Cosmology Seminars

Scheduled Seminars

• 17.09.2025 Lorenzo Giombi (U. Helsinki), (Onsite) 
  Title: Cosmological first-order phase transitions with large bubbles: beyond conformal fluid and flat spacetime
  Abstract: Using semi-analytical models, we investigate the power spectrum of gravitational waves generated by sound waves in the plasma during a first-order phase transition in new unexplored scenarios. (i) The phase transition is accompanied by a change of the equation of state from that of pure radiation. This modifies the shape of the gravitational wave power spectrum as a consequence of both sound and gravitational waves propagating across a non-conformal fluid. (ii) In addition, we investigate the novel limit where the phase transition completes with large bubbles, by which we mean that the mean bubble spacing R∗​ is a non-negligible fraction of the Hubble length 1/H∗. Since the amplitude of the gravitational wave signal increases with R∗H∗​, this is also the loud signal regime. In this regime the effects of gravity, hitherto neglected, become relevant. We here consider the first general relativistic corrections beyond the Universe expansion. This work improves the current estimation of the gravitational waves power spectrum from first order phase transitions and expands the possible scenarios of transitions that can be tested by gravitational wave detectors.
• 08.10.2025 Jasmine Thomson-Cooke (DIAS), (Onsite)
  Title: TBA
  Abstract: TBA
• 15.10.2025 Luxi Zheng (University of Tartu), (Onsite)
  Title: The spin(4) gauge theory of gravity
  Abstract: TBA
• 29.10.2025 Henda Mansour (Karlsruhe Institute of Technology), (Onsite)
  Title: TBA
  Abstract: TBA

Autumn Term 2025

Spring Term 2025

• 15.1.2025 Denis Werth (Paris, Inst. Astrophys.), (Onsite)  
  Title: Spectral Representation of Cosmological Correlators
  Abstract: Cosmological correlators hold the key to high-energy physics as they probe the earliest moments of our Universe. However, even at tree-level, perturbative calculations are limited by technical difficulties absent in flat-space Feynman diagrammatic. In this talk, after a pedagogical introduction to observational, phenomenological and theoretical aspects of cosmological correlators meant for the broad audience, I will present a new off-shell perturbative method to study and compute cosmological correlators. Using a spectral representation of massive cosmological propagators that encodes spontaneous particle production through a suitable prescription, I will show in detail how such approach not only makes the origin of the correlator singularity structure and factorisation manifest, but also renders practical analytical computations more tractable. This approach explicitly shows that complex correlators are constructed by gluing lower-point off-shell correlators, and suggests that dispersive methods hold promise for developing cosmological recursion relations, further connecting techniques from modern scattering amplitudes to cosmology. This talk will be mainly based on [arXiv:2409.02072]. With this talk, my goal will be twofold: (i) to introduce you to the physics of cosmological correlators, and (ii) to show you what I believe is some nice complex analysis with things like poles, branch cuts and the residue theorem.
• 12.2.2025 Philippa S. Cole (Sussex U.), (Onsite)
  Title: Primordial black holes: how to produce and probe them [Video]
  Abstract: Observational constraints have closed off all but one mass-window for primordial black holes making up all of the dark matter, and there are some specific conditions required for their production in the first place. However, they remain a tantalising dark matter candidate because they require no new beyond the standard model particles and they could additionally provide a lot of information about the very early universe if found. I will review the most plausible mechanisms for producing primordial black holes as well as highlight the difficulty of not over-producing them. I will also emphasise why it’s worth checking every last window for signatures of their existence, and how, with a focus on future gravitational wave experiments, we can probe the remaining viable space in a consistent way.
• 19.2.2025 Simone Blasi (DESY), (Onsite)
  Title: The role of impurities in cosmological phase transitions [Video]
  Abstract: Topological defects can play an important role in cosmology. In this talk I will discuss how defects can act as local impurities catalyzing the decay of the false vacuum. This dynamics takes place in one of the simplest extensions of the Standard Model, the xSM, where domain wall configurations associated to the new singlet scalar field are shown to exponentially enhance the tunneling rate. This dramatically changes the way the electroweak phase transition proceeds, with implications for gravitational wave emission. I will finally discuss a similar mechanism involving QCD axion strings at the time of the electroweak phase transition.
