Jan. 16, 2019, 3 p.m.
Galaxy clusters as tools for cosmology
Galaxy clusters are fundamental probes to investigate the nature of dark matter and the complex phenomena involving their baryonic content. Moreover, their number, mass and redshift distribution provide us with precious insights into the nature of our universe and of dark energy. To properly use them as a tool for cosmology, it is now of crucial importance to have large samples and a solid understanding of their observable and physical properties so as to obtain a reliable statistical sample and control over possible biases. Having this in mind, I will discuss ways (1) to detect galaxy clusters in weak lensing and optical photometric data sets; (2) to identify and characterize strong lensing features such as giant arcs in order to gain a deeper understanding of clusters; (3) to understand and exploit their weak lensing, Rees-Sciama and polarization imprints in the anisotropies of the Cosmic Microwave Background radiation. Moreover, also a method to model the images of galaxies, useful to produce realistic simulated astronomical images of galaxy clusters, is going to be discussed.
Jan. 16, 2019, 7:30 p.m.
The Art of Building Small
Exploring across the current frontiers of chemical sciences there is vast uncharted territory to experience the joy of discovery. Far beyond Nature's design, the creative power of synthetic chemistry provides unlimited opportunities to realize our own molecular world as we experience every day with products ranging from drugs to displays. In the art of building small we explore the fascinating field of molecular nanoscience . Among the major challenges ahead in the design of complex artificial molecular systems is the control over dynamic functions and responsive far-from-equilibrium behaviour. A major goal is to gain control over translational and rotary motion. The focus is on my journey in the world of molecular switches and motors creating opportunities for smart drugs, adaptive catalysts or responsive materials.
Jan. 17, 2019, 1 p.m.
Self-assembled networks of ions and molecules at solid-liquid interfaces: organisation and dynamics
The behaviour of liquid molecules near solids can be strongly influenced by the local chemical and topographical properties of the solid’s surface and does generally not behave like bulk liquid. This so-called interfacial liquid is central to countless phenomena ranging from self-assembly processes, electrochemistry, heterogeneous catalysis, heat transfer and the folding and function of biomolecules. Experimentally, nanoscale measurements of liquid-driven processes remain challenging, especially when averaging-based approaches are not possible. Atomic force microscopy (AFM) can map the molecular organisation of the interfacial liquid and gather quantitative information about its local dynamics [1-2], including around adsorbed molecules and ions. Recent results have show that simple metal ions such as Na+ and K+ ions can spontaneously form correlated nanoscale networks at the interface with immersed solids , including soft bio-membranes. This evolve over seconds, controlled by water-mediated correlative ion-ion interactions and mediate the mesoscale properties of the interface [4-5]. Over lipid membranes, ionic networks locally reduce the stiffness of the membrane, providing a spontaneous mechanism for tuning mechanical properties with nanoscale precision. The ubiquity of water-mediated interactions suggest these results have far-reaching implications for a wide range of interfacial phenomena.  Voitchovsky et al, Nat. Nano., 2010, 5, 401  Ortiz-Young et al, Nat. Commun., 2013, 4, 2482  Ricci, et al, Nat. Commun., 2014, 5, 4400  Ricci, et al, Sci. Rep., 2017, 7, 43234  Cafolla, et al, Nanoscale, 2018, 10, 11831
Jan. 30, 2019, 2 p.m.
A summary of Planck results on Isotropy and Statistics
I will provide a personal view on what Planck has shown us regarding the Isotropy and Statistics of the CMB anisotropies, paying particular attention on the tests conducted to assess their compatibility with the LCDM model. I will focus particularly on the so-called large-scale anomalies. I will also mention the limitations that we have with Planck data regarding this topic, and what is left to be done with future CMB polarization experiments, in particular with satellite missions. In this context, I will comment on the role that is being played by the European CMB community.
Jan. 31, 2019, 1 p.m.
