• Tuesday 12 January 2021 at 10.15 with Zoom: Henrique Rubira (DESY)
    A hybrid simulation of gravitational wave production in first-order phase transitions
    The LISA telescope will provide the first opportunity to probe the scenario of a first-order phase transition happening close to the electroweak scale. By now, it is evident that the main contribution to the GW spectrum comes from the sound waves propagating through the plasma. Current estimates of the GW spectrum are based on numerical simulations of a scalar field interacting with the plasma or on analytical approximations — the so-called sound shell model. In this work we present a novel setup to calculate the GW spectra from sound waves. We use a hybrid method that uses a 1d simulation (with spherical symmetry) to evolve the velocity and enthalpy profiles of a single bubble after collision and embed it in a 3d realization of multiple bubble collisions, assuming linear superposition of the velocity and enthalpy. The main advantage of our method compared to 3d hydrodynamic simulations is that it does not require to resolve the scale of bubble wall thickness. This makes our simulations more economical and the only two relevant physical length scales that enter are the bubble size and the shell thickness (that are in turn enclosed by the box size and the grid spacing). The reduced costs allow for extensive parameter studies and we provide a parametrization of the final GW spectrum as a function of the wall velocity and the fluid kinetic energy.
    Link to video:
  • Tuesday 2 February 2021 at 10.15 with Zoom: Umut Gürsoy (Utrecht U.)
    Hydrodynamics and holography of spin currents
    Collective motion of spin has been playing a prominently interesting role both in high energy and condensed matter physics, especially after the observation of global polarization of $\Lambda$ hyperons in heavy ion collisions and generation of spin current in liquid helium. In this talk I will explain how to construct the theory of spin hydrodynamics, that is, relativistic hydrodynamics comprised of the spin current and the stress tensor. We then apply this to heavy ions and find surprisingly good agreement with data. Finally, I will introduce a holographic approach to spin currents employing the Lovelock-Chern-Simons theory in five dimensions.
    Link to video:
  • Tuesday 16 February 2021 at 10.15 with Zoom: Jukka Maalampi (Jyväskylä)
    The strange history of Runar Gåsström, a Finnish-American-Soviet Physicist
    Runar Viktor Gåsström (1914-1998) was born to a Finnish immigrant family in New York. The family moved back to Finland in 1917, to immigrate ten years later to Vancouver, Canada, and further on to Petroskoi, Soviet-Karelia. Gåsström studied physics in the Moscow State University, allegedly as a student of Pjotr Kapitsa. In 1946 Gåsström, quite surprisingly, moved to Helsinki. He worked for ten years at the Department of Physics in the nuclear physics group of Lennart Simons, being e.g. responsible of the construction of a Van de Graaff generator. Gradually, some of his actions, such as his frequent and inquisitive visits in nuclear physics research centers around the Western Europe and his possible role in the defection of Italian nuclear physicist Bruno Pontecorvo to Soviet Union, as well as his background, started to raise concerns. Gåsström left Finland in 1954 and moved to Groningen, Netherlands, and then to Vienna to work for the IAEA. In 1960 he returned to Soviet Union, where he thereafter held many distiguished positions in Novosibirsk, Irkutsk, and Kaliningrad, working in the fields of nuclear electronics and radiophysics. 
    Link to video:
  • Tuesday 9 March 2021 at 10.15 with Zoom: Eoin Ó Colgáin (CQUEST & Sogang U.)
    Comments on H0
    Precision LCDM Cosmology may be in a state of flux. I will spell out some implications of Hubble tension, a persistent disagreement between local determinations and cosmological inferences of the Hubble constant.
    Link to video:
  • Tuesday 16 March 2021 at 10.15 with Zoom: Watse Sybesma (Iceland)
    Evaporation and the price of curiosity
    The Page curve arises if black hole evaporation is a unitary process. It describes entanglement between the inside and outside of the black hole as a function of time. I will describe how to compute the Page curve for an evaporating black hole in asymptotically flat spacetime by adapting the recently developed ‘island’ formula to the setting of a semi-classical two-dimensional dilaton gravity theory. Afterwards I will switch gears and use the island formula to investigate semi-classical de Sitter in two dimensions. I will consider the question if somebody can use Gibbons-Hawking radiation to decode what lies beyond their cosmological horizon. The answer to this question seems to be positive, but with the caveat that this curiosity will come at a price.
    Link to video:
  • Tuesday 23 March 2021 at 10:15 with Zoom: Masanori Hanada (Surrey)
    Color confinement, Bose-Einstein condensation, and emergent geometry in gauge/gravity duality
    Abstract: In gauge/gravity duality, the information regarding the gravitational geometry (e.g., black hole and smooth exterior geometry) has to be encoded in gauge theory. The color degrees of freedom (matrix degrees of freedom) have to play the key role, because the duality can hold even when the gauge theory side is a matrix model. In this talk, I will provide a very simple way of encoding the geometry to matrices, along the line of Witten’s work on the effective action of D-branes and strings, and the Matrix Theory conjecture by Banks, Fischler, Shenker and Susskind. Roughly speaking, eigenvalues of matrices can be identified with the location of the D-brane probe or extended objects such as black hole.

