I will show how first order phase transitions (PT) in the early universe, with relativistic bubble walls, constitute particle accelerators and colliders. These `bubbletrons' offer novel opportunities of observational access to very high energy scales, in addition to the gravitational waves from the PT. As two examples, I will discuss: i) non-adiabatic production of dark matter which is so fast to leave an imprint in the matter power spectrum, for dark matter masses of 10^8-10^9 GeV and a weak-scale PT; ii) production of relics beyond the GUT scale without the need for the universe to ever reach those temperatures.
Francesco D'Eramo