Exotic Gravitational Wave Signatures from Simultaneous Phase Transitions
  We demonstrate that the relic gravitational wave background from a multi-step
phase transition may deviate from the simple sum of the single spectra, for
phase transitions with similar nucleation temperatures $T_N$. We demonstrate
that the temperature range $\Delta T$ between the volume fractions $f(T)=0.1$
and $f(T)=0.9$ occupied by the vacuum bubbles can span $\sim 20$ GeV. This
allows for a situation in which phase transitions overlap, such that the later
bubbles may nucleate both in high temperature and intermediate temperature
phases. Such scenarios may lead to more exotic gravitational wave spectra,
which cannot be fitted that of a consecutive PTs. We demonstrate this
explicitly in the singlet extension of the Standard Model. Finally, we comment
on potential additional effects due to the more exotic dynamics of overlapping
phase transitions.
