VAST 2026: From Vacuum Stability to Gravitational Waves

Oct 21, 2026, 9:00 AM → Oct 23, 2026, 10:30 PM Europe/Madrid

Real Jardín Botánico, CSIC.

Javier Rubio (Instituto de Física de Partículas y del Cosmos), Jesús Luque del Castillo, Jose A. R. Cembranos (IPARCOS-UCM), Mariaveronica De Angelis (Universidad Complutense de Madrid), Thomas Steingasser (IFT), Tomás Mendes (Universidad Complutense de Madrid), Álvaro Cendal García (Universidad Complutense de Madrid)

The study of vacuum stability and phase transitions plays an important role in several areas of particle physics and cosmology. Questions related to the structure of the Standard Model Higgs potential, the evolution of the early Universe, and the properties of fundamental interactions are closely connected to these phenomena.

This conference aims to bring together researchers working on theoretical, phenomenological, and computational aspects of vacuum stability and phase transitions, fostering discussion and collaboration across these closely related fields.

Topics of interest include, but are not limited to:

• Electroweak vacuum stability and metastability
• Vacuum decay and tunneling in quantum field theory
• Finite-temperature field theory
• Cosmological phase transitions
• Gravitational-wave signals from the early Universe
• Beyond-the-Standard-Model scenarios and Higgs-sector extensions
• Non-equilibrium field dynamics and real-time simulations
• Topological defects and phase-transition relics
• Inflationary and post-inflationary phase transitions
• Applications to dark matter, baryogenesis, and cosmology

Confirmed plenary speakers include: 

• Giulio Barni (IFT, Madrid)
• José Ramón Espinosa (IFT, Madrid)
• David I Kaiser (MIT, Cambridge)
• Aaron Held (ENS, Paris)
• David Mulryne (Queen Mary University, London)
• Marieke Postma (NIKHEF, Amsterdam)

• Manuel Reichert (University of Sussex, Brighton)

• David Weir (HIP, Helsinki)

Organizers

• M. de Angelis — Universidad Complutense de Madrid & IPARCOS, Spain
• J. A. R. Cembranos — Universidad Complutense de Madrid & IPARCOS, Spain
• A. Cendal — Universidad Complutense de Madrid & IPARCOS, Spain
• J. Luque — Universidad Complutense de Madrid & IPARCOS, Spain
• T. Mendes — Universidad Complutense de Madrid & IPARCOS, Spain
• J. Rubio (Chair) — Universidad Complutense de Madrid & IPARCOS, Spain
• T. Steingasser (Co-Chair) — Instituto de Física Teórica UAM/CSIC, Spain
  
  

Sponsors

        

Map.pdf

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Wed, October 21

9:00 AM Registration

Plenary: Plenary 1

Convener: Thomas Steingasser (IFT)

1 Higgs vacuum stability and a gravity-induced UV completion of the Standard Model

The Standard Model of particle physics, in particular the Higgs potential, exhibits hints of fundamental quantum scale symmetry. At the same time, the asymptotic safety program has gathered substantial evidence for a nonperturbative fixed point of gravitational interactions. If this gravitational fixed point induces a UV completion of the Standard Model, it offers a compelling explanation for these observed hints of quantum scale symmetry.

My talk reviews a parameterized approach to map out the window of coupling values in which gravitational interactions induce a UV completion of the Standard-Model. This UV-complete window seems to be bounded by a critical surface: Coupling values inside this surface become asymptotically free due to the gravitational interaction, while gravity is too weak to UV-complete coupling values outside of it. Vice versa, this implies that the critical surface itself is an infrared attractor. Most strikingly, this suffices to fix several nontrivial quantitative relations among the Standard-Model coupling values, including the Higgs potential. I will summarize the status of confronting this approach with experimental data, in particular also with regards to Higgs vacuum stability.

