Nov. 6, 2019, 3 p.m.
De Sitter Meeting
Archisman Ghosh and Omar Contigiani
Nov. 13, 2019, 3 p.m.
Nov. 14, 2019, 11:30 a.m.
Chaos in the butterfly cone
A simple probe of chaos and operator growth in many-body quantum systems is the out of time ordered four point function. In a large class of local systems, the effects of chaos in this correlator build up exponentially fast inside the so called butterfly cone. It has been previously observed that the growth of these effects is organized along rays and can be characterized by a velocity dependent Lyapunov exponent. We prove a bound on this exponent that generalizes the chaos bound of Maldacena, Shenker and Stanford. We observe that many systems saturate this bound in a finite size region near the edge of the butterfly cone and the size of this region grows with the coupling. We discuss the connection to conformal Regge theory, where the velocity dependent exponent controls the four point function in an interpolating regime between the Regge and the light cone limit, and relate the aforementioned saturation of our bound to an exchange of dominance between the stress tensor and the pomeron.
Nov. 19, 2019, 1:30 p.m.
Multi-Scale Perturbation Theory for Cosmologies with Nonlinear Structure
Cosmological perturbation theory relies on the assumption that density contrasts are small. This assumption is valid for most of the history of the universe, but begins to break down at late times due to matter’s susceptibility to gravitational collapse. However, the post-Newtonian treatment of gravity is perfectly capable of dealing with highly nonlinear density contrasts, so long as the system under consideration is slow-moving and small in spatial extent - precisely the conditions present in large-scale structures such as superclusters! I will present a novel formalism for simultaneously applying a post-Newtonian expansion on short scales, while keeping a traditional approach to perturbation theory on large scales. This approach allows for the possibility of explicit coupling between terms from each sector. I will then use approximate solutions to calculate the matter bispectrum, using this statistic to highlight similarities and differences between traditional perturbation theory and our new multi-scale approach.
Nov. 20, 2019, 3 p.m.
De Sitter Meeting
Hidde Jense + Marius Cautun
Marius Cautun will present "The distribution of dark matter in hydrodynamical simulations and in the Milky Way” (https://arxiv.org/abs/1911.04557) Hidde Jense will discuss the formation of primordial black holes from an EFT for inflation and PBHs as dark matter candidates.
Nov. 27, 2019, 11:30 a.m.
Random-Matrix perspective on many-body localization
Nov. 27, 2019, 3 p.m.
Cosmological Parameters from the BOSS Galaxy Power Spectrum
I will present cosmological parameter measurements from the publicly available Baryon Oscillation Spectroscopic Survey (BOSS) data on anisotropic galaxy clustering in Fourier space. Assuming a minimal LCDM cosmology with varied massive neutrinos, fixing the primordial power spectrum tilt, and imposing the big bang nucleosynthesis (BBN) prior on the physical baryon density omega_b, the measured late-Universe parameters are: Hubble constant H_0 = 67.89 ± 1.06 km/s/Mpc, matter density fraction Omega_m = 0.295 ± 0.010, and the mass fluctuation amplitude sigma_8 = 0.721 ± 0.043. These parameters were measured directly from the BOSS data and independently of the Planck cosmic microwave background observations. I will also comment on combination of BOSS and Planck data and prospects for future spectroscopic surveys to put even tighter constraints on cosmological parameters.
Nov. 27, 2019, 7:30 p.m.
Gravitational waves: Physics at the Extreme
Jo van den Brand
The LIGO Virgo Consortium achieved the first detection of gravitational waves. A century after the fundamental predictions of Einstein, we report the first direct observations of binary black hole systems merging to form single black holes. The detected waveforms match the predictions of general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. Our observations provide unique access to the properties of space-time at extreme curvatures: the strong-field, and high velocity regime. It allows unprecedented tests of general relativity for the nonlinear dynamics of highly disturbed black holes. In 2017 the gravitational waves from the merger of a binary neutron star was observed. This discovery marks the start of multi-messenger astronomy and the aftermath of this merger was studied with 70 observatories on seven continents and in space, across the electromagnetic spectrum. The scientific impact of the recent detections will be explained. In addition key technological aspects will be addressed, such as the interferometric detection principle, optics, sensors and actuators. Attention is paid to Advanced Virgo, the European detector near Pisa. The presentation will close with a discussion of the largest challenges in the field, including plans for a detector in space (LISA), and Einstein Telescope, an underground observatory for gravitational waves science.
Nov. 28, 2019, 11:30 a.m.
Dec. 4, 2019, 3 p.m.
Weak field phenomenology of metric f(R) and hybrid metric-Palatini theories of gravity
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.
De Sitter Meeting
Guadalupe Canas Herrera + Alessandro Sonnenfeld
Dec. 13, 2019, noon
The equation of state after inflation
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.
The Physics Inside Deep Learning
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.?
Feb. 5, 2020, 7:30 p.m.
Magic Angle Graphene: a New Platform for Strongly Correlated Physics
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. 5, 2020, 7:30 p.m. CE Pablo Jarillo-Herrero (MIT) Magic Angle Graphene: a New Platform for Strongly Correlated Physics
March 4, 2020, 7:30 p.m.
Universality of Laws of Information and Unitarity
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.
Axion, anomalies and gravity
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.
The Hierarchy Problem and Naturalness
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.
Quantum Break-Time in Gravity and Cosmology
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.
- 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