    Actually there is a famous argument against such simple interpretation advocated by Polchinski in 1998. His argument used generic properties of large-N gauge theory to show that the ground-state wave function delocalizes at large N, leading to a conflict with the locality in the bulk geometry. We show that this argument is not correct: the ground-state wave function does not delocalize, and there is no conflict with the locality of the bulk geometry. In order to understand how Polchinski’s argument fails, recently-discovered connection between color confinement at large N and Bose-Einstein condensation is useful. This confinement-BEC connection has a striking consequence: in the SU(N) gauge theory, there is a partially-deconfined phase in which an SU(M)-subgroup is deconfined. Partial deconfinement gives a natural realization of the idea in BFSS Matrix Theory conjecture — extended objects, such as black hole, are realized as bound states of D-branes and strings, that look like non-commutative blocks in big matrices — in the Maldacena-type gauge/gravity duality.
    Link to video:
  • Tuesday 30 March 2021 at 10:15 with Zoom: Yago Bea (Queen Mary)
    New Insights from Real-time Dynamics
    Abstract: By using holography and numerical relativity I study the real-time dynamics of strongly coupled field theories and obtain insights that might be relevant for physical systems like the quark-gluon plasma formed in heavy-ion collisions or cosmological scenarios exhibiting phase transitions and bubble nucleation. I study the applicability of hydrodynamics in these systems.
    Links to videos: (part 1/2) and (part 2/2)
  • Tuesday 13 April 2021 at 10:15 with Zoom: Manuel Reichert (Sussex)
    Gravitational waves from first-order phase transitions with non-perturbative effective potentials
    Abstract: In this talk, I will present the gravitational-wave signals from the electroweak phase transition as well as from a dark SU(N) confinement-deconfinement phase transition. The common denominator is that the effective finite-temperature potential stems from a non-perturbative computation, in one case from the functional renormalisation group and the other case from the lattice.
    In the electroweak sector, I will detail the impact of non-perturbative higher-dimensional operators in the Higgs potential. Interestingly, the type of higher-dimensional operator seems to have no impact on the gravitational-wave signal, which leads to a universal relation between the mean-bubble separation and the strength parameter of the electroweak phase transition.
    In the dark sector, I will show how the confinement phase transition from a strongly coupled dark SU(N) sector can be computed via effective Polyakov loop models adapted to lattice data. This allows for an eagle view of the dark landscape which is spanned by the phase-transition temperature and the number of dark colours.
    The talk is based on
    – Universal gravitational-wave signatures from heavy new physics in the electroweak sector (
    – Testing the Dark Confined Landscape: From Lattice to Gravitational Waves (
  • Tuesday 20 April 2021 at 10:15 with Zoom: Mikko Laine (Bern)
    How well do we know the thermal photon rate?
    Abstract: The ALICE experiment at LHC has proposed to replace itself during Long  Shutdown 4 through a new experiment, aimed e.g. at studying the production  of very soft electromagnetic radiation (pT < 100 MeV). I review what 40+  years of theoretical research have taught us about the production of photons from a thermal QCD plasma. In particular, the prospects for constraining the  thermal photon rate through lattice QCD are discussed, pointing out that  there may still be room for new theoretical ideas in this field.
    Link to seminar:
  • Tuesday 18 May 2021 at 10:15 with Zoom: Hans-Werner Hammer (Darmstadt)
    Un-nuclear Physics
    Abstract: I will discuss a nonrelativistic version of Georgi’s “unparticle physics”. An “un-nucleus” is a field in a nonrelativistic conformal field theory characterized by a mass and a scaling dimension. Unlike the relativistic unparticle, which remains a hypothetical object, the un-nucleus is realized approximately in high-energy nuclear reactions involving emission of a few neutrons with relative energies between about 0.1 MeV and 5 MeV. Combining this observation with the known universal properties of fermions at unitarity in a harmonic trap, we predict a power-law behavior of the inclusive cross section in this kinematic regime. We compare our predictions with previous theoretical calculations of nuclear reactions and point out opportunities to measure un-nuclei at radioactive beam facilities.
    Link to seminar:
  • Tuesday 25 May 2021 at 11:15 with Zoom: Mitesh Patel (Imperial College) Note time!
    B-anomalies at LHCb
    Abstract: An update on the so-called B-anomalies will be presented, putting the recent evidence for lepton flavour universality breaking into the wider context of such measurements. The future of this field will also be discussed.
    Slides and the seminar can be found in