Oct. 4, 2018, 2 p.m.
Hyperscaling violating black holes with spherical and hyperbolic horizons
In this talk I present present a new family of charged black holes with hyperscaling violating asymptotics and non-trivial horizon topology, for arbitrary Lifshitz exponent z and hyperscaling violation parameter θ. They are (d + 2)-dimensional analytic solutions to a generalized Einstein-Maxwell-Dilaton theory with an additional vector field supporting spherical or hyperbolic horizons. The hyperbolic solutions are only consistent with the null energy condition for 1 ≤ z < 2 and θ = d(z − 1), which explains why they are absent in the pure Lifshitz case. In particular, I will focus on the phase structures of these black holes and discuss special properties of the hyperbolic solution. This talk is based on 1807.09770.
Oct. 10, 2018, 3 p.m.
Cosmic acceleration and type Ia supernovae luminosity-redshift dependence
Isaac Tutusaus Lleixa
The ΛCDM model is the current standard model in cosmology thanks to its ability to reproduce the observations. The first observational evidence for this model appeared roughly 20 years ago from the type Ia supernovae (SNIa) Hubble diagram from two different groups. However, there has recently been some debate in the literature concerning the statistical treatment of SNIa, and their ability to prove the cosmic acceleration. In this talk I will focus on the standard assumption that SNIa intrinsic luminosity is independent of the redshift. After briefly motivating the relaxation of this assumption, I will first present the conclusions that we can draw on the accelerated nature of the expansion of the Universe if we only consider low-redshift cosmological probes and an expanding universe with constant velocity: the Rh=ct model. In the second part of the talk, I will further test this exotic model by including high-redshift information coming from the cosmic microwave background. Finally, I will generalize the previous two parts and present the role of SNIa on the accelerated nature of the expansion of the Universe when we account for the main background cosmological probes in a model-independent way.
Oct. 17, 2018, 7:30 p.m.
Catastrophic Failure of Amorphous Solids
Catastrophic failure of amorphous solids is seen in earthquakes, mud slides and avalanches. I will discuss the fundamental physics of such failures, both from the mechanical and from the statistical physics points of view. I will explain the theory of mechanical yield, and connect the phenomenon to a spinodal point of a first order phase transition where critical effects can be measured and discussed. The lecture will not assume prior knowledge of the subject.
Oct. 18, 2018, 2 p.m.
Plasmons are ubiquitous in all conducting matter. We show how to model plasmons holographically, relevant for systems without quasiparticles, e.g. the ‘strange metal’ phase of high temperature superconductors. By introducing holographic electromagnetism we derive a novel set of of boundary conditions which the plasmon modes, and in fact all collective modes, must satisfy. We present the results for both bulk and 2DEG plasmons, and highlight that holographic bulk plasmons exhibit an exotic dispersion in a part of the parameter space not accessible by conventional methods.
Oct. 23, 2018, 3 p.m.
Spin-momentum locking in oxide interfaces and in Weyl semimetals
Oct. 23, 2018, 3 p.m.
Optimising growth of structure constraints for modified gravity
The study of cosmic acceleration and especially understanding its nature is nowadays one of the important branches in the field of cosmology. I would like to introduce some of the ideas that are put forward to explain this unsettling phenomenon. I will present the so called effective field theory of dark energy approach and discuss it notably in view of upcoming surveys. I will also show how constraints from cosmological data can be significantly increased for models in this approach by considering viability conditions. I will discuss the constraints we have obtained on Horndeski theories using growth of structure data. Importantly, the constraints we derive consider the first measurements of the growth factor and the r.m.s of matter fluctuations separated from each other. These have been recently provided by the VIPERS and SDSS collaborations. I will finish by discussing some forecasts for future surveys.
Oct. 24, 2018, 4:15 p.m.
Empirical signatures of universality, hierarchy and clustering in culture
Oct. 25, 2018, 1 p.m.
Phase behavior of passive and active disks and dumbbells in two dimensions
It has been recently argued  that 2D melting of hard and soft repulsive disks occurs in two steps, with a continuous BKT transition between the solid and hexatic phases, and a first order transition between the hexatic and liquid phases, when density or packing fraction are decreased at constant temperature. The hexatic phase has no positional order but quasi long-range orientational order, while the solid phase has quasi long-range positional and proper long-range orientational order. We show  that the same scenario holds for passive dumbbells, with liquid and quasi long-range orientationally ordered zones coexisting close to the liquid phase, within a narrow interval of packing fractions. When self-propulsion is switched on, our brownian dynamic simulations suggest that for dumbbells the above co-existence region continuously extends to the strong activity region where cluster aggregation had already been observed. There is therefore no discontinuity in the phase diagram between the passive and active regions with phase separation . Differently, the phase diagram of active disks  is characterized by two critical points, respectively at small and large activities, which limit the coexistence regions. The two critical points are connected by a continuous BKT transition line. 1. E. Bernard and W. Krauth, Phys. Rev. Lett. 107, 155704 (2011). 2. G. Gonnella, L. Cugliandolo, A. Suma, P. Di Gregorio, Phys. Rev. Lett. 119, 268002 (2017). 3. D. Levis, I. Pagonabarraga, G. Gonnella, L. Cugliandolo, A. Suma, P. Di Gregorio, Phys. Rev. Lett. 121, 098003 (2018).
