  Quantum mechanics makes the otherwise stable vacua of a theory metastable
through the nucleation of bubbles of the new vacuum. This in turn causes a
first order phase transition. These cosmological phase transitions may have
played an important role in settling our universe into its current vacuum, and
they may also happen in future. The most important frameworks where vacuum
decay happens contain a large number of fields. Unfortunately, calculating the
tunneling rates in these models is very time-consuming. In this paper we
present a simple approximation for the tunneling rate by reducing it to a
one-field problem which is easy to calculate. We demonstrate the validity of
this approximation using our recent code "Anybubble" for several classes of
potentials.
