One of the most successful theories of 20th century science is cosmic inflation, which preceded and set up the hot Big Bang. We also know how quantum fields generally work, and if inflation is a quantum field (which we strongly suspect it is), then there will always be more "still-inflating" space out there. Whenever and wherever inflation ends, you get a hot Big Bang. If inflation and quantum field theory are both correct, a Multiverse is a must.
