Pressure on the bubble wall in the relativistic regime

• Miguel Vanvlasselaer (VUB)

Room: Building 2a, Seminar room 2 In this talk we will review the several contributions to the pressure to the bubble wall assuming the relativistic regime, where particles can be taken as collisionless within the region of the wall. We will pay attention to leading and next-to-leading order contributions and study the effects of longitudinal modes, which undergo a discontinuity across the step wall, within the formalism of "left and right-movers".

Ultrarelativistic walls and their gravitational-wave imprint

• Bogumiła Świeżewska (University of Warsaw)

Room: Building 2a, Seminar room 2 In this talk, I will discuss a supercooled phase transition present in a model with classical conformal symmetry. I will review the details of the model and the predictions for the properties of the phase transition, dark matter, and resulting gravitational-wave spectra paying special attention to the behaviour of the bubble walls. Using this exemplary model I will aim to summarize the state of the art for ultrarelativistic walls.

Baryogenesis via relativistic bubble expansion

• Simone Blasi (Vrije U., Brussels)

Room: Building 2a, Seminar room 2 In this talk we will present a novel mechanism for baryogenesis relying on ultra relativistic bubble walls during a first order phase transition. The baryon asymmetry is sourced by the CP and baryon-number violating decay of heavy and out-of-equilibrium states which are produced as a result of high-energy particle-wall collisions. We illustrate two different realizations based on 1) light particles gaining a large mass upon crossing the bubble wall, and 2) production of heavy particles from light states scattering off the wall. The regime of the phase transition compatible with the observed baryon asymmetry (namely, moderate supercooling and relativistic walls) is complementary to standard scenarios of electroweak baryogenesis, and generically predicts a large background of gravitational waves.

Bubbles from Dark Confinement with Holography

• Nicklas Ramberg (Johannes gutenberg universität Mainz)

Room: Building 2a, Seminar room 2 In this talk, we demonstrate how to predict the gravitational wave spectra of Strongly coupled QFTs using holography and lattice data input for a pure SU(3) Yang-Mills theory with minor uncertainties. We will elaborate on how we obtain an effective potential using holography with the free energy landscape approach and formulate an effective action. Once the effective action is in our grasp, we will use this to study bubble nucleation to predict the gravitational wave spectra. Furthermore, we will discuss how the bubble wall velocity computations can be made in steady-state configurations using holographic techniques by computations of the plasma friction force.

Discussion: ultra-relativistic walls Room: Building 2a, Seminar room 2

A sonic boom in bubble wall friction?

• Thomas Konstandin (DESY (Cosmology))

Room: Building 2a, Seminar room 2 In bubble wall friction calculations, a singularity is found in the fluctuations as the wall approaches the speed of sound. We argue that a discontinuity across the speed of sound is expected on general grounds, which manifests itself as the singularity in the solution of the linearized system.

First principles determination of bubble wall velocity and local thermal equilibrium approximation

• Benoit Laurent (McGill University)

Room: Building 2a, Seminar room 2 In the first part of this talk, I will derive the fluid equations needed to compute the wall velocity from first principles. By treating the background and out-of-equilibrium perturbations in a consistent way, the resulting equations are free of the discontinuity at v_w=c_s that was observed in previous studies. I will show that the solutions can naturally be classified as deflagration/hybrid walls (v_w ~ c_s) or ultrarelativistic detonations. In the second part, I will explain how this calculation can be significantly simplified when local thermal equilibrium (LTE) is maintained in the plasma. Using this LTE assumption, the fluid equations can be reexpressed in terms of only four parameters that completely characterize a particle physics model. I will present an efficient algorithm to solve these equations and discuss the properties of their solutions. Finally, I will compute the kinetic energy fraction which is essential for predicting the gravitational wave spectrum produced during the phase transition.

Collision Integrals for Cosmological Phase Transitions

• Andrea Guiggiani (INFN,Universit`a di Firenze)

Room: Building 2a, Seminar room 2 Bubble nucleation is a key aspect of a cosmological first-order phase transition. The non-equilibrium bubble dynamics and the properties of the transition are controlled by the density perturbations in the hot plasma. We present a new spectral method devised for a fast and reliable computation of the collision integral in the Boltzmann equations. In a scalar singlet extension of the Standard Model chosen as a benchmark scenario, we test our algorithm, determining the bubble speed and profile, and we assess the impact of the out-of-equilibrium dynamics.

Hydrodynamical obstructions to bubble growth

• Mateusz Zych (Warsaw U.)

Room: Building 2a, Seminar room 2 Terminal velocity reached by bubble walls in first order phase transitions is an important parameter determining both primordial gravitational-wave spectrum and production of baryon asymmetry in models of electroweak baryogenesis. We developed a numerical code to study real-time evolution of expanding bubbles and investigated how their walls reach stationary states. Our results agree with the profiles obtained within the so-called bag model with very good accuracy, however, not all such solutions are stable and are realized in dynamical systems. Depending on the exact shape of the potential there is always a range of wall velocities where no steady state solutions exist. This behavior in deflagrations is explained by hydrodynamical obstruction where solutions that would heat the plasma outside the wall above the critical temperature and cause local symmetry restoration are forbidden. For even more affected hybrid solutions the causes are less straight forward, however, we provide a simple numerical fit allowing one to verify if a solution with a given velocity is allowed simply by computing the ratio of the nucleation temperature to the critical one for the potential in question.

