In string theory, one process of getting from one universe to another would be to use the property of quantum tunneling, which is where a particle is allowed to “jump” from one location to another across a barrier.
The uncertainty principle of quantum physics means that particles don’t have a definite location, but instead both the location and momentum of each particle are linked together with a sort of “fuzziness.” The more precisely you determine the location, the more fuzzy the momentum is, and vice versa.
This principle results in a strange phenomenon, known as quantum tunneling and shown in this figure. In this case, there is some sort of barrier (usually a potential energy barrier) that the particle shouldn’t be able to cross normally. But the graph, which represents the probability that the particle is in any given location, extends a bit across the barrier.
In other words, even when there’s an uncrossable barrier, there’s a slight chance — according to quantum mechanics — that a particle that should be on one side of the barrier may end up on the other side of the barrier. This behavior has been confirmed by experiment.
This provides a means that could in theory be used to access a parallel universe. Some cosmologists have suggested that exactly this physical mechanism is what started our own expansion as a universe.
The idea of quantum tunneling is key to the operation of electron-tunneling microscopes, which allow scientists to observe objects in incredibly fine detail.
Particles can only tunnel from a higher energy state into a lower energy state, though, so there are some limits on how this could be used, and the idea of using it to access another universe in a controlled way is way beyond current technology (or even current theory).
But for a sufficiently advanced civilization, one that has a theory that fully explains all aspects of physics and the ability to use vast amounts of energy, this sort of idea may be a possible means of getting to another universe.