Speed of sound in cosmological phase transitions and effect on gravitational waves

It is commonly expected that a friction force on the bubble wall in a first-order phase transition can only arise from a departure from thermal equilibrium in the plasma. Recently however, it was argued that an effective friction, scaling as γ2 w (with γ w being the Lorentz factor for the bubble wall velocity), persists in local equilibrium. This was derived assuming constant plasma temperature and velocity throughout the wall. On the other hand, it is known that, at the leading order in derivatives, the plasma in local equilibrium only contributes a correction to the zero-temperature potential in the equation of motion of the background scalar field. For a constant plasma temperature, the equation of motion is then completely analogous to the vacuum case, the only change being a modified potential, and thus no friction should appear. We resolve these apparent contradictions in the calculations and their interpretation and show that the recently proposed effective friction in local equilibrium originates from inhomogeneous temperature distributions, such that the γ2 w -scaling of the effective force is violated. Further, we propose a new matching condition for the hydrodynamic quantities in the plasma valid in local equilibrium and tied to local entropy conservation. With this added constraint, bubble velocities in local equilibrium can be determined once the parameters in the equation of state are fixed, where we use the bag equation in order to illustrate this point. We find that there is a critical value of the transition strength αcrit such that bubble walls run away for α>αcrit.

Section: Template Model and Resultsmentioning

confidence: 99%

Section: Model-independent Matching Equationsmentioning

confidence: 87%

Bubble wall velocities in local equilibrium

Garbrecht

Tamarit

2022

Section: Jcap07(2023)002mentioning

confidence: 85%

“…For phase transitions that are dominated by the particle content of the Standard Model, one expects the speed of sound to remain rather close to c s,b ≈ 1/ √ 3 [63,64], and, depending on the required level of accuracy, computations in the bag model might suffice. For phase transitions taking place in a hidden sector, on the other hand, deviations from c s,b = 1/ √ 3 can be significant [63]. Especially in phase transitions in a strongly coupled sector, the sound speed can be very suppressed [43,65].…”

Section: Template Model and Resultsmentioning

confidence: 99%

Model-independent bubble wall velocities in local thermal equilibrium

Beaugerie

Vis

2023

Self Cite

Accurately determining bubble wall velocities in first-order phase transitions is of great importance for the prediction of gravitational wave signals and the matter-antimatter asymmetry. However, it is a challenging task which typically depends on the underlying particle physics model. Recently, it has been shown that assuming local thermal equilibrium can provide a good approximation when calculating the bubble wall velocity. In this paper, we provide a model-independent determination of bubble wall velocities in local thermal equilibrium. Our results show that, under the reasonable assumption that the sound speeds in the plasma are approximately uniform, the hydrodynamics can be fully characterized by four quantities: the phase strength αn , the ratio of the enthalpies in the broken and symmetric phases, Ψ n , and the sound speeds in both phases, cs and cb . We provide a code snippet that allows for a determination of the wall velocity and energy fraction in local thermal equilibrium in any model. In addition, we present a fit function for the wall velocity in the case cs = cb = 1/√(3).

“…The authors of ref [59]. found that a slightly smaller speed of sound is expected in the broken phase of minimal DS models, which can lead to a sizeable suppression of the GW signal for detonations.…”

mentioning

confidence: 99%

Does NANOGrav observe a dark sector phase transition?

Bringmann,

Depta,

Konstandin

et al. 2023

Gravitational waves from a first-order cosmological phase transition, at temperatures at the MeV-scale, would arguably be the most exciting explanation of the common red spectrum reported by the NANOGrav collaboration, not the least because this would be direct evidence of physics beyond the standard model. Here we perform a detailed analysis of whether such an interpretation is consistent with constraints on the released energy deriving from big bang nucleosynthesis and the cosmic microwave background. We find that a phase transition in a completely secluded dark sector with sub-horizon sized bubbles is strongly disfavoured with respect to the more conventional astrophysical explanation of the putative gravitational wave signal in terms of supermassive black hole binaries. On the other hand, a phase transition in a dark sector that subsequently decays, before the time of neutrino decoupling, remains an intriguing possibility to explain the data. From the model-building perspective, such an option is easily satisfied for couplings with the visible sector that are small enough to evade current collider and astrophysical constraints. The first indication that could eventually corroborate such an interpretation, once the observed common red spectrum is confirmed as a nHz gravitational wave background, could be the spectral tilt of the signal. In fact, the current data already show a very slight preference for a spectrum that is softer than what is expected from the leading astrophysical explanation.