Discussion: solving the Boltzmann equations Room: Building 2a, Seminar room 2

Hydrodynamic backreaction forces in expanding bubbles

• Carlos Tamarit (Technische Universität München)

Room: Building 2a, Seminar room 2 In first-order cosmological phase transitions, it is commonly accepted that subluminal bubble expansion requires out-of-equilibrium interactions with the plasma which are captured by friction terms in the equations of motion for the scalar field. This has been disputed in works pointing out subluminal velocities in local equilibrium arising either from hydrodynamic effects in deflagrations or from the entropy change across the bubble wall in general situations. In this talk it will be argued that both effects are related and can be understood from the conservation of the entropy of the degrees of freedom in local equilibrium. Despite the lack of an explicit friction term in the equation of motion for the scalar field undergoing the phase transition, the friction effect arises from temperature gradients across the bubble wall, which are enforced by the background field dependence of the conserved entropy current in the plasma. This can lead to subluminal speeds for both deflagrations and detonations. The effects can be accounted for by simply imposing local conservation of stress-energy and including field-dependent thermal contributions to the effective potential. Although one can apply the "bag model" parameterization of the energy-momentum tensor of the plasma, determining the correct parameters still requires solving for the field profiles across the wall.

Bubble wall velocities in local thermal equilibrium

• Wen-Yuan Ai (King's College London)

Room: Building 2a, Seminar room 2 It is commonly expected that a friction force on the bubble wall can only arise from out-of-equilibrium effects. In this talk, I will discuss the bubble wall motion in local thermal equilibrium. We show that there is a nonvanishing effective friction on the wall in local thermal equilibrium provided that the plasma temperature distribution is inhomogeneous across the wall. Further, we propose a new matching condition from local entropy conservation. With this, we are able to determine bubble wall velocities in local thermal equilibrium in a model-independent way.

High-temperature effective field theories and the bubble wall speed

• Oliver Gould (University of Nottingham)

Room: Building 2a, Seminar room 2 In the study of the equilibrium properties of high-temperature phase transitions, effective field theories have been extremely fruitful. On the one hand, they make possible nonperturbative lattice simulations, which yield unambiguously reliable results up to small statistical uncertainties. On the other hand, by knitting together chains of effective field theories, perturbative predictions for first-order phase transitions can be made to converge towards lattice results to high accuracy. For the bubble wall speed, the same hierarchies of scales are present, plus more, each with their own effective description. In this talk, I will give an overview of where and how these different scales arise in computations of the bubble wall speed, and what we can learn from recent progress in the context of equilibrium physics.

Phase transition thermodynamic parameters at high precision

• Philipp Schicho (Goethe University Frankfurt)

Room: Building 2a, Seminar room 2 Together with the bubble wall velocity, uncertainties of the thermodynamic parameters of the electroweak phase transition can still be large and are subject to the finite-temperature scale hierarchy of gauge theories. While massless gauge bosons are rendered non-perturbative in the infrared, scalar bosons face slow perturbative convergence. To reliably describe the phase transition thermodynamics, both perturbative and non-perturbative methods are needed. At the intersection between these methods, one can construct a three-dimensional effective theory that systematically includes thermal resummations to all orders. Focusing on generic scalar extensions beyond the Standard Model, I determine their dimensionally reduced theory and the corresponding effective potential using the in-house software package DRalgo [1]. Finally, I present a minimal approach [2] that reconciles both gauge invariance and thermal resummation suitable for precision computations of the thermodynamic parameters of cosmological first-order phase transitions. [1] A. Ekstedt, P. Schicho, and T. V. I. Tenkanen, DRalgo: a package for effective field theory approach for thermal phase transitions, [2205.08815] [2] P. Schicho, T. V. I. Tenkanen, and G. White, Combining thermal resummation and gauge invariance for electroweak phase transition, [2203.04284]

Thermal bubble nucleation and effective field theories

• Joonas Hirvonen (University of Helsinki)

Room: Building 2a, Seminar room 2 A limiting factor in accurately determining the gravitational wave spectrum from an underlying microphysical model, in addition to the wall speed, is the nucleation rate. I will discuss recent progresses in computing nucleation rates reliably, in particular a high-temperature effective field theory framework. At high-temperatures, there is a well-known hierarchy between the thermal particles and the long-wavelength classical fields, central to wall-speed calculations as well. This scale hierarchy can be leveraged to create an effective statistical description for the nucleating bubbles. This way, one can avoid typical issues encountered in thermal nucleation calculations: double counting, stray imaginary parts, gauge dependence and diverging derivative expansions. It also eases more accurate computations. The framework is especially useful in perturbative symmetry-breaking phase transitions, in which an additional scale hierarchy between the long-wavelength classical fields can occur naturally. Although the computational methods might not be directly relevant to the computation of wall velocities, the broad ideas of length scales may become more important when pushing the computation to higher precision.

Talk: Yikun Wang Room: Building 2a, Seminar room 2

Discussion: finite-temperature precision and local thermal equilibrium Room: Building 2a, Seminar room 2