• 26.2.2025 Vinícius Oliveira (Lund U. and U. Aveiro), (Onsite)
  Title: Invisible Higgs decay from dark matter freeze-in at stronger coupling 
  Abstract: In this seminar, we will explore the invisible decay of the Higgs boson in the context of dark matter production via freeze-in at stronger coupling. Unlike traditional freeze-in models, where the dark matter coupling to the visible sector is extremely feeble, we investigate a regime where a sizable coupling is allowed without leading to thermalization. We will discuss the production of scalar and fermionic dark matter from the Standard Model thermal bath, considering the contribution of hadronic states at temperatures below the QCD scale. We will also address experimental constraints from the LHC and the prospects for detection at the HL-LHC and FCC, highlighting the relevance of invisible Higgs decay as a window into physics beyond the Standard Model.
• 5.3.2025 Jorge Sanchez Almeida (IAC, La Laguna), (Onsite)
  Title: The stellar distribution in ultra-faint dwarf galaxies suggests deviations from the collision-less cold dark matter paradigm
  Abstract: I will present the result of our work (SA+2024ApJ…973L..15S) which suggests deviations from the collisionless Cold DM (CDM) paradigm. In the standard cosmological model, the dark matter (DM) particles are collisionless and, because of this very nature, they develop halos with the characteristic central cusp known as NFW profile. Real galaxies seldom show NFW profiles but, rather, have a DM mass distribution with a central plateau or core, characteristic of self-gravitating systems in thermodynamic equilibrium. Within the standard model, the collisionless DM reaches equilibrium through baryon-driven processes able to transform the overall potential from cusp to core. Such mechanism becomes inoperative for galaxies with few stars (stellar mass <1e6 Msun), thus, finding cores in these low-mass galaxies would indicate that the DM is not collisionless, reflecting the much sought-after and currently unknown true nature of the DM  (whether it is fuzzy, SIDM, warm, or else). Measuring the DM distribution in tiny galaxies through traditional dynamical indicators is impossible, but we have developed a tool to constrain the distribution of DM from the stellar photometry alone. The tool was used to analyze the stellar cores observed in 6 Ultra Faint Dwarf (UFD) galaxies with stellar masses between 1e3 and 1e4 Msun. As a result, we know they reside in cored DM potentials rather than NFW potentials. After ruling out explanations for inferred DM cores unrelated to the nature of DM, the evidence suggests that DM particle collisions or other alternatives to CDM likely shape these galaxies. If time allows, I will also present ongoing work on (1) constraints on the SIDM cross-section imposed by the six UFDs and (2) how SIDM naturally generates extended stellar cores, as observed in most faint early-type galaxies.
• 12.3.2025 Nordic Cosmology Meeting
• 2.4.2025 Elina Keihänen (Helsinki Inst. of Phys.), (Onsite)
  Title: Euclid [Slides]
  Abstract: Euclid is the cosmology satellite of the European Space Agency (ESA). Its main goal is to solve the mystery of dark energy: what is causing the accelerated expansion of the universe. Euclid was launched in July 2023, and it has been collecting data since February 2024. One of the nine Euclid data processing centers is operating in Finland. The first public data release, the Quick Release 1 (Q1), took place on March 19th 2025. The data release includes selected data products from three deep fields of the Euclid survey, covering in total 63 square degrees and corresponding to 0.45 % of the nominal Euclid survey area. The area is too small for meaningful estimation of cosmological parameters, but a number of exciting scientific results have already been achieved with this data set. I will give a review of the status of the project, and show highlights from the Q1 release. The first cosmological results are expected in October 2026. 
• 9.4.2025 David Cabo Almeida (ICC, Barcelona U.), (Onsite)
  Title: Two Paths to the Dark Side: late reheating for Freeze-In and EFTs for Freeze-out
  Abstract: Dark Matter remains one of the major unsolved puzzles in modern physics. Two leading candidates in the quest to explain it are the so-called Feebly Interacting Massive Particles (FIMPs) and Weakly Interacting Massive Particles (WIMPs). In this talk, I will explore both scenarios. I will first discuss how, in the case of FIMPs, the assumption of zero initial abundance can imply a low reheating temperature, potentially leading to interactions that are not as feeble as typically assumed. I will illustrate the implications of this for Z’ models. I will then conclude by introducing how non-linear Effective Field Theories (EFTs) can be employed to properly capture resonance effects in WIMP studies at colliders, as well as in conventional Dark Matter searches.
• 16.4.2025 Timothy Clifton (Queen Mary, U. of London), (Remote)
  Title: Patchwork Cosmological Modelling [Video]
  Abstract: Cosmological models are usually built from the top down, with large-scale properties specified first, and small perturbations added on top. In this talk I will outline a bottom-up approach, in which we start by considering a small-scale description of the space-time around an astrophysical body, before building up to a global cosmology by patching together many such regions. The result is a set of consistency relations between gravitational physics on small and large scales, which we can use to investigate how non-linear structures affect the large-scale properties of the Universe. I will use these results to try and suggest to you that screening mechanisms not only alter the gravitational fields around non-linear structures, but could also affect the expansion rate of the Universe as a whole.