Speaker: Aaron Held (École Normale Supérieure)

aaron_21-oct-2026.pdf

2 Bubble Nucleation Rates from the Functional Renormalisation Group

The functional renormalisation group is a powerful non-perturbative tool for the computation of the vacuum and finite-temperature effective potential. It is automatically IR-finite and includes the resummation of bubble diagrams, similar to Daisy and super-Daisy resummations in perturbation theory. I will introduce this tool and, as an example, show its application to the electroweak phase transition with polynomial and non-polynomial deformations of the Higgs potential at the TeV scale. Furthermore, I will discuss the dependence on the renormalisation group scale and show how reliable nucleation rates can be obtained despite the convexity of the effective potential.

Speaker: Manuel Reichert (University of Sussex)

VAST-Reichert.pdf

11:00 AM Coffee Break

Plenary: Plenary 2

Convener: Thomas Steingasser (IFT)

3 Black Holes from a Higgs-like Field in the Radiation Era

The metastability of the Higgs potential raises a long-standing fundamental question for early-universe cosmology: what is the nonlinear fate of rare inflationary fluctuations that cross a potential barrier into a negative-energy region? Using fully nonlinear spherically symmetric numerical relativity, we investigate the evolution of such fluctuations in a radiation-dominated universe. We find a robust two-stage collapse process. First, the negative-potential core forms a primordial black hole, hiding the unstable region behind an apparent horizon. The subsequent evolution is governed by the self-gravity of the surviving barrier region and exhibits two qualitatively distinct branches. In the subcritical regime the horizon grows and engulfs the entire configuration, whereas in the supercritical regime a transient wormhole geometry develops, accompanied by bifurcating trapping horizons and the formation of an inflating child-universe branch. Despite their dramatically different internal structures, both branches leave behind an ordinary primordial black hole as viewed from the parent universe. We also provide preliminary results in which this conclusion is qualitatively similar in the case where the black hole forms during inflation and persists into the radiation phase of evolution. These results provide a new perspective on the cosmological implications of metastable Higgs-like fields and the role of gravitational collapse in shielding unstable regions of field space.

Speaker: David Mulryne (Queen Mary University of London)

VAST.pdf

4 Discussion & announcements

1:00 PM Lunch Break

Phenomenology Of Phase Transitions

Convener: Miguel Vanvlasselaer

5 Non-conformal obstructions to bubble expansion

The hydrodynamics of expanding bubbles is a key ingredient in the prediction of gravitational waves from cosmological first-order phase transitions. It fixes which expansion types are possible, and it provides a proxy for how much energy is available for gravitational wave emission. These predictions normally rely on a conformal equation of state, which need not hold in strongly coupled or QCD-like sectors. In this talk, I will relax this assumption and study the self-similar bubble solutions for a family of equations of state that interpolates between electroweak-like and QCD-like transitions. Non-conformality turns out to obstruct bubble expansion, ruling out otherwise admissible solutions and, at the same time, enabling a new expansion mode. I will discuss how both effects reshape the space of solutions and the kinetic energy fraction. If time permits, I will discuss how the collision of bubbles in a QCD-like phase transition leads to a phase-separated final state rather than a homogeneous one.

Speaker: Pedro Tarancón-Álvarez (Institute of Cosmos Sciences - University of Barcelona (ICCUB))

Presentacion_bubbles.pdf

Presentacion_bubbles.pptx

6 Can the universe be matter-dominated after a supercooled first-order phase transition?

When reheating is delayed, supercooled first-order phase transitions are often assumed to hand the Universe over to an early era of matter domination. In this talk I ask whether this assumption actually holds, and argue that it generally does not — at least not immediately. Bubble collisions leave behind a highly inhomogeneous scalar field with persistent relativistic gradients, producing an equation of state between those of matter and radiation. Using lattice simulations, we find that the equation of state is controlled by the wall Lorentz factor at collision, $\gamma_\star$: walls with larger $\gamma_\star$ populate higher-momentum modes and drive the fluid closer to radiation. Matter domination begins only after these modes redshift, at $a/a_\star \simeq \gamma_\star$, or after the field thermalises through self-scattering and number-changing processes. I close by discussing what this delay means for gravitational waves and primordial black holes.

Speaker: Henda Mansour (KIT)

HendaMansour_Vast2026.pdf

7 Standard Model Effective Field Theory and Oscillons

I show that the inclusion of a dimension-six operator in the Higgs potential has a dramatic impact on the stability of oscillons in the SU(2) bosonic sector of the Standard Model, extending their lifetime by orders of magnitude. This happens for the physical value of the ratio between the Higgs and W boson masses, mH /mW = 1.556 and for the dimension-six operator (Φ^†Φ)3 whose coupling constant is below the current upper bound.