Nov. 1, 2018, 2 p.m.
Non-geometric backgrounds in string theory
String theory has a rich structure of dualities, connecting different formulations of string theory among each other. Using dualities one finds that a string can be well-defined on spaces which are ill-defined for point-particles. Such spaces typically do not allow for a description in terms of ordinary geometry and are therefore called non-geometric. Non-geometric backgrounds are interesting for a number of reasons: they provide examples of genuine string-theory configurations, they give rise to non-commutative and non-associative structures and they provide mechanisms for the construction of models for string-phenomenology and string-cosmology. In this talk I will first give an overview on non-geometric backgrounds in string theory, and then discuss the embedding of D-branes into such spaces.
Nov. 8, 2018, 2 p.m.
The Lyapunov Spectrum of quantum thermalisation
Thermalisation in closed quantum systems occurs through a process of dephasing due to parts of the system outside of the window of observation, gradually revealing the underlying thermal nature of eigenstates. In contrast, closed classical systems thermalize due to dynamical chaos. The existence of a well-defined chaotic, semi-classical limit provides one route to define quantum chaos. We demonstrate that such ideas can be extended to many-body quantum systems that do not have a unique semi-classical limit. Projecting quantum dynamics onto variational states using the time-dependent variational principle, results in classical chaotic Hamiltonian dynamics. We study an infinite spin chain in two ways --- using the matrix product state ansatz for the wavefunction and for the thermofield purification of the density matrix --- and extract the full Lyapunov spectrum of the resulting dynamics. We show that the entanglement growth rate is related to the Kolmogorov-Sinai entropy of dynamics projected onto states with appropriate entanglement, extending previous results about initial entanglement growth to all times. The Lyapunov spectra for thermofield descriptions of thermalizing systems show a remarkable semi-circular distribution.
Nov. 9, 2018, 2 p.m.
Theoretical Cosmology Meeting
Nov. 14, 2018, 7:30 p.m.
Statistical inference and statistical physics
In recent years, ideas from statistical physics of disordered systems, notably the cavity method, have helped to develop new algorithms for important inference problems, ranging from community detection to compressed sensing, machine learning (neural networks) and generalized linear regression. The talk will review these developments and explain how they can be used, together with the replica method, to identify phase transitions in benchmark ensembles of inference problems.
Nov. 15, 2018, 11:15 a.m.
Plan B for particle physics: finding long lived particles at CERN
Nov. 20, 2018, 11:30 a.m.
Invitation to random tensors and tensor field theory
I will give an introduction to random tensors, their 1/N expansion and its consequences for large N field theory.
Nov. 21, 2018, 11:30 a.m.
Dual Shapiro steps of a phase-slip junction in the presence of a parasitic capacitance
Nov. 21, 2018, 7:30 p.m.
Wormholes and entangled states
The extended Schwarschild solution is an example of a non-traversable wormhole. It can be viewed as an entangled state of two black holes. We will describe how direct interactions between the two black holes can render the wormhole traversable. We will also discuss traversable wormhole solutions in four dimensions.
Nov. 27, 2018, 1:45 p.m.
Spectroscopy of Two-field Inflation
Nov. 28, 2018, 10 a.m.
HL 414 and Casimir
Cosmology Mini Symposium
Alexander Westphal et al.
10:00-10:30 @ HL 414 (Huygens building): Alexander Westphal: "Zoology of the swampland" (arXiv:1807.06579 and more). 10:30-11:00 @ HL 414: Gonzalo Palma: "Inflationary landscape tomography with cosmological data" (arXiv:1804.07315 and 1806.05202). 11:00-11:30 Coffee in the Casimir room (Oort building 276). 11:30-12:00 @ Casimir room: Evangelos Sfakianakis: "Multi-field effects in alpha attractors: inflation and preheating" (arXiv:1810.02804). 12:00-12:30 @ Casimir room: Guilherme Pimentel: "The cosmological bootstrap: inflationary correlators from symmetries and singularities" (arXiv:1811.00024). 12:30 - .. @ Casimir room: Lunch and extended discussions.
