The Universe may have deviated from the radiation-dominated expansion during the earliest epochs and the processes causing that can have interesting observable consequences. In this talk, I will focus on strongly supercooled phase transitions that cause a secondary period of exponential expansion of the Universe. I will describe a class of particle physics models where such transitions can happen and discuss the observable consequences of the process in the form of gravitational waves (GWs) and primordial black holes (PBHs). I will show that if the transition is slow the GW signal generated by colliding bubble walls and fluid shells can be very strong and potentially gives a good fit to the GW background recently discovered in the pulsar timing array data. I will also show that such slow transitions lead to the formation of a large PBH abundance that can constitute all dark matter and a secondary GW signal that dominates over the signal formed by the bubble walls and fluid shells if the transition is sufficiently slow.
