Towards Simulating Primordial Black Hole Formation in a Radiation Dominated Universe

Primordial black holes (PBH), unlike stellar black holes, don't form from the death of the most massive stars. Instead, these objects form as a result of cosmological density fluctuations. Generally, this occurs when an overdensity reenters the horizon in a radiation dominated epoch; as the horizon grows that mode becomes dynamical and, if the overdensity is larger than a certain threshold, can collapse into a black hole. The resulting size of the black hole is dependent on the size of the horizon— allowing for black holes to form at any size, depending only on when they are formed. Recently, excitement about these objects has grown as they are potential solutions to relevant cosmological and astrophysical problems such as supermassive black hole formation, early galaxy formation, and dark matter. To fully understand PBH formation in the very early universe, we need realistic simulations in an expanding spacetime during a radiation dominated era. To address this, we conduct simulations of PBH formation with "messy" initial conditions in a relativistic, radiation fluid using fully nonlinear numerical general relativity.