Ilia Musco : Critical gravitational collapse within an expanding medium

The well-known process of Type II critical collapse has been shown to arise within an expanding medium like the early universe, in the context of spherically-symmetric simulations of primordial black hole formation. Critical collapse is characterized, over many e-foldings, by an intermediate state, where gravity and pressure balance in such a way as to give a contraction at nearly constant compactness. This intermediate state is a crucial aspect of the critical solution and can be mathematically described with a solution having a self-similar form of the Einstein equations. The way in which this matches onto an external expanding solution has been demonstrated, obtaining the well known scaling laws for the mass and formation-time of the black holes, with the exponent whose value depends on the equation of state parameter.