  Cosmological phase transitions can proceed via the nucleation of bubbles that
subsequently expand and collide. The resulting gravitational wave spectrum
depends crucially on the properties of these bubbles. We extend our previous
holographic work on planar bubbles to circular bubbles in a strongly-coupled,
non-Abelian, four-dimensional gauge theory. This extension brings about two new
physical properties. First, the existence of a critical bubble, which we
determine. Second, the bubble profile at late times exhibits a richer
self-similar structure, which we verify. These results require a new 3+1
evolution code called Jecco that solves the Einstein equations in the
characteristic formulation in asymptotically AdS spaces. Jecco is written in
the Julia programming language and is freely available. We present an outline
of the code and the tests performed to assess its robustness and performance.
