when the universe grew exponentially in the first tiny fraction of a second after the Big Bang, some parts of space-time expanded more quickly than others. This could have created “bubbles” of space-time that then developed into other universes. The known universe has its own laws of physics, while other universes could have different laws.
initially the universe was rapidly inflating, being in an unstable energetic vacuum-like state. It became hot only later, when this vacuum-like state decayed. Quantum fluctuations produced during inflation are responsible for galaxy formation. In some places, these quantum fluctuations are so large that they can produce new rapidly expanding parts of the universe. This process makes the universe immortal and transforms it into a multiverse, a huge fractal consisting of many exponentially large parts with different laws of low-energy physics operating in each of them.