I show that the oscillon can appear as a long-lived intermediate state in sphaleron decay.

Speaker: Andrzej Wereszczyński (Jagiellonian University, Kraków, Poland)

EW_oscillon_AW_MADRID.pdf

Group Photo

4:20 PM Coffee Break

Electroweak Vacuum Stability

Convener: MARIAVERONICA DE ANGELIS

8 Linking Neutrino Masses, Dark Matter, and Higgs Criticality via Gauged $U ( 1 )_{B − L}$

$U(1)_{B-L}$ gauged extensions of the Standard Model provide a simple and phenomenological rich framework for neutrino mass generation. However, dark matter and the stability of the Higgs potential are often addressed as separate issues. In this talk, we discuss a model in which these three aspects are linked. The model contains two sterile pseudo-Dirac fermion pairs in the visible sector and an additional $Z_2$-odd pair in the dark sector. Light neutrino masses arise from a hybrid mechanism, combining a tree-level inverse seesaw in the visible sector with a scotogenic-like contribution from the dark sector.
We study the resulting active-sterile mixing and the constraints from charged lepton flavor violation and collider searches. The additional sterile fermions also affect the running of the Higgs quartic coupling, lowering the scale of vacuum instability. We show that when a metastable electroweak vacuum is assumed, the resulting bound on the Higgs mass is dictated by the BSM spectrum, linking the smallness of the Higgs mass directly to vacuum metastability. We also discuss dark matter phenomenology, were the $Z'$ resonance is the most favorable region for obtaining the observed relic abundance.

Speaker: Eduardo Peinado (Instituto de Física de la Universidad Nacional Autónoma de México)

Peinado_VAST2026Madrid2.pdf

9 Constraining the real singlet extension of the SM

It is widely appreciated that the electroweak vacuum of Standard Model
resides in a region of metastability. We discuss how absolute stability is
achieved in extensions by a real scalar singlet. Using the renormalisation
group running of couplings, we identify the parameter region for stability
up to the Planck scale, mediated by the Higgs-portal interaction.
Higgs-singlet mixing connects this improved UV behaviour to low-energy
observables. In particular, modifications of the Higgs trilinear self-coupling
can be significantly larger than those of the Higgs-Z coupling, making
Higgs-potential measurements a sensitive probe for the high-scale fate
of the vacuum. We place these results in the broader context of parameter
space compatible with a strong first-order electroweak phase transition and
discuss how precision Higgs measurements and direct searches at current
and future colliders can probe Planck-safe singlet extensions.

Speaker: Moritz Bosse (TU Dortmund)

vast_bosse.pdf

10 Higgs Vacuum Stability, Reheating, and Gravitational Waves

The Higgs non-minimal coupling to background curvature is well known to stabilise Higgs fluctuations during inflation, thus alleviating the vacuum-stability problem. However, during a post-inflationary phase of kination, the same coupling can generate a large tachyonic effective mass, dramatically worsening the stability of the Higgs vacuum. In this talk, I will show how this tachyonic instability leads to a minimal scenario of reheating through the intense non-perturbative production of Higgs quanta. Reheating proceeds entirely within the Standard Model sector, while generating a characteristic gravitational-wave signal that carries information about the Higgs effective potential at high energy scales. I will also discuss the conditions under which vacuum stability is maintained across a broad parameter space, highlighting its dependence on the precise value of the top-quark pole mass.

Speaker: Giorgio Laverda (JGU Mainz)

vast_2026_laverda.pdf

Thu, October 22

Plenary: Plenary 3

Convener: Jose A. R. Cembranos (IPARCOS-UCM)

11 Surprises in Vacuum Decay: an Update

In the standard picture, the decay of a false vacuum in a single-field potential is described by a semiclassical Euclidean bounce, but the decay landscape can be much richer. Sometimes no bounce exists and the decay proceeds via a different kind of field configurations, called pseudo-bounces. Some other potentials have more than one bounce, including antibounces, configurations with the “wrong” behavior for overshoot/undershoot algorithms that can miss them. After showing how all these different decay channels, bounces, pseudo-bounces, and antibounces, fit together I discuss the number of negative modes expected for the different solutions (to determine which ones are relevant for the decay) and present some conditions that potentials have to meet to feature more than one bounce.

