Recent advances in precision cosmology have provided detailed insights into our universe's history, from the earliest moments during the inflationary epoch to the periods following Big Bang Nucleosynthesis (BBN). However, the intermediate era—spanning potentially up to 36 orders of magnitude in time—remains largely unexplored. One intriguing possibility is that the universe underwent an early matter-dominated era (EMDE) before BBN. As an example, consider a sector similar to the Standard Model sector but whose particles were not in thermal equilibrium with Standard Model particles. When the temperature of this hidden sector falls below the mass of its lightest particle, the hidden sector becomes non-relativistic. As non-relativistic matter's energy dilutes slowly compared to radiation in an exapnding universe, this sector can come to dominate the energy density of the universe. When this occurs the universe would enter an EMDE. Standard cosmology is restored when the particle causing EMDE decays into Standard Model particles, which must occur before BBN.
If the EMDE lasts longer than 12 e-folds, then gravity has enough time to cause density perturbations to grow non-linear, leading to the formation of halos in the early universe. Halos are prone to further collapse because systems in virial equilibrium have negative heat capacity: losing energy raises their temperature, which accelerates further energy loss. As higher virial speed implies more compact structures, halos are predisposed to produce more compact systems as long as there is a mechanism for heat loss. In conventional astrophysical halos, energy loss typically occurs through radiative cooling. In EMDE scenarios without radiative channels, it may seem that compact object formation is suppressed.
In this study, we highlight that if the particle causing EMDE has self-interactions, then that alone is sufficient to form compact objects, including primordial black holes (PBHs). Specifically, self-interactions cause heat loss through particle ejections, triggering a process known as gravothermal collapse.
We also find that particle self-annihilations can inhibit gravothermal collapse, leading to the formation of cannibal star. We find for simple toy models, cannibal star formation always inhibits gravothermal collapse. If these cannibal stars can accrete surrounding matter, then they can eventually collapse into PBHs. However, further study is required to see if cannibal star can efficiently accrete surrounding matter.