Cosmology Seminars

Scheduled Seminars

• 18.03.2026 Fabio Bernardo (University of Geneva), (Onsite)
  Title: Limits of EFTs at finite temperature for strong phase transitions
  Abstract: Phase transitions are violent and interesting phenomena that could have occurred in the early universe. Possible techniques to study these phenomena can be used in the presence of a hierarchy of scales, leading to the construction of finite temperature Effective Field Theories by integrating out heavier scales. These EFTs are reliable when the dynamics are mainly encoded in the most relevant operators. I will discuss the limits of such EFTs, showing how higher-dimensional operators affect the prediction of stronger transitions, including those detectable by LISA. These considerations impact the applicability of effective theory techniques, including their use in lattice studies. I will then discuss how subtleties associated with these operators, such as gauge and renormalization group dependence, can emerge and how they are addressed only when higher loop computations are taken into account. Finally, I will compare the impact of these higher order corrections with that of higher dimensional operators on the predicted strength of the transition.
• 25.03.2026 Jaime Hoefken Zink (NCBJ, Warsaw), (Onsite)
  Title: Dark matter probes in astrophysical scenarios: compact stars and blazars
  Abstract: Dark matter is one of the most compelling pieces of evidence for physics beyond the Standard Model, yet its particle nature remains unknown. While direct detection, collider, and indirect searches probe complementary regions of parameter space, large areas—particularly in the GeV and sub-GeV mass range—remain unexplored. This seminar shows how highly energetic astrophysical environments are natural laboratories to probe particle dark matter beyond the reach of terrestrial experiments. We will focus in two kinds of relativistic systems: compact stars (neutron stars, white dwarfs), and blazars, to show how we may find visible signatures of dark matter from these scenarios. This approach connects particle physics, astrophysics, and cosmology to open new discovery channels for dark matter.
• 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.
• 15.04.2026 Lionel London (King’s College London), (Onsite)
  Title: TBA
  Abstract: TBA
• 21.04.2026 Antonino Salvino Midiri (University of Geneva), (Onsite) [at 10:15]
  Title: TBA
  Abstract: TBA
• 29.04.2026 Isak Stomberg (IFIC, València), (Onsite)
  Title: Cosmological Gravitational Waves from Phase Transitions and Simulation-Based Inference for LISA
  Abstract: A stochastic gravitational-wave (GW) background of cosmological origin is a prime target for present and forthcoming GW detectors, ranging from pulsar timing arrays to LISA. In this seminar, I will present two complementary perspectives on cosmological GW signals. First, I introduce an efficient numerical framework to model GW production from first-order cosmological phase transitions using the Higgsless approach. Using these simulations, I construct a set of GW templates within a generalized modeling scheme that extends the standard sound-wave description to include damped sources, allowing us to capture the saturation of the GW amplitude at long source durations. I will present new results for strong phase transitions, demonstrating the saturation of the GW spectrum in the limit of long-duration simulations. Second, I will briefly introduce a modern parameter-inference framework known as simulation-based inference (SBI). As a demonstration of its capabilities, I apply this approach to cosmic-string gravitational-wave signals and assess their detectability with LISA in the presence of astrophysical foregrounds.
• 13.05.2026 Brij Kishor Jashal (Rutherford Appleton Laboratory), (Remote)
  Title: TBA
  Abstract: TBA
• 20.05.2026 Prakhar Bansal (University of Michigan), (Remote) [at 15:15]
  Title: TBA
  Abstract: TBA
• 27.05.2026 Alica Rogelj (University of Bern), (Onsite)
  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.
• 18.02.2026 Mikael Chala (University of Granada), (Onsite)
  Title: Skyrmions at finite temperature [video]
  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: Solving Einstein Field Equations on a quantum computer: a quantum algorithm for numerical general relativity [video]
  Abstract: In this talk, I will present a quantum algorithm for numerical general relativity to show how some computationally expensive simulations performed on classical computers can be more efficiently tackled by quantum algorithms for solving systems of partial differential equations (pdes). I will cover the theoretical and practical aspects of this interdisciplinary project: from the identification of a suitable numerical general relativity formalism (the WEBB hyperbolic tetrad formalism), to the translation of the system of pdes into a set of quantum computing operations in the form of a quantum walk, the programming of the algorithm components for a gate-based digital quantum computer using the Qiskit software, the choice of a system to test it (Schwarzschild black hole gravitational quasinormal modes), and the tests run on both classical simulators and physical quantum computers. I will conclude by discussing on how this project fits in the landscape of quantum information applications for gravity studies and its possible future extensions and applications.
• 11.03.2026 Katherine Freese (University of Texas), (Remote)
  Title: Supermassive Dark Stars and their remnants as a possible solution to cosmic dawn puzzles [video]
  Abstract: Dark Stars would have been the first stars to form in the history of the Universe; they would have been powered by dark matter heating rather than by fusion. Dark Stars are made (almost entirely) of hydrogen and helium from the Big Bang, with 0.1% of the mass in the form of Dark Matter. The relevant types of dark matter include Weakly Interacting Massive Particles (WIMPs) and Self Interacting Dark Matter (SIDM). Although dark matter constitutes only ≲0.1% of the stellar mass, this amount is sufficient to power the star for millions to billions of years. They are very bright diffuse puffy objects (10 AU in radius) and grow to be very massive. In fact, they can grow to become ten million times as massive as the Sun and ten billion times as bright. We have found strong candidates for Supermassive Dark Stars in James Webb Space Telescope data: the spectra are a good match, and in the future with better spectra the detection of a HeII 1640 absorption line would be a smoking gun for a Dark Star vs a galaxy. JWST has made surprising discoveries: too many and too massive early galaxies, potentially in conflict with standard LCDM cosmology; unexplained gigantic black holes; and little red dots. Supermassive Dark Stars and their remnants offer solutions to all of these cosmic dawn puzzles. Once the dark matter fuel for a dark star runs out, the dark star dies, and it collapses to a giant black hole. Thus dark stars may provide seeds for they many unexplained supermassive black holes observed throughout the Universe and at early times.

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