The early universe cooled like a giant cauldron. And just like cracks can form on a freezing pond, cracks could appear in space — domain walls. These are folds in space itself, boundaries between regions with different properties. When such a wall vanished, it shook spacetime, producing gravitational waves.
The future space observatory LISA will catch this trembling using laser beams between three satellites. The authors of the study tested how accurately LISA could recover the signal from domain walls. They created precision maps: under what conditions the signal would be clear, and under what conditions it would blur into the background noise from binary stars in our galaxy and beyond. It turns out that everything is best seen when the wall vanished at temperatures between $10^3$ and $10^6$ GeV (roughly from 10 trillion to 10 billion trillion degrees). In this range, both the temperature of the event and the strength of the signal can be measured simultaneously — without confusion.
Such signals will fall in the millihertz range — the frequency at which LISA operates. Their peak power will be at least $4\times10^{-11}$ in the units used to measure gravitational wave energy. It's like hearing a whisper in a quiet room if you know what to listen for.
A temperature of $10^6$ GeV is about 10 billion times hotter than the center of the Sun.
In the novel "The Doomed City" by the Strugatsky brothers, the characters encounter an invisible wall dividing worlds — similar to domain walls in cosmology.