Gravitational wave signals of exotic phases in the early universe.

Stochastic backgrounds of gravitational waves (GWs) from the pre-BBN era can be used to probe the early stages of the universe beyond what has already been achieved with the Cosmic Microwave Background (CMB). If the source is short in duration, the long-wavelength part of the resulting GW spectrum follows a universal frequency scaling that depends on the equation of state of the universe at the time of horizon entry of the modes. The distortion of the equation of state due to massive particles becoming non-relativistic can lead to an observable dip in the GW spectrum. To illustrate this effect, a first-order chiral symmetry-breaking phase transition in the weak-confined Standard Model (WCSM) [1] is considered. This model features a large number of pions and mostly elementary fermions with masses just below the critical temperature for the phase transition. These states lead to a 20% dip in the GW power. Future sky-based GW detectors such as LISA, DECIGO, BBO, and µAres can be sensitive to such distortions in the spectrum.

References:

[1] J. Berger, A.J. Long and J. Turner, Phase of confined electroweak force in the early universe, Phys. Rev. D 100 (2019) 055005 [arXiv:1906.05157]