Nov. 29, 2018, 2 p.m.
Graphene and Boundary Conformal Field Theory
The infrared fixed point of graphene under the renormalization group flow is a relatively under studied yet important example of a boundary conformal field theory with a number of remarkable properties. It has a close relationship with three dimensional QED. It maps to itself under electric-magnetic duality. Moreover, it along with its supersymmetric cousins all possess an exactly marginal coupling -- the charge of the electron -- which allows for straightforward perturbative calculations in the weak coupling limit. I will review past work on this model and also discuss my own contributions, which focus on understanding the boundary contributions to the anomalous trace of the stress tensor and their role in helping to understand the structure of boundary conformal field theory.
Dec. 11, 2018, 12:30 p.m.
Gravitational wave multi-messenger cosmology
The detection of gravitational wave GW170817 with its electromagnetic counterparts opens a new window of multi-messenger astronomy. Gravitational waves with their electromagnetic counterparts can be used as the standard sirens to probe the cosmology. The new features of the standard sirens can provide more information of the cosmology which is unachievable for the traditional observations. On the other hand, the strongly lensed gravitational waves and the electromagnetic counterparts can be used not only for precision cosmology but also as new tool to test gravity theory.
Dec. 12, 2018, 3 p.m.
Why theory isn't dead
Famous founders of social media services or internet search engines repeatedly explain in the media that theory is dead, and the future consists of labelled data and machine learning algorithms. The mentioned machine learning algorithms are increasingly used in astronomy as well. In my goodbye-talk, I will therefore describe why and how machine learning cannot replace theoretical physics. The different points of view can of course be debated, but it is surprisingly difficult to really show with equations where machine learning is limited, and where theoretical physics must take over. To demonstrate it, I will fit all cosmological data sets simultaneously, with a single parameter, and derive the best-fitting solution analytically. I will show that this fit achieves arbitrary precision. Most statistical tests therefore prefer this universal parameter over any theoretical model. The universal parameter can only be discarded, once randomness is fully incorporated as a feature into conventional theoretical physics.
Dec. 12, 2018, 7:30 p.m.
At a Physics/Infosci intersection
I will give a very brief sociological overview of the current metastable state of scholarly research communication, and then a technical discussion of the practical implications of literature and usage data considered as computable objects, using arXiv as exemplar. Automated machine learning has provided an effective and scalable methodology for daily quality control of the physics research information feed. From the physics standpoint, there is a surprising amount of statistical mechanics in text-mining and machine learning.
Dec. 13, 2018, 2 p.m.
Exact Spectral Form Factor and Entanglement Spreading in a Minimal Model of Many-Body Quantum Chaos
I will discuss the concept of self-duality in periodically driven (Floquet) quantum Ising spin 1/2 chains which allows for some non-trivial exact computations, despite manifest non-integrability of the model. For example, I will discuss a rigorous proof of random matrix spectral form factors in the model and universal entanglement spreading which saturates the minimal cut bounds.
Dec. 19, 2018, 3 p.m.
Weak gravitational lensing with finite beams
In the standard theory of weak gravitational lensing, beams of light are assumed to be infinitesimal, i.e., very narrow compared to any relevant length scale of the problem at hand. However, this hypothesis is violated whenever light propagates through matter. Thus, one must wonder about the domain of validity of the standard lensing results, and about the magnitude of the possible corrections to it. In this talk, I will present the first theory of weak lensing with finite beams. I will start by discussing its interest regarding fundamental questions, and then turn to observational aspects; finite-beam corrections to current weak-lensing surveys will be considered, as well as novel observables.
- Dec. 19, 2018, 3 p.m. CS Pierre Fleury (Université de Genève) Weak gravitational lensing with finite beams
Jan. 16, 2019, 3 p.m.
- Jan. 16, 2019, 3 p.m. CS Matteo Maturi (ITA, Heidelberg) TBD
Jan. 30, 2019, 3 p.m.
- Jan. 30, 2019, 3 p.m. CS Patricio Vielva (Instituto de Física de Cantabria (Universidad de Cantabria - CSIC)) TBA
Feb. 27, 2019, 3 p.m.
- Feb. 27, 2019, 3 p.m. CS Alexander Vikman (CEICO) TBD
March 13, 2019, 7:30 p.m.
Testing General Relativity with Cosmological Observations
- March 13, 2019, 7:30 p.m. CE Ruth Durrer (University of Geneva) Testing General Relativity with Cosmological Observations
May 22, 2019, 7:30 p.m.
- May 22, 2019, 7:30 p.m. CE Paul Steinhard (Princeton University) TBA
June 5, 2019, 7:30 p.m.
- June 5, 2019, 7:30 p.m. CE Seth Lloyd (MIT) 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