Dark Matter and Gravity Waves from a Dark Big Bang
The Hot Big Bang is often considered as the origin of all matter and radiation in the Universe. Primordial nucleosynthesis (BBN) provides strong evidence that the early Universe contained a hot plasma of photons and baryons with a temperature $T>\text{MeV}$. However, the earliest probes of dark matter originate from much later times around the epoch of structure formation. In this work we describe a scenario in which dark matter (and possibly dark radiation) can be formed around or even after BBN in a second Big Bang which we dub the ``Dark Big Bang''. The latter occurs through a phase transition in the dark sector which transforms dark vacuum energy into a hot dark plasma of particles; in this paper we focus on a first-order phase transition for the Dark Big Bang. The correct dark matter abundance can be set by dark matter cannibalism or by pair-annihilation within the dark sector followed by a thermal freeze-out. Alternatively ultra-heavy ``dark-zilla'' dark matter can originate directly from bubble collisions during the Dark Big Bang. We will show that the Dark Big Bang is consistent with constraints from structure formation and the Cosmic Microwave Background (CMB) if it occurred when the Universe was less than one month old, corresponding to a temperature in the visible sector above $\mathcal{O}$(keV). While the dark matter evades direct and indirect detection, the Dark Big Bang gives rise to striking gravity wave signatures to be tested at pulsar timing array experiments. Furthermore, the Dark Big Bang allows for realizations of self-interacting and/or warm dark matter which suggest exciting discovery potential in future small-scale structure observations.