Adiabatic dynamics of quantum many-body systems
The concept of adiabatic evolution naturally emerges in a wide range of contexts in physics and technology, such as heat engine operation, quantised transport in mesoscopic devices, quasi-Bloch oscillations, foundations of quantum field theory, physics of black holes, de-Sitter cosmology and quantum computation, to name a few. In the quantum realm, the concept was formalized in the early days of quantum mechanics by proving the quantum adiabatic theorem. This fundamental result stated that a system can be kept arbitrarily close to the instantaneous ground state of its Hamiltonian if the latter varies in time slowly enough. To apply this theorem in practice, one normally requires a quantitative understanding of what slowly enough means. Quite remarkably, such practicable quantitative understanding is limited to a modest set of systems mostly having a small Hilbert space. We show how this gap can be bridged for a broad natural class of physical systems, namely many-body systems where a small move in the parameter space induces an orthogonality catastrophe. In this class, the conditions for adiabaticity are derived from the scaling properties of the parameter-dependent ground state without a reference to the excitation spectrum. This finding constitutes a major simplification of a complex problem, which otherwise requires solving non-autonomous time evolution in a large Hilbert space. We apply our general results to the dynamics of a mobile impurity in a one-dimensional quantum fluid and to the Grover adiabatic search algorithm. We also give an outlook of quantum adiabaticity in a wider context. In particular, we discuss how the quantum notion of adiabaticity should be generalized in order to reconcile it with the classical notion of adiabaticity.
Feb. 7, 2019, 3:30 p.m.
Electron hydrodynamics with a polygon Fermi surface
I will discuss a cartoon model for the onset of hydrodynamic behavior in an electron fluid with a polygon Fermi surface. Some interesting features of this model include new dissipative components to the viscosity tensor, a separation of fast and slow scattering rates, and the possibility of non-monotonic temperature dependence in conductance in the ballistic-to-hydrodynamic crossover in flows in channels. Our work is inspired by the possibility of electron hydrodynamics in PdCoO_2, which has an approximately hexagonal Fermi surface. We revisit data on possible hydrodynamic flow from earlier experiments in light of our model.
Feb. 13, 2019, 3:30 p.m.
Gaussian processes in cosmology: Reconstructing late-time cosmic expansion
Gaussian processes (GP) is a powerful technique for performing data analysis without assumption of the underlaying analytical model. In cosmology GP has been utilised to reconstruct the low-redshift cosmic expansion history, henceforth the dynamics of late-time evolution. We implement the extension of standard GP formalism, namely the Multi-Task Gaussian Process with the ability to perform a joint learning on several cosmological datasets simultaneously. Utilising the "low-redshift" expansion-rate we obtain constraints on H0 = 68.52 \pm 0.94 (stat) \pm 2.51 (sys) km/s/Mpc and a corresponding rd = 145.61\pm2.82(stat)\pm4.3 (sys) Mpc. Subsequently, one can further constrain q0 = -0.52\pm0.06 and the transition redshift zT = 0.64^+0.12_-0.09. Using the Om(z) diagnostic, we find that the concordance model is very consistent within the redshift range z<2 and mildly discrepant for z > 2. While GP provides flexibility, it can also lead to biases and incorrect inferences, which become more apparent with its application to reconstruct the dark energy equation of state.
Feb. 14, 2019, 11:30 a.m.
On correlation functions in JT¯-deformed CFTs
The JT¯ deformation, built from the components of the stress tensor and of a U(1) current, is a universal irrelevant deformation of two-dimensional CFTs that preserves the left-moving conformal symmetry, while breaking locality on the right-moving side. Operators in the JT¯-deformed CFT are naturally labeled by the left-moving position and right-moving momentum and transform in representations of the one-dimensional extended conformal group. We derive an all-orders formula for the spectrum of conformal dimensions and charges of the deformed CFT, which we cross-check at leading order using conformal perturbation theory. We also compute the linear corrections to the one-dimensional OPE coefficients and comment on the extent to which the correlation functions in JT¯-deformed CFTs can be obtained from field-dependent coordinate transformations.
Feb. 15, 2019, 2 p.m.
D-ITP Quantum & Topological meeting
Alexander Monin et al.
14:30: Talk 1: Alexander Monin (Lausanne), Title: Effective Field Theory for CFTs with large charge. 15:20: Talk 2: Oleksandr Gamayun (Amsterdam), Title: Relaxation in classical integrable systems. 16:30: Talk 3: Mikael Fremling (Utrecht), Title: Hall Viscosity and Composite Fermions
Feb. 21, 2019, 3:30 p.m.
Casimir Room 276
The power of conformal symmetry
Conformal symmetry plays a special role in physics. Models possessing this symmetry describe systems at criticality occurring both in many body systems and in high energy physics. Conformal symmetry may also be relevant for addressing the hierarchy problem. It is thus important to investigate conformal field theories (CFT) in depth. However, it is a complicated task for the majority of interesting CFTs are strongly coupled forbidding perturbative treatment. In this talk I present two examples when it is nevertheless possible to extract non-trivial information from CFT. First, I show that using effective field theory description in terms of superfluid its possible to derive universal behavior of dimensions and fusion coefficients of operators with large U(1) charge. Second, I show that the classical conformal symmetry for a wide class of theories — even though usually assumed to be broken by quantum corrections — can be actually preserved at quantum level.