• 23.4.2025 Iason Baldes (ENS, Paris), (Onsite)
  Title: Particle Shells from Supercooled Phase Transitions
  Abstract: Relativistic bubble walls from cosmological phase transitions necessarily accumulate expanding shells of particles. I will present our characterization of the shell properties and the processes that can prevent them from free streaming. Collision of such shells presents a possible dark matter production mechanism. These mechanisms typically lead to strong gravitational wave signals possibly detectable by experiments coming about in the medium term.
• 30.4.2025 Clare J. Burrage (Nottingham U.), (Remote)
  Title: The Hunt for Light Scalar Fields
  Abstract: In this talk I will focus on the introduction of new light scalar fields which have been suggested as possible explanations for dark matter and the accelerated expansion of the universe. I will show examples of the unusual phenomenology that can arise in such theories, and explain why properties of macroscopic objects, such as density and compactness, are important in understanding how to detect them. I’ll then show how this leads to new opportunities for precision laboratory measurements to shed light on this type of new physics, and discuss the challenges of accurately simulating their behaviour.
• 7.5.2025 Maria Olalla Olea-Romacho (King’s Coll. London), (Onsite)
  Title: Traces of cosmological phase transitions
  Abstract: Cosmological first-order phase transitions can leave a diverse range of observable imprints: from gravitational waves and collider signatures to gamma-ray signals and dark matter structures. In this talk, I explore how simple beyond-the-Standard-Model scenarios, such as the Two-Higgs-Doublet Model, can make the electroweak phase transition strongly first-order, leading to both testable collider predictions and a stochastic gravitational wave background. I also present scenarios where first-order transitions give rise to primordial black holes, as in classically conformal models, and discuss how such transitions may generate primordial magnetic fields or seed the formation of dark matter minihalos.
• 14.5.2025 Ian G. Moss (Newcastle upon Tyne U.), (Onsite)
  Title: Dark matter quantum droplets [Video]
  Abstract: Given the right set of circumstances, quantum gases are able to change character and condense into a liquid state of quantum droplets. In this talk I will present the idea that the cosmiological dark matter is made from quantum droplets of an ultralight scalar field. This alters the equation of state in a way that helps resolve some cosmological puzzles.
• 21.5.2025 Natalia Korsakova (Hannover, Max Planck Inst. Grav.), (Onsite)
  Title: Making sense of LISA data
  Abstract: Data analysis for the Laser Interferometer Space Antenna (LISA) involves sophisticated techniques to extract gravitational wave signals from highly complex datasets. LISA data includes thousands of overlapping signals that must be disentangled and analysed simultaneously—a challenge commonly addressed using what we call the global fit approach. In this talk, I will discuss classical methods for tackling this problem, with a focus on techniques based on reversible jump Markov Chain Monte Carlo (RJ-MCMC). Additionally, I will explore the potential of multi-messenger observations involving massive black hole binaries, and how we use simulation-based inference to enable fast and efficient parameter estimation in these scenarios. Finally, I will highlight the prospects for cosmology using LISA data, as well as the challenges and limitations we currently face in this area.
• 4.6.2025 Töre Boybeyi (Minnesota U.), (Onsite)
  Title: Gravitational Wave Memory over Cosmological Scales [Video]
  Abstract: We examine the cosmological impact of gravitational wave memory on distant astronomical source distributions. Using realistic models of compact binary coalescence rates extrapolated to cosmological redshifts (𝑧 ∼ 𝒪(1)), we compute cumulative tensor distortions of the metric over cosmic time. This allows us to estimate the random-walk-like gravitational wave memory effects influencing astrometric measurements. We find near-Earth gravitational wave memory contributions dominate pulsar and quasar proper motion signals, potentially detectable by upcoming high-precision astrometric surveys such as Theia. We also outline potential strategies to detect anisotropies in the quasar distribution due to gravitational wave memory accumulated over the Universe’s history. Our ongoing research aims to incorporate cosmological tail effects explicitly into our models and identify potential observational strategies.