Speaker: José R. Espinosa (IFT, Madrid)

Espinosa_VAST.pdf

12 From real-time tunneling to vacuum decay with conserved charges

Semiclassical tunneling is usually formulated in Euclidean time, but for non-trivial initial states the appropriate Euclidean saddle and boundary conditions are not always obvious. I will introduce real-time path-integral approaches to tunneling, focusing on the steadyon construction based on complex semiclassical saddles, and use it to clarify how conserved quantities are incorporated.

I will first illustrate the steadyon construction with a quantum-mechanical particle tunneling at fixed angular momentum and energy. In this framework, tunneling is described directly in real time by complex semiclassical trajectories, with the initial-state energy and conserved charge encoded in the saddle-point boundary conditions. I will then show how, for the lowest-energy state in a fixed-charge sector, this real-time construction simplifies and can be mapped onto a purely Euclidean prescription, which can subsequently be generalized to field theory.

As a concrete application, I will discuss vacuum decay in a scalar theory with a conserved global $U(1)$ charge, starting from a homogeneous charged metastable state. Fixing the charge modifies the semiclassical problem already at the level of the saddle-point boundary conditions: the usual complex-field formulation involves twisted Euclidean boundary conditions and a complex saddle, but the problem can be recast as a real two-field Euclidean boundary-value problem that is suitable for numerical solution. At finite charge, the bounce departs from the usual $O(4)$-symmetric Coleman solution, the effective tunneling barrier is reduced, and the decay exponent decreases, leading to an enhancement of the decay rate. I will also discuss the real-time dynamics following nucleation, where charge redistribution around the expanding wall can lead to a subluminal terminal velocity even in vacuum, and finally comment on the generalization to finite temperature and finite chemical potential.

Speaker: Giulio Barni (IFT-UAM/CSIC Madrid)

VAST_Giulio_Barni.pdf

11:00 AM Coffee Break

Formal Aspects Of Phase Transitions

Convener: Manuel Reichert

13 Dynamical evolution of the pressure on the bubble wall

First-order phase transitions in the early Universe are pivotal for gravitational wave production, baryogenesis, and dark matter generation. A central question is whether bubble walls reach a subjouguet or ultra-relativistic velocity - a distinction governed by hydrodynamic obstruction, where plasma heating counteracts the vacuum pressure driving the wall. Traditional analyses assume steady-state fluid profiles, but these may fail during the wall's acceleration phase. We study the dynamical evolution of the pressure on the bubble wall in local thermal equilibrium (LTE), combining analytical approximations with numerical hydrodynamic simulations. Our results reveal that the heating wave's formation time often exceeds the wall's acceleration timescale, invalidating steady-state predictions near the Jouguet velocity. We derive a revised criterion for the maximal driving pressure, which separates deflagration/hybrid regimes from detonations/runaway walls. This criterion, validated by simulations, shows that hydrodynamic obstruction is less restrictive than steady state LTE predictions suggest.

Speaker: Miguel Etienne A Vanvlasselaer (ICCUB)

VAST_MV.pdf

14 Convergence of bubble nucleation rates

Nucleation rate computations are of broad importance in many aspects of physics, especially in the accurate estimation of the phase transition (PT) parameters that enter the gravitational wave (GW) power spectrum. The nucleation rate and PT parameters rely on perturbative estimates. Therefore, they are dependent on the details of the semiclassical picture and different orders of perturbative calculation yield very different results for the bubble nucleation rate. In this talk, I will present our lattice computations of the nucleation rate for a real scalar (ϕ³+ ϕ⁴) theory that exhibits a potential barrier at tree-level. A tree-level barrier is arguably the defining feature of a strong phase transition and would, in turn, result in a stronger gravitational wave signal. Our analysis starts from the accurate estimation of the nucleation rate, extending our earlier results to show convergence with perturbation theory. Additionally, we calculate the thermodynamic phase transition parameters of the system at the percolation temperature and calibrate the accuracy of perturbative approaches.

