Primordial fractures in space-time may cause faint gravitational waves.
Astronomers have discovered evidence for domain walls in the early universe, which could have released floods of gravitational waves. The GUT phase transition of the universe may have left behind a network of boundaries between different configurations of space-time, which could have decayed into other particles, releasing a flood of gravitational waves that could persist to the present-day universe. Pulsar timing arrays have already found hints of a signal, which could be explained by domain walls decaying in the early universe. Future pulsar timing measurements should be able to distinguish this model from the traditional picture of colliding supermassive black holes.