We point out that dark matter which is produced non-adiabatically in a phase
transition (PT) with fast bubble walls receives a boost in velocity which leads
to long free-streaming lengths. We find that this could be observed via the
suppressed matter power spectrum for dark matter masses around 108−109
GeV and energy scales of the PT around 102−103 GeV. The PT should
take place at the border of the supercooled regime, i.e. approximately when the
Universe becomes vacuum dominated. This work offers novel physics goals for
galaxy surveys, Lyman-α, lensing, and 21-cm observations, and connects
these to the gravitational waves from such phase transitions, and more
speculatively to possible telescope signals of heavy dark matter decays.Comment: 5 pages plus appendice