Ultradense dark matter haloes accompany primordial black holes

Primordial black holes (PBHs) form from large-amplitude initial density fluctuations and may comprise some or all of the dark matter. If PBHs have a broadly extended mass spectrum, or in mixed PBH-particle dark matter scenarios, the extreme density fluctuations necessary to produce PBHs also lead to the formation of a much greater abundance of dark matter minihaloes that form during the radiation epoch with internal densities potentially of order $10^{12}$ M$_\odot$ pc$^{-3}$. We develop an analytic description of the formation of these ultradense haloes and use it to quantitatively compare PBH and halo distributions. PBHs that contribute only a per cent level fraction of the dark matter are accompanied by ultradense haloes that nevertheless comprise an order-unity fraction. These haloes would consist of either particle dark matter or much smaller PBHs. This finding significantly alters the predictions of many PBH scenarios, enabling a variety of new observational tests.