Speaker: Anna Kormu (University of Helsinki)

VAST 2026-Kormu.pdf

15 Toward a real-time theory of vacuum decay

False vacuum decay plays a pivotal role in the physics of the early Universe. However, we lack a satisfying theoretical understanding of this process, with existing approaches working only in imaginary (Euclidean) time, and relying on untested assumptions and approximations. In this talk, I will introduce two complementary approaches toward a real-time theory of vacuum decay: semiclassical numerical simulations on the lattice, and quantum analogue experiments in the laboratory. I will describe some of the new physical insights emerging from these approaches, and their implications for inflation, baryogenesis, and gravitational-wave cosmology.

Speaker: Alex Jenkins (University of Cambridge)

fvd-alex.pdf

16 An ode to instantons

We review a method to compute semiclassical time evolution in quantum mechanics via the path integral, illustrating how it works in paradigmatic examples. This includes the decay of general, excited metastable states from a false vacuum, generalizing a method due to Coleman for the would-be ground state. We then apply the method to compute the time-dependent decay rate of a particle that is coherently oscillating in the false vacuum region. We end with comments about the extension to quantum field theory.
Based on 2603.06575 and 2605.04139

Speaker: Dr Oliver Janssen (EPFL)

1:00 PM Lunch Break

Phenomenology Of Phase Transitions

Convener: David Mulryne

17 Filtered dark matter from strong first order phase transitions

While cooling down, the early universe is believed to have undergone symmetry breaking phase transitions. One attractive possibility for extending the Standard Model is that these phase transitions are of the first order, as they might be able to produce observable gravitational waves. We consider a dark matter freeze-out mechanism, filtering, where dark matter particles become massive in a first order phase transition. Since the mass of the dark matter increases at the phase boundary, only sufficiently energetic particles are able to enter the bubbles of the new phase, while others are reflected and will be annihilated. We focus specifically on a subset of phase transitions, deflagrations, during which the bubble walls are subsonic and slowed down as they expand. As the transition proceeds, this causes heated droplets of metastable vacuum to be formed. In the droplet stage the wall velocity and the temperature profile differ from the initial asymptotic expansion profile. We examine these parameters and evaluate how they affect the dark matter production. We have found that different amounts of dark matter are produced during the initial expansion and the final slowed down droplet stage. Taking this into account, we calculate a realistic estimate for the dark matter relic abundance produced by filtering in deflagration scenarios.

Speaker: Satumaaria Sukuvaara (University of Helsinki)

Satumaaria_Sukuvaara_VAST.pdf

18 PBH Dark Matter and GWs from Axion Inflation

A rolling axion inflaton can amplify one polarization of a gauge field through its pseudoscalar coupling. The amplified fields source scalar fluctuations and gravitational waves, and their backreaction can substantially change the inflaton’s motion. In this talk, I will present our numerical study of this coupled evolution and discuss when the homogeneous-backreaction approximation remains reliable. We assess this by comparing the inflaton gradient energy with the kinetic energy of the homogeneous field.

We find examples in which oscillations in the inflaton velocity produce potentially detectable peaks in the gravitational-wave spectrum while gradients remain subdominant. In this regime, enhanced scalar fluctuations can also form primordial black holes (PBHs) that make up all dark matter in the asteroid-mass window. Because the statistics of the rare fluctuations responsible for PBH formation remain uncertain, we consider both Gaussian and chi-squared distributions. For both benchmarks, the gravitational-wave background contains contributions sourced directly by the gauge fields and induced by scalar fluctuations, with a total signal within LISA’s projected sensitivity. For the same PBH abundance, the Gaussian case requires larger scalar fluctuations and produces a stronger gravitational-wave signal.

