Quantum Break-Time in Gravity and Cosmology

Gia Dvali

In this lecture we first explain the concept of quantum break-time. Most of the macroscopic systems in nature are well-described classically, with quantum effects being small. However, an important sub-class of macroscopic systems exhibits the phenomenon of complete quantum breaking. Namely, in such systems the quantum interactions lead to a total departure from the semi-classical description, despite the fact that the system is macroscopic. We give the evidence showing that two important examples of this sort are represented by black holes and de Sitter type cosmological space-times. Both exhibit a finite quantum break-time. We discuss some potentially-observable implications of this phenomenon for black hole physics and inflationary cosmology. We also discuss non-gravitational systems that exhibit similar phenomenon. Such are certain solitons and Bose-Einstein condensates at quantum critical point.- April 1, 2020, 2 p.m. Gia Dvali (Lorentz professor (lecture 3)) Quantum Break-Time in Gravity and Cosmology
## June 10, 2020, 7:30 p.m.

Sitterzaal## TBA

Andrew Strominger

TBA- June 10, 2020, 7:30 p.m. CE Andrew Strominger (Harvard) TBA

LS = Lorentz Seminar, Casimir room (276), Oort building

CE = Colloquium Ehrenfestii, De Sitter lecture room, Oort building

SBM = Soft & Biological Matter Seminar, Casimir room (276), Oort building

CS = Cosmology Seminar, Casimir room (276), Oort building

ST = String Theory Seminar, Casimir room (276), Oort building