• 9.6.2025 Melize Ferrus, (Onsite) at 14:15 in A315
  Title: Ringdown and the Wonders Held Within [Video]
  Abstract: Since the first detection of gravitational waves in 2015, the possibility and interest in using compact object mergers to learn more about the highly dynamic phenomenon in the universe has grown exponentially. Much of this desired information lies in the accuracy and precision of the parameters that generated the detected signal (e.g., mass of the binaries, spin). Knowing more and more about what makes up each stage of a gravitational wave can lead the way to minimizing the potential candidates for our detections. In my presentation, I will discuss research and theory on the post-merger phase of BBH mergers, including both the tails and quasinormal mode components. The aim is to learn about the importance of the tails component to the overall post-merger signal via numerical relativity-based testing and to discuss the possibility of incorporating over-tones beyond what has previously been used in models thus far via mathematical manipulation.
• 11.6.2025 Diego Cruces (Beijing, Inst. Theor. Phys.), (Onsite)
  Title: Small noise expansion in stochastic inflation [Video]
  Abstract: After a pedagogical review of the stochastic approach to inflation, I will introduce small noise expansion techniques, showing that the fully nonlinear (non-Markovian) stochastic inflationary system, may be re-cast in terms of an infinite set of Wiener processes (stochastic equations with white noises). Finally, I will explore the connection between the small noise expansion and standard cosmological perturbation theory.
• 18.6.2025 Jacopo Fumagalli (ICC, Barcelona U.), (Remote) 
  Title: Loops in inflation with enhanced scalar fluctuations
  Abstract: At small scales (below 1 Mpc), the inflationary dynamics remain largely unconstrained. This opens the door to scenarios where scalar fluctuations are significantly amplified relative to the scales probed by the Cosmic Microwave Background (CMB), potentially leading to the formation of primordial black holes and a detectable stochastic gravitational-wave background. As a result, non-standard dynamics at small scales have become a central focus of the community, pushing us into uncharted territory where traditional techniques fall short. Recently, these scenarios have been questioned at loop level, with concerns that amplified small-scale fluctuations could lead to significant loop corrections at large (for instance CMB) scales. In this talk, I will present an explicit proof demonstrating the absence of such large-scale one-loop corrections arising from short-wavelength enhanced modes. More broadly, I will argue how these discussions have underscored that many of the procedures used in standard single-field slow-roll inflation may need reconsideration, particularly in models with enhanced scalar fluctuations. Very likely, we have just opened the Pandora’s box in this field. Talk mainly based on arxiv: 2305.19263 and 2408.08296.
• 7.8.2025 Ilia Musco (Nova Gorica U.),  (Onsite), on Thursday 14:15 
  Title: Numerical Simulations of Primordial Black Holes [Video]
  Abstract: I will present a mathematical and numerical investigation on primordial black hole formation from the collapse of adiabatic cosmological perturbations, describing into details the mechanism of gravitational collapse. For a radiation dominated fluid this is described by the mechanism of critical collapse. Starting from a time independent curvature profile, specified on the super horizon regime using the gradient expansion approach, I study primordial black hole formation from adiabatic cosmological perturbations. This allows to compute the threshold and the mass distribution of primordial black holes consistently with the power spectrum of cosmological perturbations obtained from inflation. This scenario can be generalised during the quark-hadron (QCD) phase transition, when the equation of state is characterised by a softening, reducing significantly the pressure gradients around the solar mass scale. This enhances the formation of primordial black holes, in the mass range of binary systems merging into a bigger black hole, emitting gravitational waves observed by LIGO/Virgo. This gives a possible interesting explanation for the signals of the LVK catalog falling into the lower mass gap, which are difficult to be explained by black holes formed from stellar collapse.
• 20.8.2025 Hayley J. Macpherson (Chicago U.) , (Onsite) 
  Title: Fully nonlinear ray-tracing in cosmological simulations with numerical relativity [Video]
  Abstract: High-precision cosmological surveys are expected to deliver measurements at sub-percent precision. In order to ensure we correctly interpret these data, we need to be sure that our theoretical model is accurate. A key component of the standard cosmological model is the assumption of a homogeneous and isotropic space-time metric; the FLRW space-time. State-of-the-art cosmological simulations approximate the dynamics of structure formation using Newtonian gravity combined with a background FLRW space-time which evolves independently of the matter dynamics. Numerical relativity is a computational method which allows us to relax these simplifying assumptions and consider the full interaction of matter and space-time as described by general relativity. I will provide some background on the use of numerical relativity for cosmological simulations before discussing my framework to create synthetic observables in these simulations which are free from common simplifying assumptions. I will outline some work using this framework including the analysis of low-redshift cosmography methods and an in-progress study of cosmological lensing convergence.
  

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