Feb. 27, 2019, 3 p.m.
Bounce and Time Crystals: Superluminality and UV-Completion
First, I will discuss which theories can allow for a "healthy" bounce: a smooth evolution of a spatially flat Friedmann universe from contraction to expansion without ghosts and gradient instabilities. Further, I will talk about cosmological time crystals recently introduced by Wilczek. The latter are systems with limit cycles and can represent vacuum with minimally broken time-translation invariance. In particular, these systems can model Dark Energy with rapidly oscillating equation of state. I will argue that all such theories not only violate the Null Energy Condition, but also necessarily possess states with superluminality. Even though the bouncing solution can be completely subluminal. This superluminality has crucial consequences for a potential completion.
March 6, 2019, 3 p.m.
Astrophysical background of gravitational waves: from cosmology to a new era of precision astrophysics
The astrophysical background of gravitational waves (AGWB) is made up by the incoherent superposition of gravitational wave signals emitted by a large number of resolved and unresolved astrophysical sources from the onset of stellar activity until today. I present a theoretical framework to fully characterize the AGWB in terms of energy density anisotropies and polarization and I show the first numerical predictions of the angular power spectra of anisotropies in different frequency bands and of cross-correlations with electromagnetic observables e.g. weak lensing and galaxy number counts. l then illustrate and discuss potential astrophysical implications of this study.
March 7, 2019, 3:30 p.m.
Tails of steady states
I will discuss collective modes which describe the approach to asymptotic equilibrium in steady states of strongly interacting many body systems. These modes can be thought of as time independent analogues of quasinormal modes, which imprint a spatial ringdown pattern on the steady state. I will construct these modes in both hydrodynamics and holography and demonstrate their role in holographic non-equilibrium steady states corresponding to steady flows past obstacles.
March 12, 2019, 1 p.m.
Evolution and signature of cosmological magnetic fields
Magnetic fields are an ubiquitous presence in our Universe but a comprehensive theory describing their generation and evolution is still missing. A broadly accepted paradigm wants these fields to be generated by the amplification of pre-existing weaker fields during the gravitational collapse accompanying structure formation. However the mechanisms behind the generation of these seeds are yet to be understood. An intriguing possibility is that these seed fields are generated in the pre-recombination Universe and then evolve passively until the epoch of structure formation. A clean way to prove the presence of primordial magnetic fields (PMF) is to look at Cosmic Microwave Background (CMB) anisotropies induced by these fields. In this talk, I will discuss the behavior of magnetic fields in the pre-recombination epoch and their imprints on the CMB spectra. I will review the current bounds coming from the combination of various CMB probes such as the Planck satellite and the South Pole Telescope. Furthermore I will also show what we expect to learn in the future from next generation CMB experiments. Finally I will discuss how magnetic fields evolves in the post-recombination regime and their impact on structure formation
March 13, 2019, 7:30 p.m.
Testing General Relativity with Cosmological Observations
General Relativity (GR) is immensely successful. With the late discovery of gravitational waves from black hole and neutron star mergers, it has passed all the tests with flying colors. But so far, all observations have mainly tested the vacuum equations of GR. The most important non-vacuum case, cosmology, is in agreement with GR only after the introduction of two otherwise unknown components, 'Dark Matter' and 'Dark Energy' which amount to about 96% of the total energy budget of the present Universe. This let people in the field question the validity of GR for cosmology. Might it be that GR is flawed on large, cosmological scales? Or in the presence of matter in general? But how can we test Einstein's equation in the presence of matter. Can't we simply move any modification of the Einstein tensor to the right hand side and call it a 'dark matter/energy' component? In my talk I shall discuss possible ways (partially) out of this dilemma. How to test both, the left and the right hand side of Einstein's equations with cosmological observations.
March 21, 2019, 3:30 p.m.
