Gravitational collapse and the black hole shadow

Abstract

We consider a physical scenario of spherically symmetric gravitational collapse involving a domain wall. A static black hole produces a shadow of apparent radius bBHS = 3 √ 3 GM, where M is the black hole mass. Here we analyze how this shadow is generated dynamically. We consider the medium to be transparent and the shadow arising solely from gravitational effects. We discuss this phenomenon quantitatively by studying null geodesics in a dynamical spacetime. Tracing the trajectories of photons we obtain how the distribution of energies varies as a function of time and impact parameter. In the model considered the process of shadow formation takes a time of the order of Δt ∼ 40 GM