Cosmology Seminars

Scheduled Seminars

• 18.02.2026 Mikael Chala (University of Granada), (Onsite)
  Title: Skyrmions at finite temperature
  Abstract: Skyrmions are stable and topologically non-trivial field configurations that behave like localized particles. They appear in the chiral effective theory for pions, where they correspond to the baryon states. In this talk, focusing on toy models that capture different limits of the electroweak sector of the SM, I will show that skyrmions not classically stable at zero temperature can be stabilized by thermal effects. I will also speculate with the possibility that quantum effects make these particles exist also at zero temperature, potentially implying the existence of dark matter without new physics.
• 25.02.2026 Clelia Altomonte (King’s College London), (Onsite)
  Title: TBA
  Abstract: TBA
• 11.03.2026 Katherine Freese (University of Texas), (Remote)
  Title: TBA
  Abstract: TBA
• 01.04.2026 Masahide Yamaguchi (Institute for Basic Science), (Onsite) [in E204]
  Title: Tunneling rate at finite temperature
  Abstract: Cosmological phase transitions, especially first order phase transitions, have attracted renewed interest as potential sources of gravitational waves. Accurately predicting the resulting gravitational wave signal requires a reliable estimate of the transition rate, which is governed by a saddle-point configuration known as the bounce solution. The seminal work of Coleman, Glaser, and Martin established that at zero temperature, any nontrivial bounce solution to the equations of motion that minimizes the Euclidean action is O(D)-symmetric in D-dimensional spacetime. At finite temperature, however, it has not been proven that an O(D-1)-symmetric bounce solution in the spatial directions indeed yields the minimal Euclidean action, despite this assumption being widely used in the literature. In this talk, we extend the Coleman–Glaser–Martin analysis to finite temperature. We rigorously prove that for a broad class of scalar potentials, any saddle-point configuration with the least action is necessarily O(D-1)-symmetric and monotonic in the spatial directions. This result provides a firm mathematical foundation for the symmetry properties widely assumed in studies of thermal vacuum decay and cosmological phase transitions.
• 21.04.2026 Antonino Salvino Midiri (University of Geneva), (Onsite) [at 10:15]
  Title: TBA
  Abstract: TBA

Spring Term 2026

• 14.01.2026 José Correia (University of Oslo), (Onsite)
  Title: Gravitational waves from strong first order phase transitions [video]
  Abstract: Multiple extensions of the Standard Model of particle physics predict the existence of first order phase transitions occurring in the early Universe, leading to an imprint in the stochastic background of gravitational waves. When the transition occurs at the electroweak scale, this imprint will be in the expected range of LISA. In this talk we explore the gravitational wave production of strong first order phase transitions, seeking to understand the role of fluid non-linearities and their impact on the expected signal. To do so, we employ large scale simulations of two transitions: one preceded by a detonation, another by a deflagration. We then study the evolution of vortical and compressional modes, how they are intrinsically related and what their respective impacts are on the expected gravitational wave background signal. We also study the relationship of the rate of emission of gravitational wave power with non-linear decay of flow.
• 04.02.2026 Claudio Grillo (University of Milan), (Onsite)
  Title: The expansion rate and the geometry of the Universe through the time delays of time-varying sources strongly lensed by galaxy clusters [slides]
  Abstract: Current and upcoming wide-field surveys will discover thousands of new multiply imaged quasars and supernovae, several of which strongly lensed by galaxy clusters. This will offer a unique opportunity to transform time delays in lens galaxy clusters into a fundamental alternative tool for measuring the expansion rate and the geometry of the Universe. Time delay cosmography is based on well-known physics (General Relativity) and is a single-step technique. I will present the results of high-quality spectro-photometric (HST and VLT) data and high-precision strong lensing modelling in the core of the Hubble Frontier Fields galaxy cluster MACS J1149.6+2223, where the first magnified and spatially-resolved images of supernova (SN) ‘Refsdal’ at redshift 1.489 were detected. These results were exploited for a wide variety of science topics: 1) the successful prediction of the reappearance of SN ‘Refsdal’ at a specific sky position and time; 2) competitive measurements of the cosmic expansion rate and geometry, completely independent from other cosmological probes; 3) the reconstruction of the kinematics of the SN host, a regular, star-forming, rotation-dominated spiral galaxy in a 4-Gyr-old Universe; 4) a detailed study of the environment of the SN, showing a high degree of ionisation with low metallicity. By considering the independent estimates from ~10 similar cluster strong lensing studies, we will achieve the ambitious goal of a complementary measurement of the value of H0 with a 2% uncertainty. This will help clarify whether the current, hotly debated tension on the value of H0 must be ascribed to intriguing new physics or to significant systematic effects.

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