Hydrodynamic transport in the Poiseuille regime
David Rodriguez Fernandez
In the context of describing electrons in solids as a fluid in the hydrodynamic regime, we consider a flow of electrons in a channel of finite width, i.e.~a Poiseuille flow. The electrons are accelerated by a constant electric field. We develop the appropriate relativistic hydrodynamic formalism in 2+1 dimensions and show that the fluid has a finite dc conductivity due to boundary-induced momentum relaxation, even in the absence of impurities. We use methods involving the AdS/CFT correspondence to examine the system in the strong-coupling regime. We calculate and study velocity profiles across the channel, from which we obtain the differential resistance dV/dI. We find that dV/dI decreases with increasing current I as expected for a Poiseuille flow, also at strong coupling and in the relativistic velocity regime. In addition, dV/dI decreases when the coupling is increased. Furthermore, we examine the effect of the Hall-viscosity on the flow of a Fermi liquid. The liquid flows through a lead under the influence of constant electric and magnetic fields. We find that the induced Hall voltage receives an additional contribution due to Hall viscosity. We examine the general behavior of this contribution and show that its dependence on the wire width and density of the liquid can be used to differentiate it from the classical Hall effect. Finally, we present estimates of the Hall signal for GaAs in the hydrodynamic regime, which shows that this effect can be experimentally verified. ------------------------------------------------- Talk based on 1806.10635 and follow-up work involving parity breaking (to appear soon).
March 25, 2019, 1 p.m.
Chiral electrodynamics: from the early Universe to laboratories
March 27, 2019, 11:30 a.m.
Mixed axial-torsional anomaly in Weyl semimetals
- March 27, 2019, 11:30 a.m. LS Jens Bardarson (KTH Stockholm) Mixed axial-torsional anomaly in Weyl semimetals
March 27, 2019, 3 p.m.
Cosmology with neutral hydrogen intensity mapping
I will introduce the innovative technique of neutral hydrogen (HI) intensity mapping, which allows for precise cosmological measurements in the radio wavelength. After describing the current status of HI intensity mapping measurements, I will focus on forthcoming surveys using instruments like MeerKAT and the SKA to probe dark energy as well as galaxy evolution. My talk will also focus on the challenges that intensity mapping faces, and explain why cross-correlations with optical galaxy surveys will probably dominate the field for the next few years. Finally, I will introduce the GBT-eBOSS cross-correlation project, which hopes to provide the best measurements using HI intensity mapping data to date.
- March 27, 2019, 3 p.m. CS Alkistis Pourtsidou (Queen Mary University of London) Cosmology with neutral hydrogen intensity mapping
April 4, 2019, 3:30 p.m.
- April 4, 2019, 3:30 p.m. ST Eric Mefford (Polytechnique) TBA
April 10, 2019, 3 p.m.
- April 10, 2019, 3 p.m. CS Chunshan Lin (Warsaw University) TBD
April 11, 2019, 3:30 p.m.
- April 11, 2019, 3:30 p.m. ST Dmitry Bagrets (Koeln) TBA
April 25, 2019, 3:30 p.m.
- April 25, 2019, 3:30 p.m. ST Eva Llabres (CPhT Saclay) TBA
May 2, 2019, 3:30 p.m.
- May 2, 2019, 3:30 p.m. ST Gert Aarts (Swansea University) TBA
May 9, 2019, 3:30 p.m.
- May 9, 2019, 3:30 p.m. ST Manuel Vielma (University of Geneva) TBA
May 22, 2019, 7:30 p.m.
The second kind of impossible
Quasicrystals are exotic forms of matter with symmetries that were once thought to be mathematically impossible. The first known examples were synthesized in the laboratory over 35 years ago, but could Nature have beaten us to the punch? This talk will describe the decades-long search to answer this question, resulting in one of the strangest scientific stories you are ever likely to hear.
- May 22, 2019, 7:30 p.m. CE Paul Steinhardt (Princeton University) The second kind of impossible
June 5, 2019, 7:30 p.m.
- June 5, 2019, 7:30 p.m. CE Seth Lloyd (MIT) TBA
June 11, 2019, 2 p.m.
Quantum computing: past, present, and future: Lecture 1
- June 11, 2019, 2 p.m. Seth Lloyd (Lorentz Professor) Quantum computing: past, present, and future: Lecture 1
June 18, 2019, 2 p.m.
Quantum computing: past, present, and future: Lecture 2
- June 18, 2019, 2 p.m. Seth Lloyd (Lorentz Professor) Quantum computing: past, present, and future: Lecture 2
June 25, 2019, 2 p.m.
Quantum computing: past, present, and future: Lecture 3
- June 25, 2019, 2 p.m. Seth Lloyd (Lorentz Professor) Quantum computing: past, present, and future: Lecture 3
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