Worth the Effort? An Examination on the Effect of Higher Diligence Calculations of the Sound Shell Model

TL;DR

This study investigates how detailed hydrodynamic calculations, including causality adjustments, influence the gravitational wave spectrum from cosmological phase transitions, revealing increased sensitivity to physical parameters and potential for richer microphysical insights.

Contribution

It demonstrates that using the full velocity profile and recent causality adjustments enhances the understanding of spectrum sensitivity to thermal parameters, beyond simplified models.

Findings

Spectrum is highly sensitive to the speed of sound.

Small changes in thermal parameters significantly alter the spectrum.

Adjustments for causality impact the spectral shape near the peak.

Abstract

The gravitational wave spectrum arising from using the full velocity profile is well known to differ qualitatively from analytic fits to a broken power law. Former studies have shown that unlike the uncertainties arising from thermal field theory, more diligence in the hydrodynamics can sometimes have limited benefit. However, this was shown in the context of broken power law fits. We test the benefits of some recent calculations in modeling the spectrum, including new developments in adjustments of the low frequency tail to be consistent with causality, but we use the full velocity profile. We find the spectral shape information has a heightened sensitivity to the speed of sound which can be demonstrated analytically, however for our benchmark model this still results in a modest difference. The reason for a heightened sensitivity is because the velocity at the boundary is quite sensitive to the speed of sound, which in turn means a small change to the speed of sound can have a large change to the shape of the velocity profile. Furthermore, even modest changes in the product $\alpha \kappa$ can make non-trivial changes to the shape around the peak. Finally, there are many points where adjusting the infrared behavior to be consistent with causality is affecting the spectrum near its peak. All this implies that the spectrum is sensitive to five thermal parameters rather than four which gives hope that an observation of a gravitational wave spectrum from a first order cosmological phase transition could eventually give even more information about the underlying microphysics responsible.