For the Russian cosmologist, Alex Vilenkin, the multiple universes are separated and disconnected from one another. Each arises out of nothing by a process called quantum tunneling, spontaneously crossing the boundary between existence and non-existence with no expenditure of energy. Universes spring into action with precisely zero total energy, the positive energy of matter being opposite and equal to the negative energy of gravitation. Mass comes free, because energy is zero.
As Alan Guth says,
Putting [general relativity and quantum mechanics] together, one can imagine that the universe started in the total empty geometry – absolute nothingness – and then made a quantum tunneling transition to a nonempty state. Calculations show that a universe created this way would typically be subatomic in size, but that is no problem . . . Vilenkin was able to invoke inflation to enlarge the universe to its current size.
and Vilenkin explains,
[T]he state of “nothing” cannot be identified with absolute nothingness. The tunneling is described by the laws of quantum mechanics, and thus “nothing” should be subject to these laws. The laws of physics must have existed, even though there was no universe.
Classical theory dictates that the system can break apart only when it is excited with energy beyond the barrier height. In quantum theory there is a certain probability to tunnel out under the barrier.