Hydrodynamical constraints on the bubble wall velocity

We compute the terminal bubble wall velocity during a cosmological phase transition by modelling non-equilibrium effects in the plasma with the so-called “extended fluid Ansatz”. A ϕ 6 operator is included in the Standard Model effective potential to mimic effects of new physics. Hydrodynamical heating of the plasma ahead of the bubble is taken into account. We find that the inclusion of higher order terms in the fluid Ansatz is typically relevant, and may even turn detonation solutions into deflagrations. Our results also corroborate recent findings in the literature that, for a Standard Model particle content in the plasma, only deflagration solutions are viable. However, we also show that this outcome may be altered in a theory with a different particle content.

Section: Introductionsupporting

confidence: 88%

Bubble wall velocities with an extended fluid Ansatz

Dorsch,

Pinto

2024

“…In such computations, one needs to solve the scalar field equation of motion (EoM) coupled to the Boltzmann equations describing the particles in the plasma. Simplifications can be obtained by assuming local thermal equilibrium (LTE) [74][75][76][77][78][79][80][81] or in the ultrarelativistic regime [1,[82][83][84][85][86][87]. In LTE, the wall velocity can be determined with the help of an additional matching condition for the hydrodynamic quantities on both sides of the wall [77,79].…”

Section: Jcap03(2024)037mentioning

confidence: 99%

Criterion for ultra-fast bubble walls: the impact of hydrodynamic obstruction

Ai,

Nagels,

Vanvlasselaer

2024

The Bödeker-Moore thermal friction [1] is usually used to determine whether or not a bubble wall can run away. However, the friction on the wall is not necessarily a monotonous function of the wall velocity and could have a maximum before it reaches the Bödeker-Moore limit. In this paper, we compare the maximal hydrodynamic obstruction, a frictional force that exists in local thermal equilibrium, and the Bödeker-Moore thermal friction. We study the former in a fully analytical way, clarifying its physical origin and providing a simple expression for its corresponding critical phase transition strength above which the driving force cannot be balanced out by the maximal hydrodynamic obstruction. We find that for large parameter space, the maximal hydrodynamic obstruction is larger than the Bödeker-Moore thermal friction, indicating that the conventional criterion for the runaway behavior of the bubble wall may have to be modified. We also explain how to apply efficiently the modified criterion to particle physics models and discuss possible limitations of the analysis carried out in this paper.

“…where η(ϕ, T ) encapsulates the microscopic out-of-equilibrium dynamics leading to friction between the expanding bubble and the particles of the plasma [40,41,57]. The friction function η(ϕ, T ) is difficult to calculate and is often treated as a free constant parameter [53].…”

Section: Jcap03(2024)059mentioning

confidence: 99%

“…Outside the shock the fluid is at rest with the Universe frame u sh + = 0. Hybrids may not be stable solutions for some equations of state [57].…”

Section: Jcap03(2024)059mentioning

confidence: 99%

General relativistic bubble growth in cosmological phase transitions

Giombi,

Hindmarsh

2024

We use a full general relativistic framework to study the self-similar expansion of bubbles of the stable phase into a flat Friedmann-Lemaître-Robertson-Walker Universe in a first order phase transition in the early Universe. With a simple linear barotropic equation of state in both phases, and in the limit of a phase boundary of negligible width, we find that self-similar solutions exist, which are qualitatively similar to the analogous solutions in Minkowski space, but with distinguishing features. Rarefaction waves extend to the centre of the bubble, while spatial sections near the centre of the bubble have negative curvature. Gravitational effects redistribute the kinetic energy of the fluid around the bubble, and can change the kinetic energy fraction significantly. The kinetic energy fraction of the gravitating solution can be enhanced over the analogous Minkowski solution by as much as 𝒪(1), and suppressed by a factor as larger as 𝒪(10) in case of fast detonations. The amount of negative spatial curvature at the centre of the bubble is of the same order of magnitude of the naive expectation based on considerations of the energy density perturbation in Minkowski solutions, with gravitating deflagrations less negatively curved, and detonations more. We infer that general relativistic effects might have a significant impact on accurate calculations of the gravitational wave power spectrum when the bubble size becomes comparable to the cosmological Hubble radius, affecting the primary generation from the fluid shear stress, and inducing secondary generation by scalar perturbations.