Speaker: Nadir Ijaz (University of Padova, Italy)

Axion_inflation_Nadir .pdf

19 When the HL-LHC is blind, LISA is deaf (but not vice versa): 2HDM collider-cosmology synergies

A first-order electroweak phase transition in the CP-conserving type-I two-Higgs-doublet model can source a gravitational wave background in the LISA band. From extensive Monte Carlo scans over the collider-viable parameter space, we find that LISA-detectable signals arise almost exclusively where observable deviations at the HL-LHC are predicted, in particular in the H → ZZ channel for a heavy neutral CP-even scalar around 180-250 GeV. The converse fails: an audible signal requires so much tuning that a collider discovery would say little about LISA. I quantify the theoretical uncertainties behind these statements by comparing BSMPT and TransitionListener, and discuss realizations without high-temperature symmetry restoration, which combine the loudest gravitational wave signals with the weakest collider coverage.

Speaker: Carlo Tasillo (IFIC, Valencia University)

2026_10_22_Madrid_Carlo_Tasillo.pdf

20 Gravitational Waves from Critical Dynamics

The critical, universal behaviour of any second-order phase transition gives rise to stochastic anisotropic stresses. These in turn source a stochastic spectrum of gravitational waves. In this talk, I show how to derive the rate of gravitational wave production for second-order phase transitions in the simplest dynamical universality class, Model A, describing a diffusive scalar order parameter. The result is a broken power law which grows towards high frequencies.

Speaker: Oliver Gould (University of Nottingham)

VAST_Critical_Dynamics.pdf

4:20 PM Coffee Break

Formal Aspects Of Phase Transitions

Convener: Oliver Gould

21 On the validity of finite-temperature EFTs

Phase transitions are violent and fascinating phenomena that may have occurred in the early Universe. When a hierarchy of scales is present, they can be studied using finite-temperature Effective Field Theories, obtained by integrating out the heavier degrees of freedom. Such EFTs provide a reliable description as long as the dynamics are dominated by the most relevant operators. In this talk, I will discuss the limitations of this approach, showing how and when higher-dimensional operators can significantly affect the prediction of strong first-order phase transitions, including those potentially detectable by LISA. I will then discuss the validity of these techniques in the context of the widely studied scenario of supercooled phase transitions.

Speaker: Fabio Bernardo (Université de Genève)

Presentation_Fabio_Bernardo_Vast_MADRID_2.pdf

22 A consistent EFT expansion of the bubble nucleation rate

The high-temperature effective field theory (3D EFT) approach to phase transitions relies on the assumption of a power counting which organizes the EFT expansion. Most studies truncate such expansions at superrenormalizable level, yet there is increasing evidence of the relevance of higher-dimensional corrections in the strong transition regime. A consistent treatment of such corrections, preserving gauge and field redefinition invariance, involves the computation of all observables in a strict perturbative expansion. While this program has been succesful in the computation of critical parameters, these higher-order corrections have not yet been explored in the nucleation rate.

In this talk, I will show how to perform a strict perturbative expansion of the nucleation rate, consistent with the EFT power counting. This will allow us to prove invariance under field redefinitions, and to quantify the effect of higher-dimensional operators on this observable. Finally, I shall discuss the computation of the bounce order by order, as well as the evaluation of the action and functional determinants in the bounce background.

Speaker: Javier López Miras (Universidad de Granada)

VAST2026.pdf

23 Hot news on the phase structure of the SMEFT

In this talk I will discuss our recent advancements, and the technical challenges involved, in the precise determination of the high-temperature limit of the Standard Model Effective Field Theory (SMEFT). Through a comparison with lattice data, we provide evidence that the SMEFT may undergo a first-order phase transition even in the absence of direct modifications of the Higgs potential. In such case, however, our analysis implies that the phase transition is not sufficiently strong, narrowing down the new physics that allows for electroweak baryogenesis and the production of observable gravitational waves.

Speaker: Luis Gil (Universidad de Granada)

Gil_VAST26_updated.pdf

Conference Dinner

Conference dinner at "Bodega de los secretos"

Fri, October 23

Plenary: Plenary 4

Convener: Jose A. R. Cembranos (IPARCOS-UCM)

24 Bubble trouble

Everything in our universe is made of matter and not antimatter, and we do not understand why. An exciting possibility is that this asymmetry between baryons and antibaryons was created during the electroweak phase transition, in a process called electroweak baryogenesis. The scenario requires new physics at the electroweak scale, which can be tested by experiments. Much progress has been made in recent years, both on the theoretical and experimental side, with the result that the vanilla implementation of electroweak baryogenesis is now in trouble. I willl give a status update of electroweak baryogenesis, and discuss some of the ways forward.

