Numerical simulations of acoustically generated gravitational waves at a first order phase transition
Abstract
We present details of numerical simulations of the gravitational radiation produced by a first order thermal phase transition in the early Universe. We confirm that the dominant source of gravitational waves is sound waves generated by the expanding bubbles of the lowtemperature phase. We demonstrate that the sound waves have a power spectrum with a powerlaw form between the scales set by the average bubble separation (which sets the length scale of the fluid flow L_{f}) and the bubble wall width. The sound waves generate gravitational waves whose power spectrum also has a powerlaw form, at a rate proportional to L_{f} and the square of the fluid kinetic energy density. We identify a dimensionless parameter Ω_{∼GW} characterizing the efficiency of this "acoustic" gravitational wave production whose value is 8 π Ω_{∼GW}≃0.8 ±0.1 across all our simulations. We compare the acoustic gravitational waves with the standard prediction from the envelope approximation. Not only is the power spectrum steeper (apart from an initial transient) but the gravitational wave energy density is generically larger by the ratio of the Hubble time to the phase transition duration, which can be 2 orders of magnitude or more in a typical first order electroweak phase transition.
 Publication:

Physical Review D
 Pub Date:
 December 2015
 DOI:
 10.1103/PhysRevD.92.123009
 arXiv:
 arXiv:1504.03291
 Bibcode:
 2015PhRvD..92l3009H
 Keywords:

 64.60.Q;
 47.75.+f;
 95.30.Lz;
 Nucleation;
 Relativistic fluid dynamics;
 Hydrodynamics;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Phenomenology
 EPrint:
 22 pages, 13 figures