Speaker: marieke postma (Nikhef)

25 Exploring gravitational waves from strong phase transitions

A leading candidate for a stochastic source of primordial gravitational waves would be a first order phase transition. Strong electroweak-scale phase transitions in particular stand a good chance of being constrained by gravitational wave data from future missions such as LISA and Taiji, and thus it is important to understand the signal from strong transitions. However, strong transitions also lead to highly nonlinear behaviour over many length- and timescales, which makes for a challenging topic of study both for analytical and simulation approaches. In arXiv:2505.17824 we presented results of large-scale simulations for a strong detonation and deflagration, demonstrating features including the saturation of the signal. Here we extend this work to a scan across wall velocities, bubble separations and phase transition strengths. Employing the field-fluid simulation model (and our SCOTTS code) we study gravitational waves from strong hybrids, explore the role of droplets, and investigate the 'low-frequency' shallow slope. We believe that studies like this will be crucial to solving the 'inverse problem' of constraining (or detecting) a phase transition, given gravitational wave data.

Speaker: David Weir (University of Helsinki)

Link to presentation

11:00 AM Coffee break

Plenary: Plenary 5

Convener: Juan José Sanz Cillero (Universidad Complutense de Madrid & IPARCOS)

26 Baryogenesis from Exploding Primordial Black Holes

Exploding primordial black holes can source baryon asymmetry soon after the electroweak phase transition, as high-energy Hawking radiation drives ultrarelativistic shocks in the surrounding plasma. The shocks and their trailing rarefaction waves delineate two bubble-like walls around a shell of superheated fluid, in which electroweak symmetry is restored. These moving interfaces source chiral charge, which is converted to baryon number. Upon adding a simple CP-violating operator at the TeV scale, this mechanism yields the observed baryon asymmetry with minimal dependence on PBH model parameters.

Speaker: David Kaiser (MIT)

Kaiser.PBHsBAU(Oct2026).pdf

Phenomenology Of Phase Transitions

Convener: Juan José Sanz Cillero (Universidad Complutense de Madrid & IPARCOS)

27 Beyond Nambu-Goto: Towards an EFT of Cosmic Strings and Their Massive Excitations

Cosmic strings arise naturally in theories with spontaneously broken symmetries and can carry important information about the dynamics of phase transitions in the early Universe. In this talk I will present a systematic framework for deriving the effective action governing the Goldstone degrees of freedom associated with the breaking of spacetime symmetries by string defects, together with their couplings to massive bound states localized on the string. I will show how the predictions of the resulting worldsheet effective theory compare with the full field-theoretic evolution of local strings, highlighting situations in which massive excitations can significantly affect the string dynamics. Finally, I will discuss possible implications for cosmic string network evolution and the resulting gravitational wave spectrum.

Speaker: Alberto García Martín-Caro (Universidades de Vigo)

28 The Slingshot Effect: A Solution to the Magnetic Monopole Problem

The Standard Model of particle physics provides a good approximation at low energy scales. In the early hot universe, however, it is assumed that there was a so-called grand unified theory, which unified the three fundamental forces of particle physics. However, all attempts to unify these three forces lead to a possible overabundance of ’t Hooft–Polyakov magnetic monopoles. But why magnetic monopoles haven’t been observed so far?

In my presentation, I will discuss the magnetic monopole problem and present one solution in more detail: monopoles connected by cosmic strings (Langacker and Pi, 1980). I will focus particularly on the “slingshot effect,” which describes monopoles traversing the boundary between Coulomb and confining phases. During this passage, the gauge field becomes confined within a cosmic string connected to the domain wall separating the two phases. This phenomenon, which may have occurred in the early universe, could leave observable imprints such as gravitational waves. Using numerical tools, we calculated the gravitational wave spectrum and made first estimates of the observability of the slingshot effect.

Speaker: Maximilian Bachmaier (University of the Basque Country)

MaximilianBachmaier_VAST.pptx

1:05 PM End of conference